Inspect how Python searches multiple base classes for an attribute.
What You Will Learn
Python uses a consistent method resolution order (MRO), not a simple depth-first left-to-right walk. In a cooperative hierarchy, each implementation can call super() so the next implementation in the MRO participates. You can inspect Class.__mro__ to debug unexpected behavior.
Class interfaces should preserve their stated invariants. Type annotations describe intent but do not enforce runtime types on their own.
Step-by-Step Example
The example below makes the behavior visible with small input values that you can change safely.
class Base:
def label(self):
return "base"
class Left(Base):
def label(self):
return "left -> " + super().label()
class Right(Base):
def label(self):
return "right -> " + super().label()
class Combined(Left, Right):
pass
print(Combined().label())
print([item.__name__ for item in Combined.__mro__])How the Example Works
- The definitions Base, Left, Right separate the main steps into named components; calls to those components drive the example.
- The print() calls make the return values or state visible so you can compare them with the explanation and check your prediction.
Expected output from the example:
left -> right -> base
['Combined', 'Left', 'Right', 'Base', 'object']When You Would Use This
A class becomes useful when several operations must maintain rules around related data. Think about which state belongs to each instance, which operations should be public, and how errors should be reported. Avoid making the design more complex than the underlying task.
Practice: Modify the Code
Swap the order of Left and Right in Combined and predict the new MRO.
Verify the changed output against the behavior described above, including the relevant invalid or boundary case.
For complete parameters and version details, consult the official Python documentation.
Common Beginner Mistakes
- Assuming super() always means call the method on the direct parent by name.
- Calling two parents explicitly in a diamond hierarchy and accidentally initializing shared ancestors twice.
Key Takeaways
- Inspect how Python searches multiple base classes for an attribute.
- Python uses a consistent method resolution order (MRO), not a simple depth-first left-to-right walk.
- Apply this operation only after checking the assumptions and input conditions shown in the example.