Using object as key in dictionary in Python - Hash function

Question

I am trying to use an object as the key value to a dictionary in Python. I follow the recommendations from some other posts that we need to implement 2 functions: __hash__ and __eq__

And with that, I am expecting the following to work but it didn't.

class Test:
    def __init__(self, name):
        self.name = name
        
    def __hash__(self):
        return hash(str(self.name))
        
    def __eq__(self, other):
        return str(self.name) == str(other,name)
        
        
def TestMethod():
    test_Dict = {}
    
    obj = Test('abc')
    test_Dict[obj] = obj
    
    print "%s" %(test_Dict[hash(str('abc'))].name)       # expecting this to print "abc" 

But it is giving me a key error message:

KeyError: 1453079729188098211

Answer 1

You don't need to redefine hash and eq to use an object as dictionary key.

class Test:
    def __init__(self, name):
        self.name = name

test_Dict = {}

obj = Test('abc')
test_Dict[obj] = obj

print test_Dict[obj].name

This works fine and print abc. As explained by Ignacio Vazquez-Abrams you don't use the hash of the object but the object itself as key to access the dictionary value.

---

The examples you found like python: my classes as dict keys. how? or Object of custom type as dictionary key redefine hash and eq for specific purpose.

For example consider these two objects obj = Test('abc') and obj2 = Test('abc').

test_Dict[obj] = obj
print test_Dict[obj2].name

This will throw a KeyError exception because obj and obj2 are not the same object.

class Test:
    def __init__(self, name):
        self.name = name

    def __hash__(self):
        return hash(str(self.name))

    def __eq__(self, other):
        return str(self.name) == str(other.name)

obj = Test('abc')
obj2 = Test('abc')       

test_Dict[obj] = obj
print test_Dict[obj2].name

This print abc. obj and obj2 are still different objects but now they have the same hash and are evaluated as equals when compared.

Answer 2

Elements of a mapping are not accessed by their hash, even though their hash is used to place them within the mapping. You must use the same value when indexing both for storage and for retrieval.