Question

>>> a = [1, 2, 3]
>>> a.append(4)
>>> a
[1, 2, 3, 4]

But:

>>> [1, 2, 3].append(4)
>>>

Why do list methods in Python (such as insert and append) only work with defined variables?

Answer 1

In the second sample nothing is printed, because append, that was called on a list (note that append was actually performed), returns None.

Alternatively you should mention that a.append(4) also gave you a blank line (as your first sample shows), and final output of a first code sample was a representation of result of a expression, not a.append('4') expression.

Nothing is printed after append call in both cases because it is a representation of None.