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

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).

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'

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

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 :)