How I can check if class attribute was defined or derived in given class in Python?

In Python, you can check if a class attribute was defined directly in a class or inherited from a parent class by examining the class's __dict__ attribute and using hasattr().

Understanding Class Attributes

When a class inherits from another class, it can access parent class attributes. To distinguish between defined and inherited attributes, check if the attribute exists in the class's own __dict__.

Example

Let's create two classes to demonstrate defined vs inherited attributes ?

class A:
    foo = 1

class B(A):
    pass

# Check if 'foo' is defined in class A
print("Class A __dict__:", A.__dict__)
print("Class A has 'foo':", hasattr(A, 'foo'))
print("'foo' defined in A:", 'foo' in A.__dict__)

print()

# Check if 'foo' is defined in class B
print("Class B __dict__:", B.__dict__)
print("Class B has 'foo':", hasattr(B, 'foo'))
print("'foo' defined in B:", 'foo' in B.__dict__)

The output shows the difference between defined and inherited attributes ?

Class A __dict__: {'__module__': '__main__', 'foo': 1, '__qualname__': 'A'}
Class A has 'foo': True
'foo' defined in A: True

Class B __dict__: {'__module__': '__main__', '__qualname__': 'B'}
Class B has 'foo': True
'foo' defined in B: False

Key Differences

Practical Function

Here's a utility function to check attribute origin ?

def check_attribute_origin(cls, attr_name):
    if hasattr(cls, attr_name):
        if attr_name in cls.__dict__:
            return f"'{attr_name}' is defined in {cls.__name__}"
        else:
            return f"'{attr_name}' is inherited in {cls.__name__}"
    else:
        return f"'{attr_name}' not found in {cls.__name__}"

class A:
    foo = 1

class B(A):
    bar = 2

# Test the function
print(check_attribute_origin(A, 'foo'))
print(check_attribute_origin(B, 'foo'))
print(check_attribute_origin(B, 'bar'))
print(check_attribute_origin(B, 'xyz'))

'foo' is defined in A
'foo' is inherited in B
'bar' is defined in B
'xyz' not found in B

Conclusion

Use hasattr() to check if an attribute exists anywhere in the inheritance chain. Use attr_name in cls.__dict__ to check if an attribute is defined specifically in that class, not inherited.