Question

Using a class to create several instances in a dictionary, their attribute is not unique. I’d like each of them to have their own unique attribute. How do you do this?

code:

class a(object):

    attr_a = {}

    def __init__(self, a={}):

        self.attr_a = a

if __name__ == '__main__':

    b = a()
    c = a()
    b.attr_a['abc'] = 'abc'
    c.attr_a['abc'] = 'def'

    print(b.attr_a)
    print(c.attr_a)

result:

{'abc': 'def'}
{'abc': 'def'}

wanted result:

{'abc': 'abc'}
{'abc': 'def'}

Answer 1

Two problems here:

1) For instance-level attributes, as opposed to class-level attributes, don’t declare at the class level. So, remove the attr_a = {} in your class.

2) The default value a={} is evaluated at class/function declaration time, as opposed to instance-creation/function-call time, so rarely should you use a mutable object as a default value. This causes all instances of your class that are created with the default a value to share the same attr_a dict. You should use a=None as your default value, and then instantiate it inside of __init__ instead, like this:

def __init__(self, a=None):
        self.attr_a = a or {}