QUERY · ISSUE

Method Resolution Order (MRO) is not compliant

openby pfalconopened 2014-04-28updated 2024-08-28

py-core

uPy currently implements depth-first MRO, which is equivalent to one used pre-Python2.2.
References:

http://python-history.blogspot.com/2010/06/method-resolution-order.html
https://www.python.org/download/releases/2.3/mro/

CANDIDATE · PULL REQUEST

objtype: Don't treat inheritance from "object" as from native type.

closedby pfalconopened 2014-05-09updated 2014-08-09

"object" type in MicroPython currently doesn't implement any methods, and
hopefully, we'll try to stay like that for as long as possible. Even if we
have to add something eventually, look up from there might be handled in
adhoc manner, as last resort (that's not compliant with Python3 MRO, but
we're already non-compliant). Hence: 1) no need to spend type trying to
lookup anything in object; 2) no need to allocate subobject when explicitly
inheriting from object; 3) and having multiple bases inheriting from object
is not a case of incompatible multiple inheritance.

Assessment

Q Qwen · medium

Issue #525 reports that MicroPython uses depth-first MRO instead of C3 linearization (Python 2.3+ standard), which is a fundamental bug in the method resolution order implementation. PR #594 addresses an optimization related to inheritance from 'object' and mentions that the current MRO is non-compliant, but it does not implement C3 linearization. PR #594 focuses on avoiding unnecessary lookups when inheriting from 'object' and handling multiple bases inheriting from 'object', but it explicitly acknowledges the non-compliance rather than fixing it. The PR touches on MRO-related code (mp_obj_class_lookup) but does not resolve the core issue of implementing proper C3 linearization.

Suggested action

Keep both items open. Issue #525 requires a separate fix to implement C3 linearization for MRO. PR #594 is a related optimization but does not address the fundamental MRO compliance issue.

Method Resolution Order (MRO) is not compliant

objtype: Don't treat inheritance from "object" as from native type.

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