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Securing a future for anonymous functions in Python

P: n/a
GvR has commented that he want to get rid of the lambda keyword for Python 3.0.
Getting rid of lambda seems like a worthy goal, but I'd prefer to see it dropped
in favour of a different syntax, rather than completely losing the ability to
have anonymous functions.

Anyway, I'm looking for feedback on a def-based syntax that came up in a recent
c.l.p discussion:
http://boredomandlaziness.skystorm.n...in-python.html

Cheers,
Nick.

--
Nick Coghlan | nc******@email.com | Brisbane, Australia
---------------------------------------------------------------
http://boredomandlaziness.skystorm.net
Jul 18 '05 #1
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P: n/a
Nick Coghlan <nc******@iinet.net.au> writes:
Anyway, I'm looking for feedback on a def-based syntax that came up in
a recent c.l.p discussion:


Looks like just an even more contorted version of lambda. It doesn't
fix lambda's main deficiency which is inability to have several
statements in the anonymous function.
Jul 18 '05 #2

P: n/a
Paul Rubin wrote:
Nick Coghlan <nc******@iinet.net.au> writes:
Anyway, I'm looking for feedback on a def-based syntax that came up in
a recent c.l.p discussion:

Looks like just an even more contorted version of lambda. It doesn't
fix lambda's main deficiency which is inability to have several
statements in the anonymous function.


Do you consider generator expressions or list comprehensions deficient because
they don't allow several statements in the body of the for loop?

Cheers,
Nick.

--
Nick Coghlan | nc******@email.com | Brisbane, Australia
---------------------------------------------------------------
http://boredomandlaziness.skystorm.net
Jul 18 '05 #3

P: n/a

"Nick Coghlan" <nc******@iinet.net.au> wrote in message
news:ma**************************************@pyth on.org...
GvR has commented that he want to get rid of the lambda keyword for Python
3.0. Getting rid of lambda seems like a worthy goal, but I'd prefer to see
it dropped in favour of a different syntax, rather than completely losing
the ability to have anonymous functions.

Anyway, I'm looking for feedback on a def-based syntax that came up in a
recent c.l.p discussion:
http://boredomandlaziness.skystorm.n...in-python.html

Cheers,
Nick.
I think it's rather baroque, and I agree with Paul Ruben
that it needs multiple statement capability.

The syntax I prefer (and I don't know if it's actually been
suggested before) is to use braces, that is { and }.

In other words, an anonymous function looks like:
{p1, p2, p3 |
stmt1
stmt2
}

There are two reasons for using braces. One is
that it's the common syntax for blocks in a large number
of languages. The other is that it should be relatively
easy to disambiguate from dictionary literals, which are
the only other current use of braces.

The parameter list is optional, as is the bar ending
the list. The reason for the bar instead of a colon
is to help the parser in the case of a single parameter,
which would look like the beginning of a dictionary
literal. If the parser doesn't need the help, then a colon
would be more consistent, hence better.

A second issue is indentation. I'd set the indentation
boundary wherever the _second_ line of the construct
starts, as long as it's to the right of the prior
indentation boundary. The unfortunate part of this, and one of the
major stumbling blocks, is that it might take some
significant reconceptualizing of the lexer and parser,
which currently isn't set up to shift from expression
back to statement mode and then return to expression
mode.

John Roth
--
Nick Coghlan | nc******@email.com | Brisbane, Australia
---------------------------------------------------------------
http://boredomandlaziness.skystorm.net


Jul 18 '05 #4

P: n/a
John Roth wrote:
The syntax I prefer (and I don't know if it's actually been
suggested before) is to use braces, that is { and }.

In other words, an anonymous function looks like:
{p1, p2, p3 |
stmt1
stmt2
}


What's the advantage of something like that over the non-anonymous
equivalent:

def some_func(p1, p2, p3):
stmt1
stmt2

I appreciate some of the motivation, but merely avoiding giving
something a name doesn't seem like a laudible goal.

The one motivation I can see for function expressions is
callback-oriented programming, like:

get_web_page(url,
when_retrieved={page |
give_page_to_other_object(munge_page(page))})

The problem with the normal function in this case is the order of
statements is reversed:

def when_retrieved_callback(page):
give_page_to_other_object(munge_page(page))
get_web_page(url, when_retrieved=when_retrieved_callback)

Oh, and you have to type the name twice, which is annoying. For most
other functional programming constructs, list (and not generator)
comprehensions work well enough, and the overhead of naming functions
just isn't that big a deal.

I think this specific use case -- defining callbacks -- should be
addressed, rather than proposing a solution to something that isn't
necessary. Which is to say, no one *needs* anonymous functions; people
may need things which anonymous functions provide, but maybe there's
other ways to provide the same thing. Decorator abuse, for instance ;)

def get_web_page_decorator(url):
def decorator(func):
return get_web_page(url, when_retrieved=func)
return decorator

@get_web_page_decorator(url)
def when_retrieved(page):
give_page_to_other_object(munge_page(page))

Or even (given a partial function as defined in some PEP, the number of
which I don't remember):

@partial(get_web_page, url)
def when_retrieved(page):
give_page_to_other_object(munge_page(page))

It's okay not to like this proposal, I don't think I'm really serious.
But I do think there's other ways to approach this. Function
expressions could get really out of hand, IMHO, and could easily lead to
twenty-line "expressions". That's aesthetically incompatible with
Python source, IMHO.

--
Ian Bicking / ia**@colorstudy.com / http://blog.ianbicking.org
Jul 18 '05 #5

P: n/a
Ian Bicking <ia**@colorstudy.com> wrote:
I think this specific use case -- defining callbacks -- should be
addressed, rather than proposing a solution to something that isn't
necessary. Which is to say, no one *needs* anonymous functions; people
may need things which anonymous functions provide, but maybe there's
other ways to provide the same thing. Decorator abuse, for instance ;)


I'm not a big functional programming fan, so it should not come as a
surprise that I don't often use lambda. The one place I do use it is in
unit tests, where assertRaises() requires a callable. If what you're
testing is an expression, you need to wrap it in a lambda.

I suppose you could call this a special case of a callback.
Jul 18 '05 #6

P: n/a

John> In other words, an anonymous function looks like:
John> {p1, p2, p3 |
John> stmt1
John> stmt2
John> }

John> There are two reasons for using braces. One is that it's the
John> common syntax for blocks in a large number of languages.

Yeah, but it's not how blocks are spelled in Python. As Nick pointed out on
his blog, allowing statements within expressions risks making code more
difficult to read and understand.

People keep trying to make Python something it is not. It is not
fundamentally an expression-only language like Lisp, nor is it an
expression-equals-statement language like C. There are good reasons why
Guido chose the relationship between simple statements, compound statements
and expressions that he did, readability and error avoidance being key.

Skip
Jul 18 '05 #7

P: n/a
Nick Coghlan wrote:
GvR has commented that he want to get rid of the lambda keyword for Python 3.0. Getting rid of lambda seems like a worthy goal, but I'd prefer to see it dropped in favour of a different syntax, rather than completely losing the ability to have anonymous functions.


I shall either coin or reuse a new term here: "premature optimization
of the Python language."

(Please note that the word premature is not intended to be an accurate
description, but irony by analogy, so spare me any semantic
nitpicking.)

In much the same way that programmers often spend a lot of time
optimizing parts of their program that will yield very minor dividends,
while they could have spent that time working on other things that will
pay off a lot, many of the wannabe language designers here are spending
a lot of time on aspects of the language for which any improvement
would only pay small dividends.

I think the worry about anonymous functions is one of the most
widespread cases of "premature optimization of the Python Language."
One could argue about the various benefits of particular choices, maybe
even make a convincing case that one is best in accord with the design
goals of Python; but in the end, the divends are small compared to
improving other aspects of the language.
--
CARL BANKS

Jul 18 '05 #8

P: n/a
Ian Bicking <ia**@colorstudy.com> writes:
The one motivation I can see for function expressions is
callback-oriented programming, like:

get_web_page(url,
when_retrieved={page |
give_page_to_other_object(munge_page(page))})
This is my primary use case for lambda's nowadays as well - typically
just to provide a way to convert the input to a callback into a call
to some other routine. I do a lot of Twisted stuff, whose deferred
objects make heavy use of single parameter callbacks, and often you
just want to call the next method in sequence, with some minor change
(or to ignore) the last result.

