On page 209 it says: “mutable values are unhashable”
However, mutability and hash-ability are separate properties. A type is hash-able when its hash function is defined. A counter example that shows a mutable list that gets hashed and added as key to a dict:
class mylist(list): # mylist (mutable) is a list with a __hash__ function
def __hash__(self):
hash = 0
for i in self:
hash ^= i
return hash
ml= mylist()
ml.append(1)
ml.append(2)
ml.append(3)
print( ml ) # [1, 2, 3]
mydict={ ml:100, mylist([4,5,6]):200}
print( mydict ) # {[1, 2, 3]: 100, [4, 5, 6]: 200}
print( mydict[ml] ) # 100
print( mydict[mylist([4,5,6])] ) # 200
print( mylist([6,7,8]) in mydict) # False
Also a typo:
TypeError: unhashable type: ‘set’ => TypeError: unhashable type: ‘list’