Favorite non-python language trick?

[Joseph Garvin]

As someone who learned C first, when I came to Python everytime I read
about a new feature it was like, "Whoa! I can do that?!" Slicing, dir(),
getattr/setattr, the % operator, all of this was very different from C.

I'm curious -- what is everyone's favorite trick from a non-python
language? And -- why isn't it in Python?

Here's my current candidate:

So the other day I was looking at the language Lua. In Lua, you make a
line a comment with two dashes:

-- hey, this is a comment.

And you can do block comments with --[[ and ---]].

--[[
hey
this
is
a
big
comment
--]]

This syntax lets you do a nifty trick, where you can add or subtract a
third dash to change whether or not code runs:

--This code won't run because it's in a comment block
--[[
print(10)
--]]

--This code will, because the first two dashes make the rest a comment,
breaking the block
---[[
print(10)
--]]

So you can change whether or not code is commented out just by adding a
dash. This is much nicer than in C or Python having to get rid of """ or
/* and */. Of course, the IDE can compensate. But it's still neat :)

[Devan L]

With the exception of reduce(lambda x,y:x*y, sequence), reduce can be
replaced with sum, and Guido wants to add a product function.

[Christos TZOTZIOY Georgiou]

On Fri, 24 Jun 2005 21:17:09 GMT, rumours say that Ron Adam
<rr*@ronadam.co m> might have written:

I think some sort of inline or deferred local statement would be useful
also. It would serve as a limited lambda (after it's removed), eval
alternative, and as a inlined function in some situations as well I think.

Something like:

name = defer <expression>

then used as:

result = name()

The expression name() will never have arguments as it's meant to
reference it's variables as locals and probably will be replaced
directly with names's byte code contents at compile time.

Defer could be shortened to def I suppose, but I think defer would be
clearer. Anyway, it's only a wish list item for now.

This is similar:

http://groups-beta.google.com/group/...c884147852d23d
--
TZOTZIOY, I speak England very best.
"Dear Paul,
please stop spamming us."
The Corinthians

[Rocco Moretti]

Joseph Garvin wrote:

I'm curious -- what is everyone's favorite trick from a non-python
language? And -- why isn't it in Python?

I'm not aware of a language that allows it, but recently I've found
myself wanting the ability to transparently replace objects. For
example, if you have a transparent wrapper class around a certain
object, and then determine that you no longer need to wrap the object,
you can say the magic incantation, and the wrapper instance is replaced
by what it is wrapping everywhere in the program. Or you have a complex
math object, and you realize you can reduce it to a simple integer, you
can substitue the integer for the math object, everywhere.

I mainly look for it in the "object replaces self" form, but I guess you
could also have it for arbitrary objects, e.g. to wrap a logging object
around a function, even if you don't have access to all references of
that function.

Why isn't it in Python? It's completely counter to the conventional
object semantics.

[Shai]

Joseph Garvin wrote:

I'm curious -- what is everyone's favorite trick from a non-python
language? And -- why isn't it in Python?

1. Lisp's "dynamicall y scoped" variables (Perl has them, and calls them
"local", but as far as I've seen their use their is discouraged). These
are global variables which are given time-local bindings. That is,
structuring the syntax after what's used for globals,

x=10
def foo():
# No need to define x as it is only read -- same as globals
print x

def bar():
dynamic x
x = 11
foo()

def baz():
bar() # prints 11
foo() # prints 10; the binding in bar is undone when bar exits

This feature makes using "globals" sensible, providing a way to avoid
many important uses (and some say, misuses) of objects if you are so
inclined. It allows you to do some things better than objects do,
because it does to library parameters, what exceptions do to return
codes: instead of passing them in all the way from outside until a
piece of code which actually uses them, they are only mentioned where
you set them and where you really need to access them.

It would not be too hard to implement a version of this (inefficiently)
in the existing language, if frame objects could carry a modifiable
dictionary.

I suppose it is not in Python because (most) Pythoners are not looking
(hard enough) for alternatives to OOP.

