I was reading this:
http://stackoverflow.com/questions/4418741/im-able-to-use-a-mutable-object-as-a-dictionary-key-in-python-is-this-not-disa
In a User Defined Type, one can provide __hash__ that returns a integer
as a key to a dictionary.
so: d = { key : value }
What is the significance of __eq__ in this regard? Why does the article
state:
"A class that overrides __eq__() and does not define __hash__() will
have its __hash__() implicitly set to None. '
Is this behavior like __len__ and __bool__ wrt Boolean Expressions?
If one does: if x < y and __bool__ is not defined then __len__ is
checked. Here, if __hash__ is undefined __eq__ is checked? What does he
mean by 'overrides __eq__'??
Also didn't follow this
" For classes you write, this method defaults to returning a value based
off id(self), and *********** if equality is not determined by identity
for those classes, you may be surprised by using them as keys:
*******************"
"The requirement is that the hash of an object doesn't change over time,
and ********* that it keeps comparing equal (==) with its original
value. **************************"
Also if one can do x.a = 10 or 20 or whatever, and the class instance is
mutable, then why do books keep stating that keys need to be immutable?
After all, __hash__ is the guy doing all the work and maintaining
consistency for us. One could do:
class Fruit:
editable_value = ''
def __hash__(self):
if 'apple' in self.value:
return 10
elif 'banana' in self.value:
return 20
and use 'apple' 'bannana' as keys for whatever mutable data..
Are the books wrong?
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