Protocol
- Python course
- Intermediate
- Lesson 50/116
- 5 min
Learn structural typing with typing.Protocol.
What is Protocol for?
Protocol provides structural typing. Even without inheriting from a Protocol, a class is considered conforming if it has the right methods. It is imported from the typing module.
Example: class Drawable(Protocol):\n def draw(self) -> str: .... Any class that provides draw is considered a Drawable; it does not need to inherit. With @runtime_checkable you can also enable isinstance checks.
@runtime_checkable
To check a protocol with isinstance you use the @runtime_checkable decorator. It decides conformance by looking only at method signatures on the protocol.
Example: @runtime_checkable\nclass Speaker(Protocol):\n def speak(self) -> str: .... The call isinstance(obj, Speaker) returns True if obj has a speak method. This is the most practical way to check conformance without an inheritance relationship.
Practice Drawable protocol and Circle
Start with from typing import Protocol, runtime_checkable. Define @runtime_checkable and class Drawable(Protocol):; inside, write def draw(self) -> str: .... Then define class Circle: (no inheritance); in the body write def draw(self) -> str: and return "circle". Finally create c = Circle() and print conformance with print(isinstance(c, Drawable)).
Starter code
# Write the Drawable protocol and the Circle classSample solution
# Write the Drawable protocol and the Circle class
from typing import Protocol, runtime_checkable
@runtime_checkable
class Drawable(Protocol):
def draw(self) -> str: ...
class Circle:
def draw(self) -> str:
return "circle"
c = Circle()
print(isinstance(c, Drawable))Expected output
True
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the Protocol parts
| Term | Meaning |
|---|---|
Protocol |
Base class |
typing |
Module |
@runtime_checkable |
isinstance permission |
def draw(self) |
Expected method |
2 What is needed to conform to a Protocol?
- Having the right methods Correct answer
- Filling abstract methods
- Inheriting ABC
- Inheritance
Explanation: Protocol accepts having the right methods; no inheritance required.
3 Protocol import line
from typing import Protocol, ____
@runtime_checkable
class Drawable(Protocol):
def draw(self) -> str: ...from typing import Protocol, runtime_checkable
@runtime_checkable
class Drawable(Protocol):
def draw(self) -> str: ...Explanation: runtime_checkable is imported from the typing module.