AttributeError: 'NoneType' object has no attribute 'append'

Question

I have a weird problem with python passing a list as parameter to a function. Here is the code:

def foobar(depth, top, bottom, n=len(listTop)):
    print dir(top)
    print top.append("hi")
    if depth > 0:
        exit()
    foobar(depth+1, top.append(listTop[i]), bottom.append(listBottom[i]))

top = bottom = []
foobar(0, top, bottom)

It says "AttributeError: 'NoneType' object has no attribute 'append'", because top is None in foobar although dir(top) prints a full attribute and method list of a type list. So whats wrong? I just wanted to pass two lists as parameters to this recursive function.

Answer 1

You pass in the result of top.append() to your function. top.append() returns None:

>>> [].append(0) is None
True

You need to call .append() separately, then pass in just top:

top.append(listTop[i])
bottom.append(listBottom[i])
foobar(depth+1, top, bottom)

Note that the n=len(listTop) argument in the function is both redundant and only ever executed once, namely when you create the function. It won't be evaluated each time you call the function. You can omit it safely from the version you posted here in any case.

Answer 2

top.append(listTop[i]) works in place and returns None