KeyError with dict.fromkeys() and dict-like object

Question

In Python, you can use a dictionary as the first argument to dict.fromkeys(), e.g.:

In [1]: d = {'a': 1, 'b': 2}

In [2]: dict.fromkeys(d)
Out[2]: {'a': None, 'b': None}

I tried to do the same with a dict-like object, but that always raises a KeyError, e.g.:

In [1]: class SemiDict:
   ...:     def __init__(self):
   ...:         self.d = {}
   ...:
   ...:     def __getitem__(self, key):
   ...:         return self.d[key]
   ...:
   ...:     def __setitem__(self, key, value):
   ...:         self.d[key] = value
   ...:
   ...:

In [2]: sd = SemiDict()

In [3]: sd['a'] = 1

In [4]: dict.fromkeys(sd)
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)

C:\bin\Console2\<ipython console> in <module>()

C:\bin\Console2\<ipython console> in __getitem__(self, key)

KeyError: 0

What exactly is happening here? And can it be resolved, other than using something like dict.fromkeys(sd.d)?

Answer 1

To create the dict, fromkeys iterates over its argument. So it must be an iterator. One way to make it work is to add an __iter__ method to your dict-like:

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

Answer 2

instance of SemiDict is not a sequence. I'd imagine the most obvious solution would be to inherit from dict, why don't you do it?