How come self.add(a) and self + a doesn't return the same values in dictionaries?

Question

I'm taking a Python course and the exercise is to create a class in which you can append a dictionnary to another by typing : dict1 = dict1 + dict2

So this is done by creating an __add__ method in the class, because dict1 + dict2 is the same as typing dict1.__add__(dict2) , right ? Except that if I type those 2 commands they don't return the same value. This is an extract of my code :

class zDict(dict):
...
     def __add__(self, otherDict):
         otherDict = [(k,v) for k,v in otherDict.items()]
         print("passed through zDict.__add__") #just to make sure           
         for key, value in otherDict:
             self[key]=value
...

So now if we run this with 2 zDict objects, dict1 = {'a':1,'b':2,'c':3} and dict2 = {'d':4,'e':5} for examples, here are the results (Python 3.6.6) :

>>> dict1.__add__(dict2)
passed through zDict.__add__
>>> dict1
>>> {'a':1,'b':2,'c':3,'d':4,'e':5}
>>> type(dict1)
>>> <class '__main__.zDict'>

RESTART

>>> dict1 += dict2
passed through zDict.__add__
>>> dict1

>>> type(dict1)
>>> <class 'NoneType'>

So I don't understand there. I typed the same thing but got different results. dict1 = dict1 + dict2 is the same as typing dict1 += dict2, which is the same as typing dict1 = dict1.__add__(dict2), right ? And I made sure they pass through the same method with print. It can't come from the dict class, because it doesn't have any __add__ method. Maybe it passes through a method from a parent class like object, who changes the type to NoneType ? How can I check that, and prevent it ?

This is because you are not returning anything from `__add__` — Ankit Jaiswal, Nov 21 '18 at 10:16

score 0 · Answer 1 · answered Nov 21 '18 at 10:16

0

zDict.__add__() will return None (i.e. you change self in place) dict1 += dict2 is same as dict1 = dict1.__add__(dict2)

also why not use dict.update()?

answered Nov 21 '18 at 10:16

buran

13,682
10
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For some reason dict.update() returns noneType as well – julienp17 Nov 21 '18 at 11:10

score 0 · Answer 2 · answered Nov 21 '18 at 10:21

0

+= is implemented using the __iadd__ method, see this answer

answered Nov 21 '18 at 10:21

jfowkes

1,495
9
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How come self.__add__(a) and self + a doesn't return the same values in dictionaries?

2 Answers2

How come self.add(a) and self + a doesn't return the same values in dictionaries?