Why does list.append evaluate to false in a boolean context?

Question

Is there a reason being list.append evaluating to false? Or is it just the C convention of returning 0 when successful that comes into play?

>>> u = []
>>> not u.append(6)
True

A possibly better way to phrase: *why does python not use the `Builder` pattern .. so we can do `u.append(6).append(7). ..`* This is annoying. — WestCoastProjects, May 07 '17 at 01:16
@Stefan `append()` and `extend()` have different effects and are not interchangeable — WestCoastProjects, Jan 07 '20 at 18:17
@javadba What difference? My extend has the same effect as your two appends. — Stefan Pochmann, Jan 07 '20 at 18:26
extend() retains a single list if the elements were a list whereas append() will be a list of lists. In the above case they are not so it's a special case that does end up the same between extend() and append(). — WestCoastProjects, Jan 07 '20 at 18:38
@javadba Not sure what you mean, but it sounds wrong. It doesn't matter that we used numbers. If we use lists instead, my extend still has the same effect as your two appends. — Stefan Pochmann, Jan 07 '20 at 19:07
@StefanPochmann difference between append() and extend() https://stackoverflow.com/questions/252703/what-is-the-difference-between-pythons-list-methods-append-and-extend — WestCoastProjects, Jan 07 '20 at 19:25
@javadba Not sure what that shall tell me. I know the difference. But you seem mistaken about `extend`. — Stefan Pochmann, Jan 07 '20 at 19:41

score 45 · Accepted Answer · answered Nov 05 '09 at 18:29

Most Python methods that mutate a container in-place return None -- an application of the principle of Command-query separation. (Python's always reasonably pragmatic about things, so a few mutators do return a usable value when getting it otherwise would be expensive or a mess -- the pop method is a good example of this pragmatism -- but those are definitely the exception, not the rule, and there's no reason to make append an exception).

score 20 · Answer 2 · answered Nov 05 '09 at 18:25

20

None evaluates to False and in python a function that does not return anything is assumed to have returned None.

If you type:

>> print u.append(6)
None

Tadaaam :)

answered Nov 05 '09 at 18:25

Matthieu M.

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Mutators (like append, extend, sort, etc.) which update a list do not return a value. – S.Lott Nov 05 '09 at 18:41
`None` doesn't *evaluate* to `False`. – SwiftsNamesake Nov 19 '16 at 14:23
@SwiftsNamesake Try `bool(None)` – Chris_Rands Feb 15 '17 at 09:49
@Chris_Rands `None` is not a boolean, which is why you need the `bool` function to convert it (done implicitly by `not`). Yes, I know it's a quibble but it's `True`. – SwiftsNamesake Feb 15 '17 at 10:18
@SwiftsNamesake No-one said `None` is a boolean, but it does evaluate to `False` – Chris_Rands Feb 15 '17 at 10:33
@Chris_Rands I guess my quibble is with the phrase *evaluates to*, which admittedly is due to the OP. As I said I don't dispute the gist of the answer (who would) and I know that `None` is 'falsy'. – SwiftsNamesake Feb 15 '17 at 10:40
@SwiftsNamesake: I am open to suggestions, *evaluates* remains the best expression I have so far. – Matthieu M. Feb 15 '17 at 10:45
1

@SwiftsNamesake I find *evaluates* clearer than Python's own semantics, `help(bool)` says "bool(x): Returns True when the argument x is true". But `x` is true seems too similar to `x is True`, which of course has a completely different meaning – Chris_Rands Feb 15 '17 at 11:00

score 7 · Answer 3 · answered Nov 05 '09 at 18:25

because .append method returns None, therefore not None evaluates to True. Python on error usually raises an error:

>>> a = ()
>>> a.append(5)
Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    a.append(5)
AttributeError: 'tuple' object has no attribute 'append'

score 5 · Answer 4 · answered Nov 05 '09 at 18:25

5

It modifies the list in-place, and returns None. None evaluates to false.

answered Nov 05 '09 at 18:25

Matt Anderson

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score 3 · Answer 5 · answered Nov 05 '09 at 18:29

3

Actually, it returns None


>>> print u.append(6)
None
>>> print not None
True
>>>

answered Nov 05 '09 at 18:29

adam

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score 1 · Answer 6 · answered Apr 06 '18 at 08:33

Method append modifies the list in-place and the return value None

In your case, you are creating an array — [6] — on the fly, then discarding it. The variable b ends up with the return value of None.

Why?
This comply with the principle of Command–query separation devised by Bertrand Meyer.
It states that every method should either be a command that performs an action, or a query that returns data to the caller, but not both. In your example:

u.append(6)

append modified the state of [], so it’s not a best practice to return a value compliance with the principle.

In theoretical terms, this establishes a measure of sanity, whereby one can reason about a program's state without simultaneously modifying that state.

CQS is well-suited to the object-oriented methodology such as python.

This also destroys the `builder()` pattern. It is one of the many reasons python is difficult to use. — WestCoastProjects, Mar 15 '20 at 05:50

score 0 · Answer 7 · edited Jul 16 '20 at 12:59

0

The list.append function returns None. It just adds the value to the list you are calling the method from.

Here is something that'll make things clearer:

>>> u = []
>>> not u
False
>>> print(u.append(6)) # u.append(6) == None
None    
>>> not u.append(6) # not None == True
True

edited Jul 16 '20 at 12:59

answered Jan 31 '18 at 20:57

Rafael Aguilar

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Why does list.append evaluate to false in a boolean context?

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