So for example, an asynchronous sequence of operations might be like:

d = some_deferred_function()
d.addCallback(lambda x: next_function())
d.addCallback(lambda blah: third_function(otherargs, blah))
d.addCallback(lambda x: last_function())

which to me is more readable (in terms of seeing the sequence of
operations being performed in their proper order), then something like:

def cb_next(x):
return next_function()
def cb_third(blah, otherargs):
return third_function(otherargs, blah)
def cb_last(x):
return last_function()

d = some_deferred_function()
d.addCallback(cb_next)
d.addCallback(cb_third, otherargs)
d.addCallback(cb_next)

which has an extra layer of naming (the callback functions), and
requires more effort to follow the flow of what is really just a simple
sequence of three functions being called.
I think this specific use case -- defining callbacks -- should be
addressed, rather than proposing a solution to something that isn't
necessary. (...)


I'd be interested in this approach too, especially if it made it simpler
to handle simple manipulation of callback arguments (e.g., since I often
ignore a successful prior result in a callback in order to just move on
to the next function in sequence).

-- David
Jul 18 '05 #9

P: n/a
On Thu, 30 Dec 2004 23:28:46 +1000, Nick Coghlan <nc******@iinet.net.au> wrote:
GvR has commented that he want to get rid of the lambda keyword for Python 3.0.
Getting rid of lambda seems like a worthy goal, but I'd prefer to see it dropped
in favour of a different syntax, rather than completely losing the ability to
have anonymous functions.

Anyway, I'm looking for feedback on a def-based syntax that came up in a recent
c.l.p discussion:
http://boredomandlaziness.skystorm.n...in-python.html

Nit: You didn't try the code you posted ;-)
funcs = [(lambda x: x + i) for i in range(10)]

def incrementors(): ... for i in range(10):
... def incrementor(x):
... return x + i
... yield incrementor
... #funcs = list(incrementors) ... funcs2 = list(incrementors())
for f in funcs: print f(0), ...
9 9 9 9 9 9 9 9 9 9 for f in funcs2: print f(0),

...
9 9 9 9 9 9 9 9 9 9

This is an easy trap to fall into, so if the new lambda-substitute could
provide a prettier current-closure-variable-value capture than passing a dummy default
value or nesting another def and passing the value in, to provide a private closure for each,
that might be something to think about.

IMO an anonymous def that exactly duplicates ordinary def except for leaving out
the function name and having a local indentation context would maximize flexibility
and also re-use of compiler code. People could abuse it, but that's already true
of many Python features.

From your web page:
----
def either(condition, true_case, false_case):
if condition:
return true_case()
else:
return false_case()

print either(A == B, (def "A equals B"), (def "A does not equal B"))
either(thefile, (def thefile.close()), (def 0))
----

I'd rather see (:something) than (def something) for this special case,
but the full-fledged anonymous def would spell it thus:

print either(A == B, (def():return "A equals B"), (def(): return "A does not equal B"))
either(thefile, (def(): return thefile.close()), (def(): return 0))

BTW,

funcs = [(lambda x: x + i) for i in range(10)]

might be spelled

funcs = [(def(x, i=i): return x + i) for i in range(10)]

or optionally

funcs = [(
def(x, i=i):
return x + i
) for i in range(10)]

or

funcs = [(def(x, i=i):
return x + i) for i in range(10)]
or
funcs = [(def(x, i=i):
return x + i)
for i in range(10)]
or
funcs = [def(x, i=i):
return x + i
for i in range(10)]
or
funcs = [
def(x, i=i):
return x + i
for i in range(10)]

and so on. (the def defines the indentation base if the suite is indented, and the closing ')'
terminates the anonymous def explicitly, or a dedent to the level of the def or less can do it,
as in the last two examples).

This one

(def f(a) + g(b) - h(c) from (a, b, c))

would be spelled (if I undestand your example)

(def(a, b, c): return f(a)+g(b)+h(c))

which seems to me familiar and easy to understand.

BTW, there are old threads where this and other formats were discussed. I am still
partial to the full anonymous def with nesting indentation rules. Syntactic sugar
could be provided for useful abbreviations (that could possibly be expanded by the
tokenizer -- re which possibilities I haven't seen any discussion than my own
recent post, BTW), but I'd like the full capability.

Regards,
Bengt Richter
Jul 18 '05 #10

P: n/a

"Ian Bicking" <ia**@colorstudy.com> wrote in message
news:ma**************************************@pyth on.org...
John Roth wrote:
The syntax I prefer (and I don't know if it's actually been
suggested before) is to use braces, that is { and }.

In other words, an anonymous function looks like:
{p1, p2, p3 |
stmt1
stmt2
}
What's the advantage of something like that over the non-anonymous
equivalent:

def some_func(p1, p2, p3):
stmt1
stmt2

I appreciate some of the motivation, but merely avoiding giving something
a name doesn't seem like a laudible goal.


Actually, it is a laudable goal. It's always easier to understand
something when it's right in front of your face than if it's
off somewhere else.

This, of course, trades off with two other forces: avoiding
repetition and making the whole thing small enough to
understand.

So the niche is small, single use functions. The problem
with lambdas is that they're restricted to one expression,
which is too small.

Languages that are designed with anonymous functions
in mind use them very heavily. Smalltalk is the standard
example, and it's also one of the major (possibly the
only) attraction Ruby has over Python.

Python isn't designed that way, which restricts their
utility to a much smaller niche than otherwise.
The one motivation I can see for function expressions is callback-oriented
programming ...
Well, that's true, but that's a very global statement:
when you pass a function into another routine, it's
essentially a callback.

....
Function expressions could get really out of hand, IMHO, and could easily
lead to twenty-line "expressions". That's aesthetically incompatible with
Python source, IMHO.
Anything can get out of hand; there's no way of legislating
good style without restricting the language so much that it
becomes unusable for anything really interesting. Even then
it doesn't work: see COBOL as a really good example of
good intentions gone seriously wrong.

Have you ever programmed in a language that does use
anonymous functions extensively like Smalltalk?

John Roth
--
Ian Bicking / ia**@colorstudy.com / http://blog.ianbicking.org


Jul 18 '05 #11

P: n/a
David Bolen wrote:
I think this specific use case -- defining callbacks -- should be
addressed, rather than proposing a solution to something that isn't
necessary. (...)


I'd be interested in this approach too, especially if it made it simpler
to handle simple manipulation of callback arguments (e.g., since I often
ignore a successful prior result in a callback in order to just move on
to the next function in sequence).


It seems to me that what most people *actually* want, when asking for
lambdas, is a quicker and more convenient way to get closures. (At
least, that's what the vast majority of examples of lambda use seem to
be for.) Perhaps one could get a bit more traction by looking for
improved closure syntax instead of focusing on the anonymous function
aspect.

All of the suggestions for anonymous multiline functions (with embedded
indentation) seem to me to miss what seems to me to be the only
significant benefit of lambda -- its ability to be used in-line without
creating a huge ugly tangle. (I'd argue that lambdas create a *small*
ugly tangle, but apparently that's just me. ;) ) Lambdas do have some
value in defining callbacks, but that value derives almost exclusively
from the fact that they are in-line (rather than a few lines above).
Mimicking function-def indentation inside of another function's arglist
strikes me as an abomination just waiting to happen; in comparison, the
need to type a name twice seems trivial.

As a result, it seems to me that, rather than generalize lambdas into
"full" anonymous functions (with most of the negatives and few of the
positives of lambda), it would be much better to specialize them further
into inline-closure-creators, where they can serve a valuable purpose
without quite as much risk of code pollution.

Jeff Shannon
Technician/Programmer
Credit International

Jul 18 '05 #12

P: n/a
On Thu, 30 Dec 2004 15:15:51 -0800, Jeff Shannon <je**@ccvcorp.com> wrote:
David Bolen wrote:
I think this specific use case -- defining callbacks -- should be
addressed, rather than proposing a solution to something that isn't
necessary. (...)
I'd be interested in this approach too, especially if it made it simpler
to handle simple manipulation of callback arguments (e.g., since I often
ignore a successful prior result in a callback in order to just move on
to the next function in sequence).


