4 ms·

>>> my_dict[nan] = 3 >>> my_dict[nan] 3 Wait, how does that work if nan isn't equal to itself?

• gravel7623 7mo ago
  I guess because the hash of an instance stays consistent (which is used to retrieve the value from the dict). The `__eq__` method must disregard the hash and return False for all nans.
  • munchler 7mo ago
    But the hash alone shouldn't be enough to match the key. Isn't an equality check also needed to avoid a false positive? That's the idea behind a hash table, as I understand it. (I'm not a Python programmer.)
    • zahlman 7mo ago
      That equality check also considers object identity first: >>> class Evil: ... def __init__(self, hash_value): self.hash_value = hash_value ... def __eq__(self, other): return False ... def __hash__(self): return self.hash_value ... >>> e1, e2 = Evil(1), Evil(2) >>> {e1:1, e2:2} {<__main__.Evil object at ...>: 1, <__main__.Evil object at ...>: 2} >>> {e1:1, e2:2}[Evil(1)] Traceback (most recent call last): File "<stdin>", line 1, in <module> KeyError: <__main__.Evil object at ...> >>> {e1:1, e2:2}[e1] 1 I'm pretty sure that this is meant as an optimization. (But it does have to find the instance via the hash lookup first. This won't work if you e.g. return a random number from `__hash__`.)