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
Solution 1
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).
Solution 2
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 :)
Solution 3
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'
Solution 4
It modifies the list in-place, and returns None. None evaluates to false.
Solution 5
Actually, it returns None
>>> print u.append(6)
None
>>> print not None
True
>>>
Solution 6
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 Commandquery 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 its 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.
Solution 7
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
