Mad Physicist Mad Physicist - 1 year ago 102
Python Question

Why and how are Python functions hashable?

I recently tried the following commands in Python:

>>> {lambda x: 1: 'a'}
{<function __main__.<lambda>>: 'a'}

>>> def p(x): return 1
>>> {p: 'a'}
{<function __main__.p>: 'a'}

The success of both
creations indicates that both lambda and regular functions are hashable. (Something like
{[]: 'a'}
fails with
TypeError: unhashable type: 'list'

The hash is apparently not necessarily the ID of the function:

>>> m = lambda x: 1
>>> id(m)
>>> hash(m)
>>> m.__hash__()

The last command shows that the
method is explicitly defined for
s, i.e., this is not some automagical thing Python computes based on the type.

What is the motivation behind making functions hashable? For a bonus, what is the hash of a function?

Answer Source

It's nothing special. As you can see if you examine the unbound __hash__ method of the function type:

>>> def f(): pass
>>> type(f).__hash__
<slot wrapper '__hash__' of 'object' objects>

it just inherits __hash__ from object. Function == and hash work by identity. The difference between id and hash is normal for any type that inherits object.__hash__:

>>> x = object()
>>> id(x)
>>> hash(x)

You might think __hash__ is only supposed to be defined for immutable objects, but that's not true. __hash__ should only be defined for objects where everything involved in == comparisons is immutable. For objects whose == is based on identity, it's completely standard to base hash on identity as well, since even if the objects are mutable, they can't possibly be mutable in a way that would change their identity. Files, modules, and other mutable objects with identity-based == all behave this way.

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