ALL-OVER0010 · inherited_attribute_hides_a_method

Count methods a subclass writes under a name an ancestor already binds to data.

This is a deterministic rule for all languages. Read its implementation.

Definition

Read every member a base declares beside the declaration the subclass writes for it, and report a name the base binds as data where the subclass writes a method. Python resolves an instance attribute before it resolves a method, so the assignment in the ancestor wins and the method below is never reached. Nothing about it looks wrong. The method is right there, it is covered by nothing, and every call goes to whatever the ancestor stored, which is usually None and raises somewhere far away.

Both halves are needed and they never sit together. The assignment is a line inside an ancestor’s initializer and the method is a definition in another file, so only a resolved inheritance chain puts them in the same sentence.

Evidence

Each finding names the subclass, the ancestor that bound the name to data, and the method it hides. The value is the number of hidden methods.

Exceptions

A name Python rewrites into the class that wrote it, spelled with two leading underscores and no trailing ones, is left alone, because the two spellings never collide at runtime.

Only the inherited shape is reported. A class that binds a name to data and defines a method under it inside that same class is a local defect a reader can see in one screen, so it belongs to a rule about one class rather than to this family, and Pylint reports both under one message. A binding a caller makes from outside the class is invisible to any static reader, which is the other half of what Pylint’s message describes.

Examples

Bad

class Task:
    def __init__(self):
        self.run = None


class ScheduledTask(Task):
    def run(self):
        return self.execute()

Good

class Task:
    def __init__(self):
        self.runner = None


class ScheduledTask(Task):
    def run(self):
        return self.execute()

References

• Generalizes Pylint E0202 method-hidden. Open reference
• Cites “The Python Language Reference”, the standard type hierarchy and attribute lookup. Open reference
• Cites “Fluent Python”, attribute handling and the shadowing it allows