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

问题

I got the following error

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

while I tried this.

not_yet_bought_set = set()
.
.
.
for value in set_dict.itervalues():
    for item in value:
        not_yet_bought_set = not_yet_bought_set.add(item)

I dont't get why I got this error, is it because I always make not_yet_bought_set new? I make this, because when I only do

not_yet_bought_set.add(item)

there wont be all the items from all values. I do not know why.

value are sets and

not_yet_bought_set.union(value)

also generate this error

Thanks for any help.

回答1:

not_yet_bought_set.add(item)

this will return None and you are assigning it to not_yet_bought_set. So, not_yet_bought_set becomes None now. The next time

not_yet_bought_set = not_yet_bought_set.add(item)

is executed, add will be invoked on None. Thats why it fails.

To fix this, simply do this. Dont assign this to anything.

 not_yet_bought_set.add(item)

回答2:

set.add returns nothing.

>>> s = set()
>>> the_return_value_of_the_add = s.add(1)
>>> the_return_value_of_the_add is None
True

Replace following line:

not_yet_bought_set = not_yet_bought_set.add(item)

with:

not_yet_bought_set.add(item)

来源：https://stackoverflow.com/questions/19853872/attributeerror-nonetype-object-has-no-attribute-add