Mark Ransom answered on a SO question about hashes here in SO:
[...] An object is hashable if it has a hash value which never changes during its lifetime. So by the official definition, anything mutable can't be hashable, even if it has a
__hash__()function. My statement about both requirements being necessary is untrue, because being hashable already implies the requirement to be immutable.
I want to make sure, that I got that right - even as a non native speaker - so I hope someone corrects me if I got it wrong.
Assuming this class
class Author(object):
def __init__(self, id, name, age):
self.id = id
self.name = name
self.age = age
def __eq__(self, other):
return self.id==other.id\
and self.name==other.name
def __hash__(self):
return hash(('id', self.id,
'name', self.name))
I understand, that __eq__ allows me to compare objects of this class with the == operator. From Marks answer I understand, that even if my object peter = Author(1, "Peter", 33) has a __hash__ it is not hashable because I potentially could do something like peter.age = 43 which means it is not immutable. So my objects of the class Author are not hashable and therefore not usable as keys in dictionarys for example? Did I get right or does it seem, that I need more explanation? :-)