isinstance() and issubclass() return conflicting results

Question

How do you explain isinstance(Hello,object) returns True whilst issubclass(Hello,object) returns False?

>>> class Hello:
    pass

and

>>> isinstance(Hello,object)
True
>>> issubclass(Hello,object)
False
>>> a = Hello()
>>> isinstance(a,object)
True

Asked By: Tarik

||

Source

Answer 1

It’s because you are using old-style classes so it doesn’t derive from object. Try this instead:

class Hello(object):
    pass

>>> issubclass(Hello,object)
True

Old-style classes are deprecated and you shouldn’t use them any more.

In Python 3.x all classes are new-style and writing (object) is no longer required.

Answered By: Mark Byers

Answer 2

The accepted answer is correct, but seems to miss an important point. The built-in functions isinstance and issubclass ask two different questions.

isinstance(object, classinfo) asks whether an object is an instance of a class (or a tuple of classes).

issubclass(class, classinfo) asks whether one class is a subclass of another class (or other classes).

In either method, classinfo can be a “class, type, or tuple of classes, types, and such tuples.”

Since classes are themselves objects, isinstance applies just fine. We can also ask whether a class is a subclass of another class. But, we shouldn’t necessarily expect the same answer from both questions.

class Foo(object):
    pass

class Bar(Foo):
    pass

issubclass(Bar, Foo)
#>True
isinstance(Bar, Foo)
#>False

Bar is a subclass of Foo, not an instance of it. Bar is an instance of type which is a subclass of object, therefore the class Bar is an instance of object.

isinstance(Bar, type)
#>True
issubclass(type, object)
#>True
isinstance(Bar, object)
#>True

Answered By: cbare

Answer 3

My answer pertains to Python 3.

To expand upon cbare’s answer, the code below was helpful for me.

>>> class X:
...     pass
...     
>>> class Y(X):
...     pass
...     
>>> x = X()
>>> y = Y()
>>> isinstance(x, X)  # is object x an instance of class X (or any subclass)?
True
>>> isinstance(x, Y)  # is object x an instance of class Y (or any subclass)?
False
>>> isinstance(y, X)  # is object y an instance of class X (or any subclass)?
True
>>> isinstance(y, Y)  # is object y an instance of class Y (or any subclass)?
True

>>> issubclass(X, X)  # is class X a subclass of X (including class X)?
True
>>> issubclass(X, Y)  # is class X a subclass of Y (including class Y)?
False
>>> issubclass(Y, X)  # is class Y a subclass of X (including class X)?
True
>>> issubclass(Y, Y)  # is class Y a subclass of Y (including class Y)?
True

>>> issubclass(type(x), X)  # is class of object x a subclass of X (including class X)?
True
>>> issubclass(type(x), Y)  # is class of object x a subclass of Y (including class Y)?
False
>>> issubclass(type(y), X)  # is class of object y a subclass of X (including class X)?
True
>>> issubclass(type(y), Y)  # is class of object y a subclass of Y (including class Y)?
True

>>> issubclass(x.__class__, X)  # is class of object x a subclass of X (including class X)?
True
>>> issubclass(x.__class__, Y)  # is class of object x a subclass of Y (including class Y)?
False
>>> issubclass(y.__class__, X)  # is class of object y a subclass of X (including class X)?
True
>>> issubclass(y.__class__, Y)  # is class of object y a subclass of Y (including class Y)?
True

We can see that isinstance(object, class) respects inheritance / subclasses correctly.

Answered By: kevinarpe

Answer 4

So basically isinstance(new_object, some_class) returns True if the object is an instantiated class of some_class. In other words, new_object is an some_class() object. issubclass checks if the object has a subclass of some_class.

Better explained with a simple example that makes perfect sense:

class some_class():
  pass

class new_object_class(some_class):
  pass

instance_object = new_object_class()

isinstance(instance_object, new_object_class) == True
issubclass(instance_object, some_class) == True

isinstance(new_object_class,some_class) == True
issubclass(new_object_class,some_class) == True

No need for false tests to explain this.

Answered By: bauderr

isinstance() and issubclass() return conflicting results

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