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I'm trying to update a key from a python object via a subscriber/pattern, this is part of a larger code base but I would like to check if I'm doing something wrong here.

from dataclasses import dataclass
 
@dataclass
class A:
    a:float=2
    def mod(self,name):
        self.a = name
    def __hash__(self):
        return hash(self.a)

class B:
    x = {}
    def attach(self,object):
        self.x[object]= getattr(object,'mod')

    def dispatch(self):
        for _, val in self.x.items():
            val(3)

When executing

>>>a = A()
>>>b = B()
>>>print(a.a)
>>>b.attach(a)
>>>print(b.x)

As expected:

{A(a=2): <bound method A.mod of A(a=2)>}

>>>print(b.x[a])

<bound method A.mod of A(a=2)>

Then when executing the update

>>b.dispatch()
>>print(b.x)

{A(a=3): <bound method A.mod of A(a=3)>}

But when the key is verified:

>>>print(list(b.x.keys())[0] is a)

True

When trying to retrieve the method

>>>print(b.x.get(a)) 

None
Aladinoster
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1 Answers1

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Apparently the problem comes from the way hash is defined. Hashing should respect some properties. Dictionaries require hashable keys according to link

From the Official documentation

Having a hash() implies that instances of the class are immutable. Mutability is a complicated property that depends on the programmer’s intent, the existence and behavior of eq(), and the values of the eq and frozen flags in the dataclass() decorator.

Aladinoster
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  • 2