[Python] How to add known length to an iterator?

I make an iterator:

it = iter([1,2,3])


len(it) fails because object of type 'list_iterator' has no len().

I thought I could add a length by doing

it.__len__ = lambda: 3


But this also errors 'list_iterator' object has no attribute '__length__'

How can I set a length on an iterator object?

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