Override dict() on class

dict can be called with an iterable of pairs, so if you design your __iter__ to return an iterable of tuples, your example works as you'd like:

class Foo:
    def __iter__(self):
        yield from {
            'this': 'is',
            'a': 'dict'
        }.items()

dict(Foo())
{'a': 'dict', 'this': 'is'}

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If you want your class to behave like a python dictionary, in that iterating over an instance iterates over its keys, you can implement the interface defined by abc.Mapping.

You can do this either by implementing __getitem__, __iter__, and __len__, and inheriting from abc.Mapping, or by implementing all of __getitem__, __iter__, __len__ __contains__, keys, items, values, get, __eq__, and __ne__.

Although the approach in the answer from @ForeverWintr is fairly clever and works, I think it's a little obscure since it takes advantage of some of the arcane details about the attributes of the argument passed to the dict class constructor.

For that reason a better approach might be what I was saying in my comments about just adding a method that does what you want (plus shows how simply giving it a name indicating exactly what's going on makes it more understandable).

Example code:

class Foo:
    def to_dict(self):
        return {'this': 'is', 'more': 'clear'}

print( Foo().to_dict() )  # -> {'this': 'is', 'more': 'clear'}