It seems to me that what most people *actually* want, when asking for
lambdas, is a quicker and more convenient way to get closures. (At
least, that's what the vast majority of examples of lambda use seem to
be for.) Perhaps one could get a bit more traction by looking for
improved closure syntax instead of focusing on the anonymous function
aspect.

Not sure what you mean by closure here. To me it means the necessary
external environment needed to be captured for use by a function definition
getting exported from its definition environment. I.e., it is something
a function uses, and part of the function definition, but it isn't the
function itself. I would compare a closure more to a callable class instance's
self attributes, except that the latter are more flexible.

In fact, for a callback, a constructor call creating a suitable
callable class instance could sometimes work well as a substitute
for a lambda expression, ISTM. (I.e., when it is not important to
show the code in line, and the differences are in initialization parameters
rather than code).

All of the suggestions for anonymous multiline functions (with embedded
indentation) seem to me to miss what seems to me to be the only
significant benefit of lambda -- its ability to be used in-line without
creating a huge ugly tangle. (I'd argue that lambdas create a *small*
ugly tangle, but apparently that's just me. ;) ) Lambdas do have some
value in defining callbacks, but that value derives almost exclusively
from the fact that they are in-line (rather than a few lines above). They do let you define _code_ inline, which a constructor call doesn't do
(unless you pass a string to compile etc -- not cool).
Mimicking function-def indentation inside of another function's arglist
strikes me as an abomination just waiting to happen; in comparison, the
need to type a name twice seems trivial. Self-restraint can avoid abominations ;-)

As a result, it seems to me that, rather than generalize lambdas into
"full" anonymous functions (with most of the negatives and few of the
positives of lambda), it would be much better to specialize them further
into inline-closure-creators, where they can serve a valuable purpose
without quite as much risk of code pollution.


There's always the temptation to be enforcer when being persuader
is not the easiest ;-)

(BTW, again, by closure, do you really mean deferred-action-thingie?)

Regards,
Bengt Richter
Jul 18 '05 #13

P: n/a
Bengt Richter wrote:
On Thu, 30 Dec 2004 15:15:51 -0800, Jeff Shannon <je**@ccvcorp.com> wrote:
Mimicking function-def indentation inside of another function's arglist
strikes me as an abomination just waiting to happen; in comparison, the
need to type a name twice seems trivial.


Self-restraint can avoid abominations ;-)


It can, but given the prevalence of lambda abominations (such as the
many that the Martellibot described among Cookbook submissions), is
there any reason to believe that there would be *more* restraint in
using a less-constrained feature?? :)
As a result, it seems to me that, rather than generalize lambdas into
"full" anonymous functions (with most of the negatives and few of the
positives of lambda), it would be much better to specialize them further
into inline-closure-creators, where they can serve a valuable purpose
without quite as much risk of code pollution.


(BTW, again, by closure, do you really mean deferred-action-thingie?)


My understanding of "closure" (which may well be wrong, as I've
inferred it entirely from past discussions on this newsgroup) is that
it's a callable with some or all of its parameters already set; e.g.,

def one_and(foo):
def closure(arg):
return foo(1, arg)
return closure

incr = one_and(operator.add)

The function one_and() returns a closure, here bound to incr. It is
essentially a (partially) deferred action thingy, if you want to use
technical terms. ;) I suppose that one could look at it as the
environment in which to call a given function, exported for later use...

My thesis here is that one of the most common (legitimate) uses of
lambda is as an adapter, to create an intermediary that allows a
callable with a given signature to be used in places where a different
signature is expected -- that is, altering the number or order of
arguments passed to a given callable (and possibly also capturing the
current value of some other variable in the process). I feel that
it's more fruitful to focus on this "adapter" quality rather than
focusing on the "anonymous function" quality.

Jeff Shannon
Technician/Programmer
Credit International

Jul 18 '05 #14

P: n/a
Bengt Richter wrote:
This is an easy trap to fall into, so if the new lambda-substitute could
provide a prettier current-closure-variable-value capture than passing a dummy default
value or nesting another def and passing the value in, to provide a private closure for each,
that might be something to think about.


I forgot about that little trap. . .

Cheers,
Nick.

--
Nick Coghlan | nc******@email.com | Brisbane, Australia
---------------------------------------------------------------
http://boredomandlaziness.skystorm.net
Jul 18 '05 #15

P: n/a
Carl Banks wrote:
Nick Coghlan wrote:
In much the same way that programmers often spend a lot of time
optimizing parts of their program that will yield very minor dividends,
while they could have spent that time working on other things that will
pay off a lot, many of the wannabe language designers here are spending
a lot of time on aspects of the language for which any improvement
would only pay small dividends.


Whereas I see it as wannabe language designers only being able to tinker at the
edges of Python, because GvR already got the bulk of the language right.
Anonymous functions are the major core construct that doesn't 'fit right', so a
disproportionate amount of time is spent playing with ideas about them.

Cheers,
Nick.

--
Nick Coghlan | nc******@email.com | Brisbane, Australia
---------------------------------------------------------------
http://boredomandlaziness.skystorm.net
Jul 18 '05 #16

P: n/a
On Thu, 30 Dec 2004 17:39:06 -0800, Jeff Shannon <je**@ccvcorp.com> wrote:
Bengt Richter wrote:
On Thu, 30 Dec 2004 15:15:51 -0800, Jeff Shannon <je**@ccvcorp.com> wrote:
Mimicking function-def indentation inside of another function's arglist
strikes me as an abomination just waiting to happen; in comparison, the
need to type a name twice seems trivial.
Self-restraint can avoid abominations ;-)


It can, but given the prevalence of lambda abominations (such as the
many that the Martellibot described among Cookbook submissions), is
there any reason to believe that there would be *more* restraint in
using a less-constrained feature?? :)

Maybe. If people are determined to overcome limitations that needn't exist,
they are likely to invent horrible hacks ;-)
As a result, it seems to me that, rather than generalize lambdas into
"full" anonymous functions (with most of the negatives and few of the
positives of lambda), it would be much better to specialize them further
into inline-closure-creators, where they can serve a valuable purpose
without quite as much risk of code pollution.
(BTW, again, by closure, do you really mean deferred-action-thingie?)


My understanding of "closure" (which may well be wrong, as I've
inferred it entirely from past discussions on this newsgroup) is that
it's a callable with some or all of its parameters already set; e.g.,

def one_and(foo):
def closure(arg):
return foo(1, arg)
return closure

incr = one_and(operator.add)

The function one_and() returns a closure, here bound to incr. It is
essentially a (partially) deferred action thingy, if you want to use
technical terms. ;) I suppose that one could look at it as the
environment in which to call a given function, exported for later use...


Quoting from "Essentials of Programming Languages" (Friedman, Wand, Haynes):
(using *xxx yyy etc* for italics)
"""
In order for a procedure to retain the bindings that its free variables had
at the time it was created, it must be a *closed* package, independent of
the environment in which it is used. Such a package is called a closure.
In order to be self-contained, a closure must contain the procedure body,
the list of formal parameters, and the bindings of its free variables.
It is convenient to store the entire creation environment, rather than
just the bindings of the free variables. We sometimes say the procedure
*is closed over* or *closed in* its creation environment. We represent
closures as records.
"""

So it looks like you infer better than I remember, and attachment to
faulty memory has led me to resist better inferences ;-/

Closure is the name for the whole thing, apparently, not just the environment
the procedure body needs, which was the aspect that I (mis)attached the name to.

(Representing closures as records doesn't really belong in that paragraph IMO,
since it is not really part of the definition there, just a choice in that stage
of exposition in the book, using scheme-oriented examples. But I quoted verbatim.).

On the subject CLtL finally (after some stuff beyond my current caffeine level) says,
"""
The distinction between closures and other kinds of functions is somewhat pointless,
actually, since Common Lisp defines no particular representation for closures and
no way to distinguish between closures and non-closure functions. All that matters
is that the rules of lexical scoping be obeyed.
"""
I guess that clinches it ;-)

Might be a good python glossary wiki entry.