2. Prolog's ability to add operators to the language. Though this
facility is quite clanky in Prolog (because there is no elegant way to
specify precedence), the idea is appealing to me. It would allow a
better implementation of my (awkward, granted) recipe for adding logic
programming constructs to Python. It is not in the language because it
might fragmentize it, and because it is very hard to make
recursive-descent parsers like CPython's programmable this way.

3. Lisp's Macros, of course, which have been mentioned already in this
thread. Even Boo-like macros, which are nowhere as strong as Lisp's,
would be very useful. Not in the language, besides its being hard in
any non-lisp-like language, for the reasons mentioned for adding
operators.

On the other hand, there's no end to the features I wish I could copy
from Python to other languages...

[Scott David Daniels]

Rocco Moretti wrote:

Joseph Garvin wrote:

I'm not aware of a language that allows it, but recently I've found
myself wanting the ability to transparently replace objects....
I mainly look for it in the "object replaces self" form, but I guess you
could also have it for arbitrary objects, e.g. to wrap a logging object
around a function, even if you don't have access to all references of
that function.

Why isn't it in Python? It's completely counter to the conventional
object semantics.

Actually this is the old (and terrifying) Smalltalk message 'becomes:'.
There is a concrete reason it is not in python: objects are represented
as pointers to their data structures, do not have identical sizes, and
therefore cannot be copied into each others data space. Smalltalk
implementations often have a level of indirection that allows it to
simply tweak an indirection table to implement this method.

The reason I find it terrifying is that I can be passed an object,
place it in a dictionary (for example) based on its value, and then
it can magically be changed into something else which does not fit
in that spot in the dictionary.

--Scott David Daniels
Sc***********@A cm.Org

[Mike Meyer]

"Shai" <sh**@platonix. com> writes:

Joseph Garvin wrote:

I'm curious -- what is everyone's favorite trick from a non-python
language? And -- why isn't it in Python?
1. Lisp's "dynamicall y scoped" variables (Perl has them, and calls them
"local", but as far as I've seen their use their is discouraged). These
are global variables which are given time-local bindings. That is,
structuring the syntax after what's used for globals,

Perl started life with nothing but dynamically scoped variables. They
added lexical scoping after they realized what a crock dynamic scoping
was.
x=10
def foo():
# No need to define x as it is only read -- same as globals
print x

def bar():
dynamic x
x = 11
foo()

def baz():
bar() # prints 11
foo() # prints 10; the binding in bar is undone when bar exits

This feature makes using "globals" sensible, providing a way to avoid
many important uses (and some say, misuses) of objects if you are so
inclined. It allows you to do some things better than objects do,
because it does to library parameters, what exceptions do to return
codes: instead of passing them in all the way from outside until a
piece of code which actually uses them, they are only mentioned where
you set them and where you really need to access them.

It would not be too hard to implement a version of this (inefficiently)
in the existing language, if frame objects could carry a modifiable
dictionary.

I suppose it is not in Python because (most) Pythoners are not looking
(hard enough) for alternatives to OOP.

Last time I checked, dynamic binding variables were frowned on in LISP
systems as well. Scheme doesn't have them. Common LISP requires
special forms to use them.

The problem with the given use case is that it lets every routine in
the call chain substitute it's own variable for the library parameter
you want to use, with no local indication that this is going
on. This makes bugs in dynamically scoped variables a PITA to find.

Given that it's a feature I don't want programmers using, I'd only be
willing to see it added to the language if you can show that it has no
overhead so long as you don't use it. I'm not sure that can be done.

<mike
--
Mike Meyer <mw*@mired.or g> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.

[Chris Rebert (cybercobra)]

My personal favorite would be ruby's iterators and blocks.
Instead of writing a bunch of repetitive list comprehensions or
defining a bunch of utility functions, you just use the iterators
supported by container objects.
For instance,

[f(x) for x in y]

could be written in Ruby as

y.collect |x| do
#body of f
end

You don't have to use a lambda or define f() externally.
Best of all, Ruby's containers come with many iterators for common
cases builtin.

Joseph Garvin wrote:

As someone who learned C first, when I came to Python everytime I read
about a new feature it was like, "Whoa! I can do that?!" Slicing, dir(),
getattr/setattr, the % operator, all of this was very different from C.

