list = [1, 2, 3]
print(list.append(4)) ## WRONG, print does not work, append() returns None
## RIGHT:
list.append(4)
print(list) ## [1, 2, 3, 4]
I'm learning Python and I'm not sure if this problem is specific to the language and how append is implemented in Python.
append is a mutating (destructive) operation (it modifies the list in place instead of of returning a new list). The idiomatic way to do the non-destructive equivalent of append would be
>>> l = [1,2,3]
>>> l + [4]
[1,2,3,4]
>>> l
[1,2,3]
to answer your question, my guess is that if append returned the newly modified list, users might think that it was non-destructive, ie they might write code like
m = l.append("a")
n = l.append("b")
and expect n to be [1,2,3,"b"]