My thesis here is that one of the most common (legitimate) uses of
lambda is as an adapter, to create an intermediary that allows a
callable with a given signature to be used in places where a different
signature is expected -- that is, altering the number or order of
arguments passed to a given callable (and possibly also capturing the
current value of some other variable in the process). I feel that
it's more fruitful to focus on this "adapter" quality rather than
focusing on the "anonymous function" quality.

I see what you are saying (I think), but I think I'd still like a full
anonymous def, whatever adapter you come up with. And I prefer to be persuaded ;-)

Regards,
Bengt Richter
Jul 18 '05 #17

P: n/a
Jeff Shannon wrote:
My thesis here is that one of the most common (legitimate) uses of
lambda is as an adapter, to create an intermediary that allows a
callable with a given signature to be used in places where a different
signature is expected -- that is, altering the number or order of
arguments passed to a given callable (and possibly also capturing the
current value of some other variable in the process). I feel that it's
more fruitful to focus on this "adapter" quality rather than focusing on
the "anonymous function" quality.


Maybe the 'functional' module proposed in PEP 309[1] could provide such
functions?

py> def ignoreargs(func, nargs, *kwd_names):
.... def _(*args, **kwds):
.... args = args[nargs:]
.... kwds = dict((k, kwds[k])
.... for k in kwds if k not in kwd_names)
.... return func(*args, **kwds)
.... return _
....
py> def f(x, y):
.... print x, y
....
py> ignoreargs(f, 2)(1, 2, 3, 4)
3 4
py> ignoreargs(f, 2, 'a', 'b')(1, 2, 3, 4, a=35, b=64)
3 4

Steve

[1] http://python.fyxm.net/peps/pep-0309.html
Jul 18 '05 #18

P: n/a
bo**@oz.net (Bengt Richter) writes:
print either(A == B, (def "A equals B"), (def "A does not equal B"))
either(thefile, (def thefile.close()), (def 0))


I'd really rather have some reasonable macro facility, than to resort
to using anonymous functions and deferred evaluation for common things
like that.

Jul 18 '05 #19

P: n/a
John Roth wrote:
I appreciate some of the motivation, but merely avoiding giving
something a name doesn't seem like a laudible goal.
Actually, it is a laudable goal. It's always easier to understand
something when it's right in front of your face than if it's
off somewhere else.


Naming the function doesn't move it far away. It changes the order (you
have to define it before you use it), and it introduces a name.
The one motivation I can see for function expressions is
callback-oriented programming ...


Well, that's true, but that's a very global statement:
when you pass a function into another routine, it's
essentially a callback.


Sure, technically. But I'm thinking of real use cases. One I'm
familiar with is things like map and filter. These are generally better
handled with list expressions (and MUCH more readable as such, IMHO).
Another is control structures, ala Ruby or Smalltalk. IMHO, we have all
the control structures we need -- while, for, if. Most "novel" control
structures in Ruby or Smalltalk are just another take on iterators. The
exception being callbacks, and perhaps some other lazy evaluation
situations (though outside of what I think of as "callbacks", I can't
think of any lazy evaluation situations off the top of my head).

So that's why I think callbacks are important; callbacks in the style of
Twisted Deferred, GUI events, etc.
Function expressions could get really out of hand, IMHO, and could
easily lead to twenty-line "expressions". That's aesthetically
incompatible with Python source, IMHO.


Anything can get out of hand; there's no way of legislating
good style without restricting the language so much that it
becomes unusable for anything really interesting. Even then
it doesn't work: see COBOL as a really good example of
good intentions gone seriously wrong.


OK, I should go further -- even a two-line expression (i.e., an
expression that contains meaningful vertical whitespace) is
aesthetically incompatible with Python source. Which covers any
anonymous function that is more powerful than lambda. I'm not arguing
that it can be abused, but more that it isn't any good even when it's
not being abused.
Have you ever programmed in a language that does use
anonymous functions extensively like Smalltalk?


Yep, I've done a fair amount of Smalltalk and Scheme programming. I
don't expect Python to act like them. I appreciate the motivation, but
I don't think their solution is the right one for Python.

--
Ian Bicking / ia**@colorstudy.com / http://blog.ianbicking.org
Jul 18 '05 #20

P: n/a
David Bolen wrote:
So for example, an asynchronous sequence of operations might be like:

d = some_deferred_function()
d.addCallback(lambda x: next_function())
d.addCallback(lambda blah: third_function(otherargs, blah))
d.addCallback(lambda x: last_function())

which to me is more readable (in terms of seeing the sequence of
operations being performed in their proper order), then something like:

def cb_next(x):
return next_function()
def cb_third(blah, otherargs):
return third_function(otherargs, blah)
def cb_last(x):
return last_function()

d = some_deferred_function()
d.addCallback(cb_next)
d.addCallback(cb_third, otherargs)
d.addCallback(cb_next)

which has an extra layer of naming (the callback functions), and
requires more effort to follow the flow of what is really just a simple
sequence of three functions being called.


But this sequence contains an error of the same form as the "fat":

while test() != False:
...code...

The right sequence using lambda is:
d = some_deferred_function()
d.addCallback(next_function)
d.addCallback(lambda blah: third_function(otherargs, blah))
d.addCallback(last_function)

And I would write it as:

def third_function_fixed_blah(blah):
def call_third(otherargs):
return third_function(otherargs, blah)
return call_third

d = some_deferred_function()
d.addCallback(next_function)
d.addCallback(third_function_fixed_blah, otherargs)
d.addCallback(last_function)

The name gives you the chance to point out that the argument order is
tweaked. In many such cases, I use curry (ASPN recipe #52549), which
should show up in Python as "partial" in the "functional" module
according to PEP 309 <http://www.python.org/peps/pep-0309.html>
(accepted but not included). I suppose it will show up in Python 2.5.

Programming is a quest is for clear, easy-to-read code, not quick,
easy-to-write code. Choosing a name is a chance to explain what you
are doing. lambda is used too often in lieu of deciding what to write.

--Scott David Daniels
Sc***********@Acm.Org
Jul 18 '05 #21

P: n/a
David Bolen wrote:
Ian Bicking <ia**@colorstudy.com> writes:

The one motivation I can see for function expressions is
callback-oriented programming, like:

get_web_page(url,
when_retrieved={page |
give_page_to_other_object(munge_page(page))})

This is my primary use case for lambda's nowadays as well - typically
just to provide a way to convert the input to a callback into a call
to some other routine. I do a lot of Twisted stuff, whose deferred
objects make heavy use of single parameter callbacks, and often you
just want to call the next method in sequence, with some minor change
(or to ignore) the last result.

So for example, an asynchronous sequence of operations might be like:

d = some_deferred_function()
d.addCallback(lambda x: next_function())
d.addCallback(lambda blah: third_function(otherargs, blah))
d.addCallback(lambda x: last_function())


Steven proposed an ignoreargs function, and the partial function offers
the other side (http://www.python.org/peps/pep-0309.html). So this
would become:

d = some_deferred_function()
d.addCallback(ignoreargs(next_function, 1))
d.addCallback(partial(third_function, otherargs))
d.addCallback(ignoreargs(last_function, 1))

I'm not sure this is "better" than it is with lambda. It's actually
considerably less readable to me. Hmm... well, that makes me less
excited about those...

--
Ian Bicking / ia**@colorstudy.com / http://blog.ianbicking.org
Jul 18 '05 #22

P: n/a
Scott David Daniels <Sc***********@Acm.Org> writes:
David Bolen wrote:
So for example, an asynchronous sequence of operations might be like:
d = some_deferred_function()
d.addCallback(lambda x: next_function())
d.addCallback(lambda blah: third_function(otherargs, blah))
d.addCallback(lambda x: last_function())
which to me is more readable (in terms of seeing the sequence of
operations being performed in their proper order), then something like:
def cb_next(x):
return next_function()
def cb_third(blah, otherargs):
return third_function(otherargs, blah)
def cb_last(x):
return last_function()
d = some_deferred_function()
d.addCallback(cb_next)
d.addCallback(cb_third, otherargs)
d.addCallback(cb_next)
which has an extra layer of naming (the callback functions),
and
requires more effort to follow the flow of what is really just a simple
sequence of three functions being called.
But this sequence contains an error of the same form as the "fat":


"this" being which of the two scenarios you quote above?
while test() != False:
...code...
I'm not sure I follow the "error" in this snippet...
The right sequence using lambda is:
d = some_deferred_function()
d.addCallback(next_function)
d.addCallback(lambda blah: third_function(otherargs, blah))
d.addCallback(last_function)


By what metric are you judging "right"?