I'm curious -- what is everyone's favorite trick from a non-python
language? And -- why isn't it in Python?

Here's my current candidate:

So the other day I was looking at the language Lua. In Lua, you make a
line a comment with two dashes:

-- hey, this is a comment.

And you can do block comments with --[[ and ---]].

--[[
hey
this
is
a
big
comment
--]]

This syntax lets you do a nifty trick, where you can add or subtract a
third dash to change whether or not code runs:

--This code won't run because it's in a comment block
--[[
print(10)
--]]

--This code will, because the first two dashes make the rest a comment,
breaking the block
---[[
print(10)
--]]

So you can change whether or not code is commented out just by adding a
dash. This is much nicer than in C or Python having to get rid of """ or
/* and */. Of course, the IDE can compensate. But it's still neat :)

[Mike Meyer]

[Lots of quoted text left in...]

I started thinking about this, and realized that there was a way to do
what you wanted, with no execution time overhead, and without
providing ways to radically change the program behavior behind the
scenes.

Mike Meyer <mw*@mired.or g> writes:

"Shai" <sh**@platonix. com> writes:

Joseph Garvin wrote:

I'm curious -- what is everyone's favorite trick from a non-python
language? And -- why isn't it in Python?
1. Lisp's "dynamicall y scoped" variables (Perl has them, and calls them
"local", but as far as I've seen their use their is discouraged). These
are global variables which are given time-local bindings. That is,
structuring the syntax after what's used for globals,

Perl started life with nothing but dynamically scoped variables. They
added lexical scoping after they realized what a crock dynamic scoping
was.

That's a bit harsher than I intended. I mean that having nothing but
dynamically scoped variables - like early Perl and LISP - is a
crock. I'll add that dynamic scoping as the default is a crock. But
you aren't asking for those.

x=10
def foo():
# No need to define x as it is only read -- same as globals
print x

def bar():
dynamic x
x = 11
foo()

def baz():
bar() # prints 11
foo() # prints 10; the binding in bar is undone when bar exits

Here's the problem with that. Consider this script:

import foo
x = 10
def bar():
print x

foo.foogle(bar)

If foo.foogle includes "dynamic x" and then invokes bar, bar could
print anything. This makes the behavior of bar unpredictable by
examining the sourc, with no hint that that is going on.
Given that it's a feature I don't want programmers using, I'd only be
willing to see it added to the language if you can show that it has no
overhead so long as you don't use it. I'm not sure that can be done.

Here's a proposal for dynamically bound variables that you should be
able to implement without affecting the runtime behavior of code that
doesn't use it.

Instead of dynamic meaning "all references to the named variable(s)
will be dynamic until this function exits", have it mean "the named
variable(s) will be dynamic in this function." Whether it should only
check local variables in the calling routines, check local + global,
or check for all free variables, is an open question.

I.e. - your example would be written:

x = 10
def foo():
dynamic x
print x

def bar():
x = 11
foo()

def baz():
bar() # prints 11
foo() # Possibly an error?

For my example above, bar would *always* print 10. Nothing that
foo.foogle did would change that. However, you could write:

import foo
def bar():
dynamic x
print x

foo.foogle(bar)

In this case, bar will print whatever foo.foogle sets x to - and it's
noted in the source to bar. This means that functions that don't
declare a dynamic variable can be compiled to the same code they are
compiled to now.

Would this version provide the functionality you wanted?

<mike
--
Mike Meyer <mw*@mired.or g> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.

[BJörn Lindqvist]

I like C++ templates so that you can ensure that a list only contain
items of one type. I also like the JMP instruction in x86 assembler,
you could do some nasty tricks with that.

--
mvh Björn

[Terry Reedy]

"Devan L" <de****@gmail.c om> wrote in message
news:11******** **************@ o13g2000cwo.goo glegroups.com.. .

With the exception of reduce(lambda x,y:x*y, sequence), reduce can be
replaced with sum, and Guido wants to add a product function.

The update function is not at all limited to sums and products, but can be
any callable with the appropriate signature. The new any() and all()
functions are examples that use other updates.

Terry J. Reedy