method resolution order (MRO)

The method resolution order (MRO) is the order that Python follows to look up attributes and methods in a class hierarchy. It determines which method or attribute to use when names collide in multiple inheritance scenarios.

Python uses the C3 linearization algorithm to define this ordering.

Example

Say you have the following hyerarchy of classes and you need to know the MRO for class D:

Python 
>>> class A:
...     def speak(self):
...         return "A"
...

>>> class B(A):
...     pass
...

>>> class C(A):
...     def speak(self):
...         return "C"
...

>>> class D(B, C):
...     pass
...

>>> D.__mro__
(
    <class '__main__.D'>,
    <class '__main__.B'>,
    <class '__main__.C'>,
    <class '__main__.A'>,
    <class 'object'>
)

>>> D.mro()
[
    <class '__main__.D'>,
    <class '__main__.B'>,
    <class '__main__.C'>,
    <class '__main__.A'>,
    <class 'object'>
]

You can view a class’s MRO using either the .__mro__ attribute or the .mro() class method. You have to read the sequence from left to right—top to button in this example.

Python searches each class in that order until it finds the requested attribute or method. If the attribute or method doesn’t exist, then you get an AttributeError exception.

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For additional information on related topics, take a look at the following resources:

By Dan Bader and Leodanis Pozo Ramos • Updated April 23, 2025 • Reviewed by Leodanis Pozo Ramos
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