Adding an instance variable to a base class in Python

Question

I have a base class A, inherited by classes B and C, from which I was trying to set an instance variable. Such variable is used by methods from base class A as follows.

class A(object):
    def foo(self):
        print(self.value)

class B(A):
    value = "B"

class C(A):
    value = "C"

>>> b = B()
'B'
>>> c = C()
'C'

I understand function foo will only be evaluated during execution, which is fine as long as I do not invoke foo straight from an instance of A. Yet, I fail to grasp how value = "B" and value = "C" manage to become self.value = "B" and self.value = "C".

Sorry if this is naive question; I have been far from python for quite a while now, and really had not seen anything quite like it. I'm using Python version 2.7.12.

They are not instance variables, they are class-level variables being accessed from an instance. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Oct 26, 2017 at 3:45
@juanpa.arrivillaga But should not this access happen through value instead of self.value? Why does self.value work? — Rubens
– Rubens, Commented Oct 26, 2017 at 15:09
No. value is not defined in any scope. That would raise a NameError. You can use MyClass.value or my_instance.value, but just value will try to find a local variable or a global variable. When a attribute on an instance is accessed, first it checks the namespace of the instance, if it isn't found, it checks the namespace of the class, and then all the classes in the MRO until the name is resolved or an AttributeError is raised. That's inheritance in a nutshell. See this question — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Oct 26, 2017 at 16:17

Aaron Bentley · Accepted Answer · 2017-10-26 17:46:15Z

1

You cannot add an instance variable to a base class in Python, because instance variables can only be added to instances, not classes.

In your example, "value" is a class variable. In Python, when there is no instance variable with a given name, it falls back to the class variable with that name. However, if you assigned a new value to b.value, that would create an instance variable on b. B.value would be unaffected, as would any new instances of B.

I didn't find an authoritative reference for the behavior, but here's an article about it. https://www.toptal.com/python/python-class-attributes-an-overly-thorough-guide

edited Oct 26, 2017 at 17:46

answered Oct 26, 2017 at 4:16

Aaron Bentley

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3 Comments

Rubens Over a year ago

Ah, I really did not know that when there is no instance variable with a given name, it falls back to the class variable with that name. Would you suggest some reference, so I can better understand this default behavior definition? Thank you in advance.

Aaron Bentley Over a year ago

I didn't find an authoritative reference before my original post, so I didn't include one. I've updated my answer to include an article.

juanpa.arrivillaga Over a year ago

@Rubens all the details are in the data model, so for example this snippet: "Custom class types are typically created by class definitions (see section Class definitions). A class has a namespace implemented by a dictionary object. Class attribute references are translated to lookups in this dictionary, e.g., C.x is translated to C.__dict__["x"] (although there are a number of hooks which allow for other means of locating attributes). When the attribute name is not found there, the attribute search continues in the base classes."

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