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.

In your example, top and bottom refer to the same list, and this will lead to (maybe) unexpected results in your code. (t = b = [];t.append(1);print b) — jorgeca
– jorgeca, Commented Nov 25, 2012 at 21:18
This wasn't the reason of the described problem, but it solved another one i didn't noticed yet. So thanks either way. — Cravid
– Cravid, Commented Nov 26, 2012 at 9:14
Glad it helped. And that's why I didn't post it as an answer ;) — jorgeca
– jorgeca, Commented Nov 26, 2012 at 14:39

Martijn Pieters · Accepted Answer · 2012-11-25 21:15:48Z

12

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.

answered Nov 25, 2012 at 21:15

Martijn Pieters

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1 Comment

Cravid Over a year ago

Yeah, i thought it would return the list... so thanks, that solved it. That n is only set on creation is deliberate.

applicative_functor · Accepted Answer · 2012-11-25 21:15:07Z

2

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

answered Nov 25, 2012 at 21:15

applicative_functor

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