In my scenario, the functions next_function and last_function are not
written to expect any arguments, so they can't be passed straight into
addCallback because any deferred callback will automatically receive
the result of the prior deferred callback in the chain (this is how
Twisted handles asynchronous callbacks for pending operations).
Someone has to absorb that argument (either the lambda, or
next_function itself, which if it is an existing function, needs to be
handled by a wrapper, ala my second example).

Your "right" sequence simply isn't equivalent to what I wrote.
Whether or not next_function is fixable to be used this way is a
separate point, but then you're discussing two different scenarios,
and not two ways to write one scenario.

-- David
Jul 18 '05 #23

P: n/a
David Bolen wrote:
Scott David Daniels <Sc***********@Acm.Org> writes:
while test() != False:
...code...

I'm not sure I follow the "error" in this snippet...


The code is "fat" -- clearer is:
while test():
...code...
The right sequence using lambda is:
d = some_deferred_function()
d.addCallback(next_function)
d.addCallback(lambda blah: third_function(otherargs, blah))
d.addCallback(last_function)

By what metric are you judging "right"?


By a broken metric that requires you to mis-understand the original code
in the same way that I did. It was an idiotic response that required
more careful reading than I am doing this morning. The thing I've seen
in too much code (and though I saw in your code) is code like:

requires_function(lambda: function())

rather than:

requires_function(function)

It happens quite often, and I'm sure you've seen it. But I got your
code wrong, and for that I apologize.

--Scott David Daniels
Sc***********@Acm.Org
Jul 18 '05 #24

P: n/a
bo**@oz.net (Bengt Richter) writes:
Closure is the name for the whole thing, apparently, not just the
environment the procedure body needs, which was the aspect that I
(mis)attached the name to.
Which brings me to the point where I'd welcome more flexibility in
writing to variables outside the local scope. This limitation most
often kicks in in closed-over code in function objects, although it's
a more general issue in Python's scoping.

As we know, you can't write to variables that are both non-local and
non-global (globals you can declare "global"). Now that effectively
makes free variables read-only (although, the objects they point to
can _still_ be mutated).

Allowing write access to variables in a closed-over lexical scope
outside the innermost scope wouldn't hurt because:

1) if you need it, you can already do it -- just practice some
cumbersome tricks make suitable arrangements (e.g. the classical
accumulator example uses an array to hold the counter value instead
of binding it directly to the free variable;

2) if you don't need or understand it, you don't have to use it;

3) and at least in function instances: if you accidentally do, it'll
change the bindings within your closure only which is definitely
less dangerous than mutating objects that are bound inside the
closure.

It must be noted, however, that such behaviour would change the way of
hiding nested variable names:

Now it's safe (though maybe lexically confusing) to use the same
variable names in inner functions. This could happen with common names
for temporary variables like "i", "x", "y".

On the other hand, one could introduce a way to declare variables from
global scope or from local scope, with default from lexical scope. (If
you want to explicitly hide an outer binding, you'd declare "local
foo", for example. You can already do "global foo".)
I see what you are saying (I think), but I think I'd still like a
full anonymous def, whatever adapter you come up with. And I prefer
to be persuaded ;-)


I elaborated on this one in a post a few days ago. Indeed, it is
mostly a minor issue that _can_ be worked around(1). The problem is
that it eventually becomes irritating, when repeated all the time, to
name functions even if the name isn't used elsewhere.

It also creates an implicit dependency from the function call (one of
whose arguments points to the once-named function) to the once-named
function. That is, when you refactor some of your code, you must keep
two things paired all the time in your cut+paste maneuvers.
br,
S

(1) Everything can be worked around. In contrast: you can work around
the lack of a syntactic language by typing in machine code manually.
Sound stupid? Yes, it was done decades ago. How about using C to write
a "shell script" equivalent that you need, as a workaround to the
problem of lacking a shell? Stupid? Yes, but less -- it's been done.
How about writing callbacks by passing in a function pointer and a
data pointer, if you don't want to use a language like Python that
automates that task for you? Stupid? Yes, but not much -- it's been
done all the time. How about implementing an _anonymous_ function by
naming it, if you can't do it otherwise?
Jul 18 '05 #25

P: n/a
Ian Bicking <ia**@colorstudy.com> writes:
But I do think there's other ways to approach this. Function
expressions could get really out of hand, IMHO, and could easily lead
to twenty-line "expressions". That's aesthetically incompatible with
Python source, IMHO.


You can already write unaesthetic hundred-line Python functions, if
you want to. Python's syntax doesn't yet impose a restriction on the
number of sequential statements :-) It sounds artificial to impose
such restrictions on these hypothetical "inline blocks", even if by
only allowing them to be plain expressions.

IMHO, the most pythonic way to write an "inline-block" is by reusing
existing keywords, using Python-like start-of-blocks and ending it by
indentation rules:

map (def x:
if foo (x):
return baz_1 (x)
elif bar (x):
return baz_2 (x)
else:
global hab
hab.append (x)
return baz_3 (hab),
[1,2,3,4,5,6])

and for one-liners:

map (def x: return x**2, [1,2,3,4,5,6])

As a side-effect, we also

- got rid of the "lambda" keyword;

- strenghtened the semantics of "def": a "def" already defines a
function so it's only logical to use it to define anonymous
functions, too;

- unified the semantics: function is always a function, and functions
return values by using "return". When learning Python, I learned the
hard way that "lambda"s are expressions, not functions. I'd pay the
burden of writing "return" more often in exchange for better
consistency.
my two cents,
br,
S
Jul 18 '05 #26

P: n/a
Simo Melenius wrote:
map (def x:
if foo (x):
return baz_1 (x)
elif bar (x):
return baz_2 (x)
else:
global hab
hab.append (x)
return baz_3 (hab),
[1,2,3,4,5,6])


I think this would probably have to be written as:

map (def x:
if foo(x):
return baz_1(x)
elif bar(x):
return baz_2(x)
else:
global hab
hab.append(x)
return baz_3(hab)
, [1,2,3,4,5,6])

or:

map (def x:
if foo(x):
return baz_1(x)
elif bar(x):
return baz_2(x)
else:
global hab
hab.append(x)
return baz_3(hab)
,
[1,2,3,4,5,6])

Note the placement of the comma. As it is,
return baz_3(hab),
returns the tuple containing the result of calling baz_3(hab):

py> def f(x):
.... return float(x),
....
py> f(1)
(1.0,)

It's not horrible to have to put the comma on the next line, but it
isn't as pretty as your version that doesn't. Unfortunately, I don't
think anyone's gonna want to revise the return statement syntax just to
introduce anonymous functions.

Steve
Jul 18 '05 #27

P: n/a
Steven Bethard <st************@gmail.com> writes:
Simo Melenius wrote:
map (def x:
if foo (x):
return baz_1 (x)
elif bar (x):
return baz_2 (x)
else:
global hab
hab.append (x)
return baz_3 (hab),
[1,2,3,4,5,6])
I think this would probably have to be written as:

.... return baz_3(hab)
, [1,2,3,4,5,6])
or: .... return baz_3(hab)
,
[1,2,3,4,5,6])

Note the placement of the comma. As it is,
return baz_3(hab),
returns the tuple containing the result of calling baz_3(hab):
That one didn't occur to me; creating a one-item tuple with (foo,) has
been odd enough for me: only few times I've seen also the parentheses
omitted.

I did ponder the unambiguousness of the last line, though. One could
suggest a new keyword like "end", but keyword bloat is bad.

(Of course, if we trade the "lambda" keyword for another, new keyword
we're not exactly _adding_ keywords... :))
It's not horrible to have to put the comma on the next line, but it
isn't as pretty as your version that doesn't. Unfortunately, I don't
think anyone's gonna want to revise the return statement syntax just
to introduce anonymous functions.


