CODEWITHSUNDEEP
pythonpython-3.xdictionaryhashcpython
user459872
user459872

Reputation: 24907

Strange behavior with python dictionary

I was learning about python hash function and came across the following behavior.

>>> hash(-1)
-2
>>> hash(-2)
-2

SO already has a great post which answers why: Why do -1 and -2 both hash to -2 in CPython?

Since python dictionary uses hash of the key to store the value, the following output is expected because both True and 1 has same hash:

>>> my_dict = { True: "true", 1: "one"}
>>> my_dict
{True: 'one'}
>>> hash(True)
1
>>> hash(1)
1

But if I try with the following, I expect the output as { -1: "Minus Two"}, since -1 and -2 has same hash. But that was not the case.

>>> my_dict = { -1: "Minus One", -2: "Minus Two"}
>>> my_dict
{-1: 'Minus One', -2: 'Minus Two'}
>>> hash(-1)
-2
>>> hash(-2)
-2

What is the reason for this behavior?. Don't dictionaries use the hash of the key to store it's value?

NOTE: I know it's CPython specific, but I'm curious to know the reason for this behavior.

Upvotes: 3

Views: 214

Answers (1)

Seth
Seth

Reputation: 2369

Not only does it check the hash value of an object, it also checks the equality of it. You can see this in effect with this example:

>>> class Samesies:
...     def __hash__(self):
...             return 1
...     def __eq__(self, other):
...             return True
...
>>> {Samesies(): 1, Samesies(): 2}
{<__main__.Samesies object at 0x0000023997ACEFA0>: 2}

EDIT: The reason that 1 == True and hash(1) == hash(True) is due to the fact that bool actually subclasses int.

Upvotes: 3

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