@override Decorator

You're refactoring a logging system to add file-specific functionality. You have a Logger base class and a FileLogger subclass. You need to override the log() method to handle file-specific logging logic. However, in your haste, you make a typo, naming the method log_to_file() instead of log().

Without any checks, your code will run, but the base class's log() method will be called, not your intended override. This can lead to subtle, hard-to-debug bugs, as the parent method may not have the file-writing logic you've implemented.

class Logger:
    def log(self, message: str) -> None:
        print(f"LOG: {message}")

class FileLogger(Logger):
    # OOPS! Typo here: method name doesn't match base class
    def log_to_file(self, message: str, filename: str) -> None:
        with open(filename, 'a') as f:
            f.write(f"FILE LOG: {message}\\n")

# The user's code expects FileLogger.log to be called, but it won't be!
logger = FileLogger()
logger.log("System started.")
# This will print "LOG: System started."
# NOT write to a file as intended!

The @override decorator from the typing module is a simple yet powerful tool for preventing this exact problem. It explicitly marks a method as an intended override of a parent class method.

If you use @override and the method doesn't actually exist in any of the parent classes, type checkers will raise an error. This turns a silent, insidious bug into an error that you can fix right away. By flagging any misspellings or non-existent overrides, it encourages safer object-oriented programming (OOP) practices by reducing accidental bugs.

Here is a fixed version of that same code:

from typing import override


class Logger:
    def log(self, message: str) -> None:
        print(f"LOG: {message}")


class FileLogger(Logger):
    @override  # This will catch the typo!
    def log_to_file(self, message: str, filename: str) -> None:  # Error: no matching method in parent
        with open(filename, 'a') as f:
            f.write(f"FILE LOG: {message}\\n")

Any popular type checker would help notice this and tell you about the error:

mypy - Most widely used Python type checker
pyright/Pylance - Microsoft's type checker (default in VS Code)
Pyre - Meta's type checker
pytype - Google's type checker

The errors would look like this:

# mypy error: Method "log_to_file" marked with @override does not override anything [misc] 
# pyright/Pylance Method "log_to_file" overrides nothing 
# Pyre Invalid override [40]: `log_to_file` is not defined in base classes of `FileLogger`

Any modern Python development setup with type checking enabled will catch these @override mistakes before your code ever runs, turning silent bugs into immediate, fixable errors.