There might not be a return statement: the anonymous function might
conditionally return earlier and have side-effects at the end of the
block (to implicitly return None). So the block-ending would need to
fit after any statement and be strictly unambiguous.
br,
S
Jul 18 '05 #28

P: n/a
Steven Bethard wrote:
Simo Melenius wrote:
map (def x:
if foo (x):
return baz_1 (x)
elif bar (x):
return baz_2 (x)
else:
global hab
hab.append (x)
return baz_3 (hab),
[1,2,3,4,5,6])

I think this would probably have to be written as:


Right the comma plus other things make this difficult for a parser to
handle correctly. Other people have already come up with working solutions.

We have a special way to pass a multiline closure as a parameter to a
function. Put it outside the parameter list.

First, the single-line way using curly braces:

newlist = map({x as int | return x*x*x}, [1,2,3,4,5,6])

Then the multi-line way. I had to add an overload of map to support
reversing the order of parameters (list first, then the closure):

newlist = map([1,2,3,4,5,6]) def (x as int):
return x*x*x

for item in newlist:
print item

Jul 18 '05 #29

P: n/a
On 01 Jan 2005 00:56:30 +0200, Simo Melenius <fi****************@iki.fi-spam> wrote:
Steven Bethard <st************@gmail.com> writes:
Simo Melenius wrote:
> map (def x:
> if foo (x):
> return baz_1 (x)
> elif bar (x):
> return baz_2 (x)
> else:
> global hab
> hab.append (x)
> return baz_3 (hab),
> [1,2,3,4,5,6])
I think this would probably have to be written as:

...
return baz_3(hab)
, [1,2,3,4,5,6])
or:

...
return baz_3(hab)
,
[1,2,3,4,5,6])

Note the placement of the comma. As it is,
return baz_3(hab),
returns the tuple containing the result of calling baz_3(hab):


That one didn't occur to me; creating a one-item tuple with (foo,) has
been odd enough for me: only few times I've seen also the parentheses
omitted.

I did ponder the unambiguousness of the last line, though. One could
suggest a new keyword like "end", but keyword bloat is bad.

ISTM you don't need "end" -- just put the def expression in parens,
and let the closing paren end it, e.g.:

map((def x:
if foo (x):
return baz_1 (x)
elif bar (x):
return baz_2 (x)
else:
global hab
hab.append (x)
return baz_3 (hab)), [1,2,3,4,5,6])

(Of course, if we trade the "lambda" keyword for another, new keyword
we're not exactly _adding_ keywords... :))
It's not horrible to have to put the comma on the next line, but it
isn't as pretty as your version that doesn't. Unfortunately, I don't
think anyone's gonna want to revise the return statement syntax just
to introduce anonymous functions.


There might not be a return statement: the anonymous function might
conditionally return earlier and have side-effects at the end of the
block (to implicitly return None). So the block-ending would need to
fit after any statement and be strictly unambiguous.

Just use parens as necessary or when in doubt ;-)

Regards,
Bengt Richter
Jul 18 '05 #30

P: n/a

"Simo Melenius" <fi****************@iki.fi-spam> wrote in message
news:m2***************@geist.local...
bo**@oz.net (Bengt Richter) writes:
Which brings me to the point where I'd welcome more flexibility in
writing to variables outside the local scope.


This idea was discussed extensively on PyDev perhaps 2 years ago. (You can
check the archived summaries if interested enough ;-) As I remember, there
was no consensus either on the desireability of such a mechanism or on the
syntax (several were proposed).

Terry J. Reedy

Jul 18 '05 #31

P: n/a
bo**@oz.net (Bengt Richter) writes:
ISTM you don't need "end" -- just put the def expression in parens,
and let the closing paren end it, e.g.:


I first rejected any parens as not being native to how
classes/toplevel functions/control blocks are written in Python.
However, this looks quite clear to me. I'd expect that in most cases
it would be difficult to mistake a block of statements in parentheses
with e.g. tuple notation. And parensing works with the old "lambda"
already, of course.
br,
S
Jul 18 '05 #32

P: n/a
Doug Holton <a@b.c> writes:
Steven Bethard wrote:
Simo Melenius wrote:
map (def x:

Oops, I found a typo alreay. I meant to write "def (x):" -- no name
for anonymous functions but just the argument list, please :)
Right the comma plus other things make this difficult for a parser to
handle correctly. Other people have already come up with working
It is difficult since in Python we don't enjoy the ugly luxury of
stuffing statements inside {}s. But as Bengt pointed out, parentheses
don't actually look bad at all in this case.
Then the multi-line way. I had to add an overload of map to support
reversing the order of parameters (list first, then the closure):

newlist = map([1,2,3,4,5,6]) def (x as int):
return x*x*x


That doesn't seem to scale if you want to pass more than one anonymous
function arguments to the function or call an arbitrary function with
parameters in arbitrary order.
br,
S
Jul 18 '05 #33

P: n/a
Nick Coghlan <nc******@iinet.net.au> writes:
Do you consider generator expressions or list comprehensions deficient
because they don't allow several statements in the body of the for
loop?


I don't see what it would mean to do otherwise.
Jul 18 '05 #34

P: n/a
Paul Rubin wrote:
Nick Coghlan <nc******@iinet.net.au> writes:
Do you consider generator expressions or list comprehensions deficient
because they don't allow several statements in the body of the for
loop?

I don't see what it would mean to do otherwise.


Exactly the same as a suite would in the innermost portion of the equivalent
explicit generator (i.e. where the implicit "yield <expr>" currently ends up).

If you could put a suite inside a function expression (i.e. replacing the
implicit "return <expr>") why not inside generator expressions as well?

Cheers,
Nick.

--
Nick Coghlan | nc******@email.com | Brisbane, Australia
---------------------------------------------------------------
http://boredomandlaziness.skystorm.net
Jul 18 '05 #35

P: n/a
On Thu, 30 Dec 2004 23:28:46 +1000, Nick Coghlan
<nc******@iinet.net.au> wrote:
GvR has commented that he want to get rid of the lambda keyword for Python 3.0.
Getting rid of lambda seems like a worthy goal,


Can I ask what the objection to lambda is?
1) Is it the syntax?
2) Is it the limitation to a single expression?
3) Is it the word itself?

I can sympathise with 1 and 2 but the 3rd seems strange since a
lambda is a well defined name for an anonymous function used in
several programming languages and originating in lambda calculus
in math. Lambda therefore seems like a pefectly good name to
choose.

So why not retain the name lambda but extend or change the syntax
to make it more capable rather than invent a wholly new syntax
for lambdas?

Slightly confused, but since I only have time to read these
groups regularly when I'm at home I have probably missed the bulk
of the discussion over the years.

Alan G.
Author of the Learn to Program website
http://www.freenetpages.co.uk/hp/alan.gauld
Jul 18 '05 #36

P: n/a
Alan Gauld wrote:
Can I ask what the objection to lambda is?
1) Is it the syntax?
2) Is it the limitation to a single expression?
3) Is it the word itself?

I can sympathise with 1 and 2 but the 3rd seems strange since a
lambda is a well defined name for an anonymous function used in
several programming languages and originating in lambda calculus
in math. Lambda therefore seems like a pefectly good name to
choose.


I think that the real objection is a little bit of 1), and something
that's kinda close to 2), but has nothing to do with 3).

The issue isn't that lambdas are bad because they're limited to a
single expression. The issue is that they're an awkward special case
of a function, which was added to the language to mollify
functional-programming advocates but which GvR never felt really "fit"
into Python. Other, more pythonic functional-programming features
have since been added (like list comprehensions and iterators).

It seems to me that in other, less-dynamic languages, lambdas are
significantly different from functions in that lambdas can be created
at runtime. In Python, *all* functions are created at runtime, and
new ones can be defined at any point in execution, so lambdas don't
get that advantage. Thus, their advantages are limited to the fact
that they're anonymous (but names are treated differently in Python
than in most other languages, so this is of marginal utility), and
that they can be created inline. This last bit makes them suitable
for creating quick closures (wrapping a function and tweaking its
parameters/return values) and for creating a delayed-execution object
(e.g. callbacks), so there's a lot of pressure to keep them, but
they're still a special case, and "special cases aren't special enough
to break the rules".

