When does Python fall back onto class __dict__ from instance __dict_

Please see the below snippet:

class Foo:
    class_var = "hi"

foo = Foo()
assert foo.class_var is Foo.class_var
assert "class_var" in Foo.__dict__
assert "class_var" not in foo.__dict__

All assertions here pass, though I am not sure if it's surprising that the identity assertion passes.

When and how does Python fall back onto a class __dict__ from an instance __dict__?

CodePudding user response：

As far as I know when foo.class_var is called the following steps happen:

Python starts to look for class_var in the namespace of the foo object.
If it finds it, it returns it.
In this case however it doesn't find it so it looks in the type of foo, which is Foo.
It finds it in Foo and returns it.

CodePudding user response：

You should know the difference between a "class attribute" and an "instance attribute", in your example, you have class_var which is class attribute, in the first assert statement, it check if class_var is in the instance of Foo , it pass, that because foo is of type Foo let's try another more clear examples here:

class Foo:
    var1 = "Hello"
    def __init__(self,var2):
        self.var2 = var2
foo = Foo("Hello")
assert foo.var1 is Foo.var1
assert "var1" in Foo.__dict__
assert "var2" in foo.__dict__
assert "var1" in foo.__dict__

notice something, the last assert statement is gonna raise an error, because var1 here is a class attribute, not instance attribute.