Protocol in Python

Protocols in Python

Introduced in PEP 544, in Python 3.8.

A way to formalize Duck Typing in python using Static Type Checkers

Example:

from typing import Protocol

class Printable(Protocol):
    def print_info(self) -> None:
		... # '...' indicates an abstract method that must be implemented

Any class that implements the print_info method automatically conforms to the Printable class. Unlike Abstract Base Classes(ABCs),

  • protocols are implicit, does not inherit a class
  • isinstance() does not work by default. Using the @runtime_checkable decorator enables this.
  • offers greater flexibility as you only care about the structure of the object, not its inheritance hierarchy
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