Jeff Shannon
Technician/Programmer
Credit International

Jul 18 '05 #37

P: n/a
Jeff Shannon <je**@ccvcorp.com> writes:
It seems to me that in other, less-dynamic languages, lambdas are
significantly different from functions in that lambdas can be created
at runtime.


What languages are those, where you can create anonymous functions
at runtime, but not named functions?! That notion is very surprising
to me.
Jul 18 '05 #38

P: n/a
Paul Rubin wrote:
Jeff Shannon <je**@ccvcorp.com> writes:
It seems to me that in other, less-dynamic languages, lambdas are
significantly different from functions in that lambdas can be created
at runtime.


What languages are those, where you can create anonymous functions
at runtime, but not named functions?! That notion is very surprising
to me.


Hm, I should have been more clear that I'm inferring this from things
that others have said about lambdas in other languages; I'm sadly
rather language-deficient (especially as regards *worthwhile*
languages) myself. This particular impression was formed from a
recent-ish thread about lambdas....

http://groups-beta.google.com/group/...4b&mode=thread

(line-wrap's gonna mangle that, but it's all one line...)

Looking back, I see that I've mis-stated what I'd originally
concluded, and that my original conclusion was a bit questionable to
begin with. In the referenced thread, it was the O.P.'s assertion
that lambdas made higher-order and dynamic functions possible. From
this, I inferred (possibly incorrectly) a different relationship
between functions and lambdas in other (static) languages than exists
in Python.

Jeff Shannon
Technician/Programmer
Credit International

Jul 18 '05 #39

P: n/a
Alan Gauld wrote:
GvR has commented that he want to get rid of the lambda keyword for Python 3.0.
Getting rid of lambda seems like a worthy goal,

Can I ask what the objection to lambda is?
1) Is it the syntax?
2) Is it the limitation to a single expression?
3) Is it the word itself?

I can sympathise with 1 and 2 but the 3rd seems strange since a
lambda is a well defined name for an anonymous function used in
several programming languages and originating in lambda calculus
in math. Lambda therefore seems like a pefectly good name to
choose.


I agree with keeping lambda functionality, and I don't care what name is
used, but there are people who do not like "lambda":
http://lambda-the-ultimate.org/node/...9#comment-3069
The word "lambda" is meaningless to most people. Of course so is "def",
which might be why Guido van Robot changed it to "define":
http://gvr.sourceforge.net/screen_shots/

Even a simple word like "type" can be difficult to explain to beginners:
http://lambda-the-ultimate.org/node/view/337

Python is easier for beginners to learn than other mainstream
programming languages (like java or C++), but that's not to say it
doesn't have some stumbling blocks for beginners of course:
http://www.linuxjournal.com/article/5028
So why not retain the name lambda but extend or change the syntax
to make it more capable rather than invent a wholly new syntax
for lambdas?
Yes, I agree, and either keep the "lambda" keyword or else reuse the
"def" keyword for anonymous methods. See this page Steven Bethard
created: http://www.python.org/moin/AlternateLambdaSyntax

I really don't think anyone should worry about lambda disappearing.

By the way, you've done great work with your learning to program site
and all the help you've given on the python-tutor list:
Alan G.
Author of the Learn to Program website
http://www.freenetpages.co.uk/hp/alan.gauld

Jul 18 '05 #40

P: n/a

"Alan Gauld" <al********@btinternet.com> wrote in message
news:f1********************************@4ax.com...
On Thu, 30 Dec 2004 23:28:46 +1000, Nick Coghlan
<nc******@iinet.net.au> wrote:
GvR has commented that he want to get rid of the lambda keyword for
Python 3.0.
Getting rid of lambda seems like a worthy goal,
Can I ask what the objection to lambda is?
1) Is it the syntax?
2) Is it the limitation to a single expression?
3) Is it the word itself?


Depending on the person, any of the 3. Plus add
4) The constant complaints re: 2)
I can sympathise with 1 and 2 but the 3rd seems strange since a
lambda is a well defined name for an anonymous function used in
several programming languages and originating in lambda calculus
in math.
And that is why 'lambda' is wrong -- in Python, it is only an abbreviation
for a restricted group of def statements, which is *not* its 'well defined'
usage. Hence complaints re 2. If the syntax were def x: x + 2, etc, I
suspect the complaints would be far fewer.

For some new programmers, 'lambda' is as meaningless as elle*. All other
Python keywords are English words or obvious abbreviations thereof.
So why not retain the name lambda but extend or change the syntax
to make it more capable rather than invent a wholly new syntax
for lambdas?


That you suggest this illustrates, to me, what is wrong with the name ;-)

Terry J. Reedy

* The Spanish name for the letter 'll', pronounced el-yea, which comes
after 'l' in their alphabet.

Jul 18 '05 #41

P: n/a
On Thu, 06 Jan 2005 21:02:46 -0600, Doug Holton <a@b.c> wrote:
used, but there are people who do not like "lambda":
http://lambda-the-ultimate.org/node/...9#comment-3069
The word "lambda" is meaningless to most people. Of course so is "def",
which might be why Guido van Robot changed it to "define":
http://gvr.sourceforge.net/screen_shots/
The unfamiliar argument doesn't work for me. After all most
people are unfamiliar with complex numbers (or imaginary) numbers
but python still provides a complex number type. Just because the
name is unfamiliar to some doesn't mean we shouldn't use the
term if its the correct one for the concept.

Hopefully anyone who wants to use anonymous functions will know
that such are called lambdas and hopefully will have studied
lambda calculus to at least some level - certainly CS majors and
software engineering types should have...
Python is easier for beginners to learn than other mainstream
programming languages
Absolutely, but it has to decide (and soon I think) how important
that role is in the development of the language. Many of the more
recent features are beginner hostile - slots, properties, meta
classes, decorators etc... So is Python going to consciously try
to remain beginner friendly (which it remains by simply ignoring
the newer fatures!) or deliberately go for the "fully featured"
general purpose audience?
Yes, I agree, and either keep the "lambda" keyword or else reuse the
"def" keyword for anonymous methods. See this page Steven Bethard
created: http://www.python.org/moin/AlternateLambdaSyntax
I agree, I'm much more concerned about the idea of losing
anonymous functions (aka lambdas) than about losing the name
lambda, its just that the name is so descriptive of what it does!
( In fact it was seeing the name lambda appearing in a Lisp
programme I was reading that got me started in Lambda calculus
many years ago...)
By the way, you've done great work with your learning to program site
and all the help you've given on the python-tutor list:


Aw shucks! Thanks ;-)

Alan G.
Author of the Learn to Program website
http://www.freenetpages.co.uk/hp/alan.gauld
Jul 18 '05 #42

P: n/a
Okay, I tried to post this previously but ran into the new Google
groups system which appears to have sent my original response into the
ether... Oops. Trying again.

Alan Gauld wrote:
On Thu, 30 Dec 2004 23:28:46 +1000, Nick Coghlan
<nc******@iinet.net.au> wrote:
GvR has commented that he want to get rid of the lambda keyword for Python 3.0. Getting rid of lambda seems like a worthy goal,


Can I ask what the objection to lambda is?
1) Is it the syntax?
2) Is it the limitation to a single expression?
3) Is it the word itself?

I can sympathise with 1 and 2 but the 3rd seems strange since a
lambda is a well defined name for an anonymous function used in
several programming languages and originating in lambda calculus
in math. Lambda therefore seems like a pefectly good name to
choose.

So why not retain the name lambda but extend or change the syntax
to make it more capable rather than invent a wholly new syntax
for lambdas?

Slightly confused, but since I only have time to read these
groups regularly when I'm at home I have probably missed the bulk
of the discussion over the years.

Alan G.
Author of the Learn to Program website
http://www.freenetpages.co.uk/hp/alan.gauld


Hi Alan:

Having taken some calculus (derivatives, limits, some integrals) but
never even heard of lambda calculus, to me, lambda means absolutely
NOTHING. Less than nothing.

Actually, in my encounters with others code in Python (including
reading near 1000 recipes for the recent edition of the Cookbook), what
I've discovered is that what lambda mostly means to me is:
"I don't wanna write clear, elegant Python - I wanna write fancy,
clever, obfuscated code, and lambda lets me do that!"

