Hello,
def test(data):
i = ? This is the line I have trouble with
if i==1: return data
else: return data[:i]
a,b,c,d = test([1,2,3,4])
How can I set i based on the number of output arguments defined in
(a,b,c,d)?
Thank you,
Darren
66  5007 
On Sun, 14 Nov 2004 17:12:24 -0500, Darren Dale <dd**@cornell.e du>
declaimed the following in comp.lang.pytho n: Hello,
def test(data):
i = ? This is the line I have trouble with
if i==1: return data
else: return data[:i]
a,b,c,d = test([1,2,3,4])
How can I set i based on the number of output arguments defined in
(a,b,c,d)?
This is rather confusing...
What do you expect to receive for
a,b,c,d = test([1,2,3,4,5])
Note, you are only passing ONE argument into the function, and
apparently wanting a tuple in return...
Problem: in the case of mismatched lengths (4 items in
destination, and 5 in the list) you get an exception. Otherwise you
could just use
a,b,c,d = tuple([1,2,3,4])
If you are actually trying to work with an unknown number of
input arguments (rather than a single argument of a list)...
def test(*data):
.... print len(data)
.... test([1,2,3,4])
1 test(1,2,3,4)
4
def test(*data):
.... return data
.... test([1,2,3,4])
([1, 2, 3, 4],) test(1,2,3,4)
(1, 2, 3, 4) def test(*data):
.... return list(data)
.... test([1,2,3,4])
[[1, 2, 3, 4]] test(1,2,3,4)
[1, 2, 3, 4]
Thank you,
Darren
-- =============== =============== =============== =============== == <
wl*****@ix.netc om.com | Wulfraed Dennis Lee Bieber KD6MOG <
wu******@dm.net | Bestiaria Support Staff <
=============== =============== =============== =============== == <
Home Page: <http://www.dm.net/~wulfraed/> <
Overflow Page: <http://wlfraed.home.ne tcom.com/> <
Darren Dale wrote: Hello,
def test(data):
i = ? This is the line I have trouble with
if i==1: return data
else: return data[:i]
a,b,c,d = test([1,2,3,4])
How can I set i based on the number of output arguments defined in
(a,b,c,d)?
Something like this:
def test(*args, **kwargs):
i = len(args) + len(kwargs)
should work. But note that the usage example you gave will result in i
having a value of 1 -- you're passing in a single argument (which is a
list).
Of course, if you're always going to be passing a sequence into your
function, and you want to get the length of that sequence, then it's
pretty simple:
def test(data):
i = len(data)
return data[:i]
Note, however, that this function is effectively a no-op as it stands.
Presumably you're intending to do something to transform data, which may
change its length? Otherwise, it would be simpler to just modify the
list (or a copy of it) in-place in a for loop / list comp, and not worry
about the length at all.
Jeff Shannon
Technician/Programmer
Credit International
Fernando Perez <fp*******@yaho o.com> wrote: suspect that by playing very nasty tricks with sys._getframe() , the dis
and the inspect modules, you probably _could_ get to this information, at
least if the caller is NOT a C extension module. But I'm not even 100%
sure this works, and it would most certainly the kind of black magic I'm
sure you are not asking about. But given the level of expertise here, I
better cover my ass ;-)
Yep, that's the cookbook recipe Jp was mentioning -- Sami Hangaslammi's
recipe 284742, to be precise. Yep, it _is_ going into the printed 2nd
edition (which I'm supposed to be working on right now -- deadline
closing in, help, help!-).
Alex
Alex Martelli wrote: Fernando Perez <fp*******@yaho o.com> wrote:
suspect that by playing very nasty tricks with sys._getframe() , the dis
and the inspect modules, you probably _could_ get to this information, at
least if the caller is NOT a C extension module. But I'm not even 100%
sure this works, and it would most certainly the kind of black magic I'm
sure you are not asking about. But given the level of expertise here, I
better cover my ass ;-)
Yep, that's the cookbook recipe Jp was mentioning -- Sami Hangaslammi's
recipe 284742, to be precise. Yep, it _is_ going into the printed 2nd
edition (which I'm supposed to be working on right now -- deadline
closing in, help, help!-).
Well, I feel pretty good now: I didn't see Jp's mention of this, and just
guessed it should be doable with those three tools. I just looked it up, and
it seems it's exactly what I had in mind :) Cute hack, but I tend to agree
with Scott Daniels' comment that this kind of cleverness tends to promote
rather unreadable code. Maybe I just haven't seen a good use for it, but I
think I'd rather stick with more explicit mechanisms than this.
Anyway, is it true that this will only work for non-extension code? If you are
being called from a C extension, dis & friends are toast, no?
Cheers,
f
Fernando Perez <fp*******@yaho o.com> wrote: Yep, that's the cookbook recipe Jp was mentioning -- Sami Hangaslammi's
recipe 284742, to be precise. Yep, it _is_ going into the printed 2nd
edition (which I'm supposed to be working on right now -- deadline
closing in, help, help!-).
