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pythonsetattributeerror

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

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.

Solution 
  • 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)