It's because cpython shortcuts through identity: https://github.com/python/cpython/blob/main/Objects/object.c#L724-L731 https://github.com/python/cpython/blob/m

It's because cpython shortcuts through identity: https://github.com/python/cpython/blob/main/Objects/object.c#L724-L731 https://github.com/python/cpython/blob/main/Objects/object.c... /* Quick result when objects are the same. Guarantees that identity implies equality. */ if (v == w) { if (op == Py_EQ) return 1; else if (op == Py_NE) return 0; } `math.nan` is always itself, and thus this shortcut is taken and the key works.