How to Fix Marker Interface Errors

In this tutorial, you'll learn about How to Fix Marker Interface Errors. We cover key concepts, practical examples, and best practices.

Fix marker interface errors when capability checked via isinstance or hasattr scattered across code.

Quick Fix

Wrong

class Savable: pass
class Report(Savable):
    def save(self): print('Save')
def save_all(items):
    for item in items:
        if hasattr(item,'save'): item.save()  # duck typing, fragile

Checking hasattr() or isinstance() scattered. No explicit contract. Refactoring breaks silently.

Right

from abc import ABC
class Savable(ABC):
    pass
class Report(Savable):
    def save(self): print('Save')
def save_all(items):
    for item in items:
        if isinstance(item,Savable): item.save()
        else: raise TypeError(f'{type(item).__name__} not savable')

Marker interface explicitly marks capability. isinstance checks are centralized and clear.

Prevention

Marker interface marks class with capability. Empty interface as tag for type checking.

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