Question

As you know, python allows us simply override dict.__getitem__ method so we can do something different in there when someone tries to retrieve any value from it.

I want to do some code when one MyDict(dict) class instance is passed to update method of another python dict instance. See below:

class MyDict(dict):
    def __getitem__(self, item):
        print "Doing some stuff here"
        return dict.__getitem__(self, item)

d1 = MyDict({'1': 1, '2': 2})
d2 = {}
# I want to have d1.__getitem__ called, but it does not work :-(
d2.update(d1)

Answer 1

All you need is to subclass MyDict from object and create .keys() method for it. See below:

class MyDict(object):
    def __init__(self, items=()):
        self._dict = dict(items)

    def keys(self):
        return self._dict.keys()

    def __getitem__(self, item):
        print "Doing some stuff for item:", item
        return self._dict[item]

    def __setitem__(self, item, value):
        self._dict[item] = value

    # You can add some more dict methods

d1 = MyDict({'1': 1, '2': 2})
d2 = {}
# Now you will see some stuff executed for each
# value extracted from d1 while updating d2
d2.update(d1)