CodeQL documentation

Inconsistent method resolution order

ID: py/inconsistent-mro
Kind: problem
Severity: error
Precision: very-high
Tags:
   - reliability
   - correctness
Query suites:
   - python-security-and-quality.qls

Click to see the query in the CodeQL repository

Python 2.3 introduced new-style classes (classes inheriting from object). New-style classes use the C3 linearization method to determine a method resolution ordering (MRO) for each class. The C3 linearization method ensures that for a class C, if a class C1 precedes a class C2 in the MRO of C then C1 should also precede C2 in the MRO of all subclasses of C. It is possible to create a situation where it is impossible to achieve this consistency and this will guarantee that a TypeError will be raised at runtime.

Recommendation

Use a class hierarchy that is not ambiguous.

Example

The MRO of class X is just X, object. The program will fail when the MRO of class Y needs to be calculated because object precedes X in the definition of Y but the opposite is true in the MRO of X.

class X(object):
    def __init__(self):
        print("X")
class Y(object,X):
    def __init__(self):
        print("Y")