How can I work out the class hierarchy given an object instance in Python?

Question

Is there anyway to discover the base class of a class in Python?

For given the following class definitions:

class A:
   def speak(self):
      print "Hi"

class B(A):
   def getName(self):
      return "Bob"

If I received an instance of an object I can easily work out that it is a B by doing the following:

instance = B()
print B.__class__.__name__

Which prints the class name 'B' as expected.

Is there anyway to discover that the instance of an object inherits from a base class as well as the actual class?

Or is that just not how objects in Python work?

Answer 1

b = B()
b.__class__
b.__class__.__base__
b.__class__.__bases__
b.__class__.__base__.__subclasses__()

I strongly recommend checking out ipython and use the tab completion :-)

Answer 2

If instead of discovery of the baseclass (i.e reflection) you know the desired class in advance you could use the following built in functions

eg:

# With classes from original Question defined
>>> instance = A()
>>> B_instance = B()
>>> isinstance(instance, A)
True
>>> isinstance(instance, B)
False
>>> isinstance(B_instance, A) # Note it returns true if instance is a subclass
True
>>> isinstance(B_instance, B)
True
>>> issubclass(B, A)
True

isinstance( object, classinfo) Return true if the object argument is an instance of the classinfo argument, or of a (direct or indirect) subclass thereof. Also return true if classinfo is a type object and object is an object of that type. If object is not a class instance or an object of the given type, the function always returns false. If classinfo is neither a class object nor a type object, it may be a tuple of class or type objects, or may recursively contain other such tuples (other sequence types are not accepted). If classinfo is not a class, type, or tuple of classes, types, and such tuples, a TypeError exception is raised. Changed in version 2.2: Support for a tuple of type information was added.

issubclass( class, classinfo) Return true if class is a subclass (direct or indirect) of classinfo. A class is considered a subclass of itself. classinfo may be a tuple of class objects, in which case every entry in classinfo will be checked. In any other case, a TypeError exception is raised. Changed in version 2.3: Support for a tuple of type information was added.

Answer 3

The inspect module is really powerful also:

>>> import inspect

>>> inst = B()
>>> inspect.getmro(inst.__class__)
(<class __main__.B at 0x012B42A0>, <class __main__.A at 0x012B4210>)

Answer 4

Another way to get the class hierarchy is to access the mro attribute:

class A(object):
    pass
class B(A):
    pass
instance = B()
print(instance.__class__.__mro__)
# (<class '__main__.B'>, <class '__main__.A'>, <type 'object'>)

Note that in Python 2.x, you must use "new-style" objects to ensure they have the mro attribute. You do this by declaring

class A(object):

instead of

class A():

See http://www.python.org/doc/newstyle/ and http://www.python.org/download/releases/2.3/mro/ for more info new-style objects and mro (method resolution order).

In Python 3.x, all objects are new-style objects, so you can use mro and simply declare objects this way:

class A():