What happens when I loop a dict in python

Question

I know Python will simply return the key list when I put a dict in for...in... syntax.

But what what happens to the dict?

When we use help(dict) , we can not see __next()__ method in the method list. So if I want to make a derived class based on dict :

class MyDict(dict)
    def __init__(self, *args, **kwargs):
        super(MyDict, self).__init__(*args, **kwargs)

and return the value list with for...in...

d = Mydict({'a': 1, 'b': 2})
for value in d:

what should I do?

Answer 1

Naively, if all you want is for iteration over an instance of MyClass to yield the values instead of the keys, then in MyClass define:

def __iter__(self):
    return self.itervalues()

In Python 3:

def __iter__(self):
    return iter(self.values())

But beware! By doing this your class no longer implements the contract of collections.MutableMapping , even though issubclass(MyClass, collections.MutableMapping) is True. You might be better off not subclassing dict , if this is the behaviour you want, but instead have an attribute of type dict to hold the data, and implement only the functions and operators you need.