Well, I feel pretty good now: I didn't see Jp's mention of this, and just
guessed it should be doable with those three tools. I just looked it up, and
it seems it's exactly what I had in mind :) Cute hack, but I tend to agree
with Scott Daniels' comment that this kind of cleverness tends to promote
rather unreadable code. Maybe I just haven't seen a good use for it, but I
think I'd rather stick with more explicit mechanisms than this.
Yeah, but "once and only once" is a great principle of programming. Any
time you have to say something _TWICE_ there's something wrong going on.
So,
a, b, c, d = lotsa[:4]
_should_ properly give the impression of a code smell, if your "once and
only once" sense is finely tuned. What's the business of that ':4' on
the RHS? Showing the compiler that you can count correctly?! You're
having to tell twice that you're getting four items into separate
variables, once by listing exactly four variables on the LHS, and
another time by that ':4' on the RHS. IMHO, that's just as bogus as
struct.unpack's limitation of not having any way to indicate explicitly
'and all the rest of the bytes goes here', and for similar reasons.
I think it would be better to have a way to say 'and all the rest'.
Lacking that, some automated way to count how many items are being
unpacked on the LHS is probably second-best.
Anyway, is it true that this will only work for non-extension code? If
you are being called from a C extension, dis & friends are toast, no?
Yep, whenever your function isn't being called as the only item on the
RHS of a multiple assignment, then counting how many items are on the
LHS are being unpacked in that inexistent or unapplicable multiple
assignment is right out. Presumably, any way to count the number of
items in this fashion will need a way to indicate "not applicable",
though it's not obvious whether raising an exception, or returning a
clearly bogus value such as 0, is most useful.
Another case where a specific number of items is requested, which is not
(syntactically speaking) a multiple assignment, is assignment to an
_extended_ slice of, e.g., a list (only; assignment to a slice with a
stride of 1 is happy with getting whatever number of items are coming).
I don't particularly LIKE writing:
L[x:y:z] = len(L[x:y:z]) * [foo]
i.e. having to extract the extended slice first, on the RHS, just to
gauge how many times I must repeat foo. However, if I squint in just
the right way, I _can_ try to convince myself that this _isn't_ really
violating "once and only once"... and I do understand how hard it would
be to allow a 'how many items are needed' function to cover this
case:-(.
Alex al*****@yahoo.c om (Alex Martelli) wrote:
Fernando Perez <fp*******@yaho o.com> wrote:
Yep, that's the cookbook recipe Jp was mentioning -- Sami Hangaslammi's
recipe 284742, to be precise. Yep, it _is_ going into the printed 2nd
edition (which I'm supposed to be working on right now -- deadline
closing in, help, help!-).
Well, I feel pretty good now: I didn't see Jp's mention of this, and just
guessed it should be doable with those three tools. I just looked it up, and
it seems it's exactly what I had in mind :) Cute hack, but I tend to agree
with Scott Daniels' comment that this kind of cleverness tends to promote
rather unreadable code. Maybe I just haven't seen a good use for it, but I
think I'd rather stick with more explicit mechanisms than this.
Yeah, but "once and only once" is a great principle of programming. Any
time you have to say something _TWICE_ there's something wrong going on.
So,
a, b, c, d = lotsa[:4]
_should_ properly give the impression of a code smell, if your "once and
only once" sense is finely tuned. What's the business of that ':4' on
the RHS? Showing the compiler that you can count correctly?! You're
having to tell twice that you're getting four items into separate
variables, once by listing exactly four variables on the LHS, and
another time by that ':4' on the RHS. IMHO, that's just as bogus as
struct.unpack's limitation of not having any way to indicate explicitly
'and all the rest of the bytes goes here', and for similar reasons.
The slicing on the right is not so much to show the compiler that you
know how to count, it is to show the runtime that you are looking for
a specified slice of lotsa. How would you like the following two cases
to be handled by your desired Python, and how would that make more sense
than what is done now?
a,b,c = [1,2,3,4]
a,b,c = [1,2]
I think it would be better to have a way to say 'and all the rest'.
Lacking that, some automated way to count how many items are being
unpacked on the LHS is probably second-best.
Is it worth a keyword, or would a sys.getframe()/bytecode hack be
sufficient? In the latter case, I'm sure you could come up with such a
mechanism, and when done, maybe you want to offer it up as a recipe in
the cookbook *wink*.
Another case where a specific number of items is requested, which is not
(syntactically speaking) a multiple assignment, is assignment to an
_extended_ slice of, e.g., a list (only; assignment to a slice with a
stride of 1 is happy with getting whatever number of items are coming).
I don't particularly LIKE writing:
L[x:y:z] = len(L[x:y:z]) * [foo]
I much prefer...
for i in xrange(x, y, z):
L[i] = foo
But then again, I don't much like extended list slicing (I generally
only use the L[:y], L[x:] and L[x:y] versions).
- Josiah
On Mon, 15 Nov 2004 19:44:06 -0800, Josiah Carlson wrote: I think it would be better to have a way to say 'and all the rest'.
Lacking that, some automated way to count how many items are being
unpacked on the LHS is probably second-best.