I've seen maybe 1 case in 10 where lambda appears, to me, to allow
clearer, cleaner code than defining a function. Most cases-it's an
ugly, incomprehensible hack.

So, if people really need anonymous functions in Python (and
apparently, they do), then at least lets get a word that actually
*means* something. Every other word in Python has an obvious meaning.
lambda doesn't.

So, I guess I don't like the word itself - any more than I like how
it's (mostly) used.

Anna Martelli Ravenscroft

Jul 18 '05 #43

P: n/a
"Anna" <an*******@gmail.com> writes:
Having taken some calculus (derivatives, limits, some integrals) but
never even heard of lambda calculus, to me, lambda means absolutely
NOTHING. Less than nothing.


Lambda calculus is from mathematical logic, but more to the point
"lambda" has been the term used in Lisp for this operation since time
immemorial. Since the kinds of programmers who want to use anonymous
functions have probably been exposed to Lisp at one time or another,
"lambda" should not cause any confusion.
Jul 18 '05 #44

P: n/a
Paul Rubin wrote:
"Anna" <an*******@gmail.com> writes:
Having taken some calculus (derivatives, limits, some integrals) but
never even heard of lambda calculus, to me, lambda means absolutely
NOTHING. Less than nothing.

Lambda calculus is from mathematical logic, but more to the point
"lambda" has been the term used in Lisp for this operation since time
immemorial. Since the kinds of programmers who want to use anonymous
functions have probably been exposed to Lisp at one time or another,
"lambda" should not cause any confusion.


That's probably not the most enthusiastic evangelical approach to
functional programming I've ever heard :-)

Perhaps what we really need is a good Lisp subsystem for Python?

regards
Steve
--
Steve Holden http://www.holdenweb.com/
Python Web Programming http://pydish.holdenweb.com/
Holden Web LLC +1 703 861 4237 +1 800 494 3119
Jul 18 '05 #45

P: n/a
Steve Holden <st***@holdenweb.com> writes:
Perhaps what we really need is a good Lisp subsystem for Python?


I've thought the other way around, it would be nice to have a Python
subsystem for Lisp.
Jul 18 '05 #46

P: n/a
"Anna" <an*******@gmail.com> writes:
Having taken some calculus (derivatives, limits, some integrals) but
never even heard of lambda calculus, to me, lambda means absolutely
NOTHING. Less than nothing.
And before you took calculus, the chances are that derivatives, limits
and integrals meant less than nothing to you.

But now, I am quite sure, you know that in Python lambda is a keyword
which creates anonymous functions. Now that you know what lambda does,
what's the problem with it? (It certainly doesn't mean "Less than
nothing" to you now.)
So, I guess I don't like the word itself
Fair enough. I guess there are people out there who might have a
distaste for the word "class" or "def" or any of the other words which
are keywords in Python.
Every other word in Python has an obvious meaning. lambda doesn't.


Obvious to whom?

The meaning of every word is obvious, once you have been taught it;
and a complete mystery if you have not.

What do you make of "seq[2:-2]"? It means "less than nothing" to the
uninitiated. Just like lambda.

Getting students in my Python courses to understand "seq[2:-2]" takes
about as much (maybe even a bit more) effort as getting them to
understand lambda[*]. But once they have been taught these features,
they can handle them just fine.

[*] Funnily enough, getting them to understand that "lambda x: fn(x)"
is just a very silly way of writing "fn", can be quite a struggle
at times ... but that's probably a consequence of the context in
which lambda is introduced.
Jul 18 '05 #47

P: n/a
Alan Gauld wrote:
On Thu, 06 Jan 2005 21:02:46 -0600, Doug Holton <a@b.c> wrote:
used, but there are people who do not like "lambda":
http://lambda-the-ultimate.org/node/...9#comment-3069
The word "lambda" is meaningless to most people. Of course so is "def",
which might be why Guido van Robot changed it to "define":
http://gvr.sourceforge.net/screen_shots/

The unfamiliar argument doesn't work for me. After all most
people are unfamiliar with complex numbers (or imaginary) numbers
but python still provides a complex number type. Just because the
name is unfamiliar to some doesn't mean we shouldn't use the
term if its the correct one for the concept.


I'm not sure this is really a fair comparison. What's the odds that if
you're unfamiliar with complex numbers that you're going to have to read
or write code that uses complex numbers? Probably pretty low. I don't
think I've ever had to read or write such code, and I *do* understand
complex numbers. Lambdas, on the other hand, show up in all kinds of
code, and even though I hardly ever use them myself, I have to
understand them because other people do (over-)use them.

Steve
Jul 18 '05 #48

P: n/a
Jacek Generowicz wrote:
[*] Funnily enough, getting them to understand that "lambda x: fn(x)"
is just a very silly way of writing "fn", can be quite a struggle
at times ... but that's probably a consequence of the context in
which lambda is introduced.


If you genuinely taught them that, you may have done them a disservice:

Py> def f(x):
.... print x
....
Py> f1 = f
Py> f2 = lambda x: f(x)
Py> f1("hi")
hi
Py> f2("hi")
hi
Py> def f(x):
.... print x * 2
....
Py> f1("hi")
hi
Py> f2("hi")
hihi

Cheers,
Nick.

--
Nick Coghlan | nc******@email.com | Brisbane, Australia
---------------------------------------------------------------
http://boredomandlaziness.skystorm.net
Jul 18 '05 #49

P: n/a
Paul Rubin wrote:
"Anna" <an*******@gmail.com> writes:
Having taken some calculus (derivatives, limits, some integrals) but
never even heard of lambda calculus, to me, lambda means absolutely
NOTHING. Less than nothing.

Lambda calculus is from mathematical logic, but more to the point
"lambda" has been the term used in Lisp for this operation since time
immemorial.


I think that's part of the problem though - people familiar with lambda calculus
and Lisp's lambdas want Python's lambdas to be equally capable, and they just
plain *aren't*.

If you have a complex function, the Pythonic way is to give it a meaningful
name. Having a way to defer evaluation of a simple expression *is* quite handy,
but 'lambda' is the wrong name for it - the parallels to lambda calculus and
Lisp's lambda functions are likely to be misleading, rather than helpful.

Add in the fact that there are many, many Python programmers with non-CS
backgrounds, and the term 'lambda' sticks out like a sore thumb from amongst
Python's other English-based keywords. 'def' is probably the second-most cryptic
when you first encounter it, but it is a good mnemonic for "define a function",
so it's still easy to parse. "Lambda is the term mathematicians use to refer to
an anonymous function" is nowhere near as grokkable ;)

For me, the alternative syntax discussion is based on 3 of the 4 mentioned reasons:

1. The syntax
I don't like re-using colons as something other than suite delimiters - it
breaks up the affected expression too much (particularly function calls). Code
with dict literals inside function calls bugs me for the same reason (it's OK
when the literal is separated out into an assignment statement for the dict).
It's also too easy to write lambdas which look ambiguous, even though they
technically aren't.
Finally, Python has a reputation as "executable pseudocode". Lambda
expressions don't read like any sort of psuedocode you're likely to see outside
a maths department.

2. The limitation to a single expression
I consider this no more of a problem than the restriction to a single
expression in the main loop of a generator expression or a list comprehension.
When those get too complicated, you switch to using a real for loop somewhere.
Deferred expressions are no different - when the guts get too complicated,
switch to a named function.

3. The word 'lambda' itself
This _is_ one of my objections for the reasons stated above: for people
unfamiliar with the term, they don't know what it is; for people familiar with
the term, it isn't what they think it should be.
Python already has a perfectly good keyword for functions, which has the
additional virtue of being half the length of lambda (this matters, since this
is a keyword that gets embedded in expressions - all the other keywords
currently in that category are three letters or less: and, or, is, in, for)

4. People complaining about 2
Oh hell yes, this bugs me. And I think changing the syntax and calling them
"deferred expressions" instead of "lambdas" would go a long way towards
eliminating the griping.

Cheers,
Nick.

--
Nick Coghlan | nc******@email.com | Brisbane, Australia
---------------------------------------------------------------
http://boredomandlaziness.skystorm.net
Jul 18 '05 #50

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