first hash past the post

Numeric types in Python have the interesting property that their hash() is often their value:

>>> hash(1)

1

>>> hash(1.0)

1

Python also considers floating point and integers of the same value to be equal:

>>> 1 == 1.0

True

Two things with the same hash that are equal count as the same key in a dict:

>>> 1 in {1.0: 'cat'}

True

However, it is possible for the key to be either an int or a float:

>>> {1: 'first'}

{1: 'first'}

>>> {1.0: 'second'}

{1.0: 'second'}

Whichever key is used first sticks.  Later writes to the dict can change the value, but the int or float key remains:

>>> {1: 'first', 1.0: 'second'}

{1: 'second'}

>>> {1.0: 'first', 1: 'second'}

{1.0: 'second'}