[issue20135] FAQ need list mutation answers
Roundup Robot added the comment: New changeset 138f54622841 by R David Murray in branch '3.4': #20135: FAQ entry for list mutation. (See also 6375bf34fff6.) https://hg.python.org/cpython/rev/138f54622841 New changeset 3d924bbfdcbc by R David Murray in branch 'default': Merge: #20135: FAQ entry for list mutation. (See also 90b07d422bd9.) https://hg.python.org/cpython/rev/3d924bbfdcbc New changeset 2b9db1fce82e by R David Murray in branch '2.7': #20135: FAQ entry for list mutation. https://hg.python.org/cpython/rev/2b9db1fce82e -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
R. David Murray added the comment: I accidentally committed the patch early to 3.4/3.5. I've now addressed Ezio's review comment per my suggestion on the review, and committed it to 2.7 as well. Thanks everyone for your contributions. -- resolution: - fixed stage: commit review - resolved status: open - closed ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
R. David Murray added the comment: Here is my version of Ezio's patch. -- Added file: http://bugs.python.org/file36748/mutable_faq.patch ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Ezio Melotti added the comment: Here is a new patch that includes an additional paragraph about mutation and non-mutation operations. -- Added file: http://bugs.python.org/file36236/issue20135-2.diff ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Francis MB added the comment: A) On the example: +Also note that some operations (e.g. ``y.append(10)``/``y += [10]`` or +``y.sort()``) mutate are you saying: 1) y.append(10) divided by y += [10] or 2) y.append(10) and y += [10] I don't want to split hairs but reading it fast confused me a bit B) The paragraph: + etc.), we can use some specific operations to mutate it and all the variables + that refer to it will see + the change; Couldn't be (one newline less): + etc.), we can use some specific operations to mutate it and all the variables + that refer to it will see the change; -- nosy: +francismb ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
R. David Murray added the comment: I think the example would be clarified by speaking about mutation operations versus non-mutation operations. After all: x = [1] y = x x = x + [2] x [1, 2] y [1] At that point including a list += operation would also be beneficial. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Ezio Melotti added the comment: Good point, I'll try to add that to the FAQ. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Berker Peksag added the comment: LGTM. -- nosy: +berker.peksag stage: needs patch - patch review ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Changes by Ezio Melotti ezio.melo...@gmail.com: -- assignee: docs@python - ezio.melotti nosy: +rhettinger stage: patch review - commit review ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Roundup Robot added the comment: New changeset 3881c12fa3ae by Ezio Melotti in branch '2.7': #20135: move FAQ about mutable default arguments to the programming FAQs page. http://hg.python.org/cpython/rev/3881c12fa3ae New changeset 3b7b0f5aac1e by Ezio Melotti in branch '3.4': #20135: move FAQ about mutable default arguments to the programming FAQs page. http://hg.python.org/cpython/rev/3b7b0f5aac1e New changeset f2a36b01ac02 by Ezio Melotti in branch 'default': #20135: merge with 3.4. http://hg.python.org/cpython/rev/f2a36b01ac02 -- nosy: +python-dev ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Ezio Melotti added the comment: I moved the FAQ about mutable default arguments to the programming FAQs page. I was going to do a review about new FAQ, but it since I had several comments I just tried to rewrite it and make a new patch. Do you think this is clear enough? -- Added file: http://bugs.python.org/file35875/issue20135.diff ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Mark Lawrence added the comment: +1 from me, I thought it was crystal clear. -- nosy: +BreamoreBoy versions: +Python 3.5 -Python 3.3 ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
M. Volz added the comment: Thanks David, I've updated the patch to move the default values question into the programming FAQ where you recommended it go, and also changed the title of the mutable list question. -- Added file: http://bugs.python.org/file34604/sortFAQ_defaultval.patch ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Éric Araujo added the comment: I agree; if someone goes to the docs, “Programming FAQ” looks more enticing than “Design and History”. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
R. David Murray added the comment: M. Votz: I like your entry, but I think it should be titled Why did changing list 'a' also change list 'b'? That's the form I've always encountered this question in. Then the answer can start of with It didn't, 'a' and 'b' are the same list. and then go on with your text. Since Éric concurs, lets go ahead and move that other answer. Probably to right before How can I pass optional or keywords parameters from one function to another? -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
