Updating Python Class fields

Question

This is a basic question that can shed some light on how class fields are updated. I came across the following situation:

class MyClass():
    def __init__(self):
        self.a=0
        self.my_dict={'a':self.a}

c=MyClass()
print('c.a=', c.a)
print('c.my_dict[\'a\']=', c.my_dict['a'])
c.my_dict['a']=1
print('c.a=', c.a)
print('c.my_dict[\'a\']=', c.my_dict['a'])

Results:

c.a= 0
c.my_dict['a']= 0
c.a= 0
c.my_dict['a']= 1

c.a has the same value. Why? Shouldn't c.a reference the same location in memory as c.my_dict['a']?

Answer 1

If, after your code, you look at the variables in your class, hopefully it clarifies things:

>>> vars(c)
{'a': 0, 'my_dict': {'a': 1}}

When you assign self.my_dict = {'a': self.a}, you read the current value of a, then make a dict {'a': that_value}. There is no reference to the variable a.

If you wanted to store such a reference, you could do something like the following:

class MyClass():
    def __init__(self):
        self.a = [0]
        self.my_dict = {'a': self.a}

c = MyClass()
>>> vars(c)
{'a': [0], 'my_dict': {'a': [0]}}

c.my_dict['a'][0] = 1
>>> vars(c)
{'a': [1], 'my_dict': {'a': [1]}}