Inheritance and isinstance

Question

I just noticed that

isinstance(myob, MyClass)

does not only return True when myob is an instance of MyClass but also if myob is an instance of a class that inherits from MyClass.

To be more clear, consider the following:

class Book(object):
    def __init__(self, cover)
        self._cover = cover

class Novel(Book):
    def __init__(self, cover, category):
        Book.__init__(self, cover)
        self._category = category

When instanciating Novel as follows:

novel = Novel('hardcover', 'police')

then

print(isinstance(novel, Book))

and

print (isinstance(novel , Novel))

both print True.

Why is that so? In my sense, novel is a Novel instance, not a Book one...

Also, relevant to this :

In order to get the "grand-mother" (sic) class, I do:

print(novel.__class__.__bases__)

Is there a more direct way?

A novel is a specialization of a book, but it still is a book. — Nils Werner, Oct 24 '18 at 15:19
Because Novels *are* Books. That's the relationship that inheritance describes. — Carcigenicate, Oct 24 '18 at 15:19
Possible duplicate of [What are the differences between type() and isinstance()?](https://stackoverflow.com/questions/1549801/what-are-the-differences-between-type-and-isinstance) — Torxed, Oct 24 '18 at 15:26
This is OOP-standard. Isn't a novel still a book? Isn't a penguin still both a bird and an animal? — tobias_k, Oct 24 '18 at 15:31

timgeb · Accepted Answer · 2018-10-24T15:42:12.580

This transitive behavior is how it should work intuitively ...

>>> class Text:
...:    pass
...:
...:
>>> class Book(Text):
...:    pass
...:
...:
>>> class Novel(Book):
...:    pass
...:
...:
>>> n = Novel()
>>> isinstance(n, Novel)
>>> True
>>> isinstance(n, Book)
>>> True
>>> isinstance(n, Text)
>>> True

... because a Novel is-a Novel, but also is-a Book and is-a Text.

If you want to know whether a class (or instance of a class) is a direct ancestor of another class, you can use the __subclasses__ method of class objects.

>>> Text.__subclasses__()
>>> [__main__.Book]
>>> Book.__subclasses__()
>>> [__main__.Novel]
>>> Novel.__subclasses__()
>>> []
>>> 
>>> Novel in Text.__subclasses__()
>>> False
>>> type(n) in Text.__subclasses__()
>>> False
>>> Novel in Book.__subclasses__()
>>> True
>>> type(n) in Book.__subclasses__()
>>> True

edit: YourClass.__bases__ also gives you all direct parent classes.

>>> Novel.__bases__
>>> (__main__.Book,)

score 4 · Answer 2 · answered Oct 24 '18 at 15:31

Inheritance is "is a" relationship - if Duck inherit from Bird then obviously a duck (instance of Duck) is ALSO a bird (instance of Bird), so the behaviour you observe is really the expected one (and this holds for all class-based OOPLs).

If you want to check the exact type of an object, you can get it using type(obj) - which will return the class of object - and do an identity test against the desired class, ie:

obj = 42
print(type(obj) is int)

Inheritance and isinstance

2 Answers2