Is it worth a keyword, or would a sys.getframe()/bytecode hack be
sufficient?
Who needs a keyword?
a, b, *c = [1, 2, 3, 4]
a, b, *c = [1, 2]
In the latter case I'd expect c to be the empty tuple.
Clear parallels to function syntax: def f(a, b, *c):
.... print c
.... f(1, 2, 3, 4)
(3, 4) f(1, 2)
()
No parallel for **, but... *shrug* who cares?
Josiah Carlson <jc******@uci.e du> wrote: a, b, c, d = lotsa[:4]
_should_ properly give the impression of a code smell, if your "once and
only once" sense is finely tuned. What's the business of that ':4' on
the RHS? Showing the compiler that you can count correctly?! You're
... The slicing on the right is not so much to show the compiler that you
know how to count, it is to show the runtime that you are looking for
a specified slice of lotsa. How would you like the following two cases
to be handled by your desired Python, and how would that make more sense
than what is done now?
a,b,c = [1,2,3,4]
a,b,c = [1,2]
I would like to get exceptions in these cases, which, being exactly what
IS done now, makes exactly as much sense as itself. Note that there is
no indicator in either of these forms that non-rigid unpacking is
desired. Assuming the indicator for 'and all the rest' were a prefix
star, then:
a, b, *c = [1, 2, 3, 4]
a, b, *c = [1, 2]
should both set a to 1, b to 2, and c respectively to [3, 4] and []. Of
course there would still be failing cases:
a, b, *c = [1]
this should still raise -- 'a, b, *c' needs at least 2 items to unpack.
I think it would be better to have a way to say 'and all the rest'.
Lacking that, some automated way to count how many items are being
unpacked on the LHS is probably second-best.
Is it worth a keyword, or would a sys.getframe()/bytecode hack be
sufficient? In the latter case, I'm sure you could come up with such a
mechanism, and when done, maybe you want to offer it up as a recipe in
the cookbook *wink*.
Two recipes in the 2nd ed of the CB can be combined to that effect
(well, nearly; c becomes an _iterator_ over 'all the rest'), one by
Brett Cannon and one by Sami Hangaslammi. Not nearly as neat and clean
as if the language did it, of course.
Another case where a specific number of items is requested, which is not
(syntactically speaking) a multiple assignment, is assignment to an
_extended_ slice of, e.g., a list (only; assignment to a slice with a
stride of 1 is happy with getting whatever number of items are coming).
I don't particularly LIKE writing:
L[x:y:z] = len(L[x:y:z]) * [foo]
I much prefer...
for i in xrange(x, y, z):
L[i] = foo
Not the same semantics, in the general case. For example:
L[1:-1:2] = ...
rebinds (len(L)/2)-1 items; your version rebinds no items, since
xrange(1, -1, 2) is empty. To simulate the effect that assigning to an
extended slice has, you have to take a very different tack:
for i in slice(x, y, z).indices(len( L)):
L[i] = foo
and that's still quite a bit less terse and elegant, as is usually the
case for fussy index-based looping whenever it's decently avoidable.
But then again, I don't much like extended list slicing (I generally
only use the L[:y], L[x:] and L[x:y] versions).
It may be that in your specific line of work there is no opportunity or
usefulness for the stride argument. Statistically, however, it's
somewhat more likely that you're not taking advantage of the
opportunities because, given your dislike, you don't even notice them --
as opposed to the opportunities not existing at all, or you noticing
them, evaluating them against the lower-level index-based-looping
alternatives, and selecting the latter. If you think that extended
slices are sort of equivalent to xrange, as above shown, for example,
it's not surprising that you're missing their actual use cases.
Alex
Jeremy Bowers <je**@jerf.or g> wrote:
On Mon, 15 Nov 2004 19:44:06 -0800, Josiah Carlson wrote: I think it would be better to have a way to say 'and all the rest'.
Lacking that, some automated way to count how many items are being
unpacked on the LHS is probably second-best.
Is it worth a keyword, or would a sys.getframe()/bytecode hack be
sufficient?
Who needs a keyword?
a, b, *c = [1, 2, 3, 4]
a, b, *c = [1, 2]
I'll post the same thing that was posted by James Knight on python-dev
about this precise syntax...
James Knight wrote: Two threads on the topic, with rejections by Guido:
http://mail.python.org/pipermail/pyt...er/030349.html
http://mail.python.org/pipermail/pyt...st/046684.html
Guido's rejection from
http://mail.python.org/pipermail/pyt...t/046794.html: For the record, let me just say that I'm -1 on adding this feature
now. It's only a minor convenience while it's a fairly big change to
the parser and code generator, but most of all, Python's growth needs
to be reduced. If we were to add every "nice" feature that was
invented (or present in other languages), Python would lose one of its
main charms, which is that it is really a rather simple language that
can be learned and put to production quickly.
So while I appreciate the idea (which BTW has been proposed before) I
think it's not worth adding now, and if you don't hear from me again
on this topic it's because I haven't changed my mind...
Guido hasn't updated his stance, so don't hold your breath.
- Josiah This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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