How to get the parents of a Python class?

Question

How can I get the parent class(es) of a Python class?

Footlight

Answer 1

Use the following attribute:

cls.__bases__

From the docs:

The tuple of base classes of a class object.

Example:

>>> str.__bases__
(<class 'object'>,)

Another example:

>>> class A(object):
...   pass
... 
>>> class B(object):
...   pass
... 
>>> class C(A, B):
...   pass
... 
>>> C.__bases__
(<class '__main__.A'>, <class '__main__.B'>)

Jacklyn

To get bases of an instantiated object do type(C()).__bases__ as mentioned further below – Indusium

Answer 2

If you want all the ancestors rather than just the immediate ones, use cls.__mro__.

For versions of Python earlier than 3.5, use inspect.getmro:

import inspect
print inspect.getmro(cls)

Usefully, this gives you all ancestor classes in the "method resolution order" -- i.e. the order in which the ancestors will be checked when resolving a method (or, actually, any other attribute -- methods and other attributes live in the same namespace in Python, after all;-).

Hardwick

You can also just use cls.__mro__ (at least in Python 3.5) – Jodoin

@naught101, plz turn it into a full answer. I almost missed it, and so will, I think lots of other people. – Wallenstein

cls.mro() also works – Archiearchiepiscopacy

Answer 3

The fastest way to get all parents, and in order, is to just use the __mro__ built-in.

For instance, repr(YOUR_CLASS.__mro__).

The following:

import getpass
getpass.GetPassWarning.__mro__

...outputs, in order:

(<class 'getpass.GetPassWarning'>, <type 'exceptions.UserWarning'>, <type 'exceptions.Warning'>, <type 'exceptions.Exception'>, <type 'exceptions.BaseException'>, <type 'object'>)

There you have it. The "best" answer may have more votes but this is so much simpler than some convoluted for loop, looking into __bases__ one class at a time, not to mention when a class extends two or more parent classes. Importing and using inspect just clouds the scope unnecessarily.

Selfdenial

@John Smith stackoverflow.com/users/139885/john-smith, I hope you see this answer. If you like it, please let me know with an upvote! – Selfdenial

In fact, inspect.getmro just calls __mro__ on object, as you can see in github.com/python/cpython/blob/… . Using getmro produces cleaner and more readable code. Though skipping a function call is indeed faster. – Pteridophyte

How can I test if a parent class of an instance is the one I'm expecting? I've tried isinstance(pre_switch.__class__.__mro__[1], nengo.base.Process) but it returns False. – Tortuga

I'm finding this helpful with climbing the hierarchy of my Sqlalchemy joined table inheritance orm models. Super simple and exactly what I was looking for. – Grodno

__mro__ also contains the class itself, such that A in A.__mro__ is True, the same does not hold for __bases__, so take that in mind if you want to check whether a class is a strict subclass of another. – Polynomial

Answer 4

New-style classes have an mro method you can call which returns a list of parent classes in method resolution order.

Lochia

What counts as a new-style class? It seems I can use this with Django models, but anything simply inheriting from object doesn't seem to respond to mro. – Ong

considering an object x, we can get the method resolution order with the call type(x).mro() we can consider if x has ClassX as a base class with: ClassX in type(x).mro() – Sacrarium

Answer 5

Use bases if you just want to get the parents, use __mro__ (as pointed out by @naught101) for getting the method resolution order (so to know in which order the init's were executed).

Bases (and first getting the class for an existing object):

>>> some_object = "some_text"
>>> some_object.__class__.__bases__
(object,)

For mro in recent Python versions:

>>> some_object = "some_text"
>>> some_object.__class__.__mro__
(str, object)

Obviously, when you already have a class definition, you can just call __mro__ on that directly:

>>> class A(): pass
>>> A.__mro__
(__main__.A, object)

Calvo

Answer 6

If you want to ensure they all get called, use super at all levels.

Linguistics

Once you use super you have to use it in all levels anyway, which is why you should document it's use explicitly. Also you might want to know that super doesn't work on every class... – Lochia

Answer 7

If you have a variable and want to get its class and parent classes use type() method which will give class for a variable

val="happy coding"
print(type(val).__mro__)

Output:

(<class 'str'>, <class 'object'>)

Decima

Answer 8

This funciton will print the all the classes of an object, while in each step the next object will the left most parent.

def print_root_left(class_):
    while True:
      print(class_)
      # Check there if are no bases then we have reached the root class
      if not class_.__bases__:
        break
      class_=class_.__bases__[0] # use the left most parent


example = "hello" 
print_root_left(example.__class__)

Fulmis

Answer 9

def class_hierarchy(class_name):
    import re
    ancestor = successor = class_name
    level = 0
    print ("The", successor, "has the next parents:\n")
    while True:
        try: 
            ancestor = re.search(r"'(.*)'", str(successor.__bases__[0]))
            print (level, ancestor.group(1))
            successor = eval(ancestor.group(1))
            level+=1
        except: 
            break
#example        
class_hierarchy(IndexError)   

Owe