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pythontypesdictionarycustomizationinternals

How to change the behavior of a python dictionary's __setattr__?

In Python, everything has a class. Therefore dict also has a class.

So, in theory, I should be able to change the implementation of the keyvalue assignment behavior.


Example:

d = dict()
d['first'] = 3    # Internally d['first'] is stored as 6 [i.e. value*2 if value is INT]

print d['first']  # should print 6

d['second'] = 4 

print d['second'] # should print 8


I noticed that most objects have attributes listed in OBJECT.__dict__ or vars(OBJECT). But this isn’t the case for dict or list.

How can I get the desired behavior by overriding dict.__setattr__() method?

Solution

  • It is __setitem__ that have to be overriden in this case - and it is as simples as:

    class MyDict(dict):
    def __setitem__(self, key, value):
         dict.__setitem__(self, key, 2 * value)

    Example:

    >>> m  = MyDict()
>>> m[0] = 5
>>> m
{0: 10}

    __setattr__ controls how object attributes themselves (not key/value pairs) are attributed.