M. Volz added the comment: I've written and attached a patch for the list mutation question. I took a little bit of a different tack on this and addressed a more general question whilst still including list mutation as an example. I know the language can get a little fussy on these sorts of things- let me know if there's anything that could be better worded. As for the second issue, as Ezio mentioned there's already an FAQ question regarding that- would there be any objection to just copying it into the programming section? Or should it be left where it is? -- Added file: http://bugs.python.org/file34336/defaultdoc_listmut.patch ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Éric Araujo added the comment: Resources: http://nedbatchelder.com/text/names.html http://python.net/~mwh/hacks/objectthink.html -- nosy: +eric.araujo ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
R. David Murray added the comment: I'm in favor of moving the FAQ answer Ezio identified to the Programming section, myself, but I'd like concurrence from at least one other committer before we do it. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Changes by M. Volz marie...@gmail.com: -- nosy: +mvolz ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Ezio Melotti added the comment: http://docs.python.org/3/faq/design.html#why-are-default-values-shared-between-objects -- nosy: +ezio.melotti ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
R. David Murray added the comment: I *thought* there was a FAQ entry for that. Didn't think to look for it in the design section though :(. Nor is the title likely to catch the eye of someone wondering why their default argument modifications are being unexpectedly persistent. Also, the fact that it starts out with the sentence This type of bug commonly bites neophyte programmers is, IMO, a clue it should be in the programming section. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Fran Bull added the comment: I read the FAQ last night and I couldn't see these answered there either. I would like to try submitting a patch for this one, probably this evening. It will likely be two FAQs in the programming section that go something like: Why does changing one list change another different list? This happens: a = [1, 2, 3] b = a b.append(4) print a [1, 2, 3, 4] because variables are just names for things, in this case 'a' is the list we first defined and then b = a says that 'b' is also a name for that list. They are both the same list. Why are my default args wrong? This happens: from datetime import datetime class A(object): ... def __init__(self, created_time=datetime.now()): ... self.created_time = created_time ... an_a = A() another_a = A() an_a.created_time datetime.datetime(2014, 1, 16, 10, 40, 54, 33283) another_a.created_time datetime.datetime(2014, 1, 16, 10, 40, 54, 33283) because default arguments are evaluated when they're read for the first time by the interpreter. Usually when the class is imported. A good way to get the above to do what you want is to use a default argument of None and check for it, like: class B(object): ... def __init__(self, created_time=None): ... if created_time is None: ... created_time=datetime.now() ... self.created_time = created_time ... a_b = B() another_b = B() a_b.created_time datetime.datetime(2014, 1, 16, 10, 44, 44, 956710) another_b.created_time datetime.datetime(2014, 1, 16, 10, 44, 51, 71311) Feedback appreciated, particularly I'm not sure if this: 'default arguments are evaluated when they're read for the first time by the interpreter' is exactly the right language. I guess I'll look it up. -- nosy: +Fran.Bull ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
R. David Murray added the comment: Thanks for working on this. Technically, defaults are evaluated when the 'def' statement is executed, which is normally, but not always, at the time that the module is first imported (counterexample: nested function definitions). The answer should also explicitly cover the more common mistake made with default keyword values of passing a mutable object and having it get persistently modified,and the question phrasing should reflect that. The datetime example is a great first example before getting to the more common one, though. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Fran Bull added the comment:
Perhaps a 3rd FAQ something like this?:
Why is changing a list in one instance of a class also changing it in another
instance of the same class?
This happens:
class A(object):
... def __init__(self, fruit=[]):
... self.fruit = fruit
...
an_A = A()
an_A.fruit.append('apple')
another_A = A()
print another_A.fruit
['apple']
another_A.fruit.append('banana')
print another_A.fruit
['apple', 'banana']
print an_A.fruit
['apple', 'banana']
print an_A.fruit is another_A.fruit
True
print an_A.fruit is A().fruit
True
because of a combination of the above two FAQs, first the default argument is
evaluated when the 'def' statement is executed and creates an instance of list.
After that new instances of A have a variable 'fruit' that is the name for that
instance of list.
I'm not sure whether I should be using as general terms as possible for the Q,
i.e. 'mutable object' instead of 'list'. I'll reread
http://docs.python.org/devguide/documenting.html and the FAQs before writing
the patch.
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[issue20135] FAQ need list mutation answers
Changes by Fran Bull fran.b...@gmail.com: -- keywords: +patch Added file: http://bugs.python.org/file33502/20135.patch ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue20135] FAQ need list mutation answers
Changes by Terry J. Reedy tjre...@udel.edu: -- stage: - needs patch title: mutate list - FAQ need list mutation answers ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue20135 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
