4.6. Match Constant

A constant value pattern works like the literal but for certain named constants. Note that it must be a qualified (dotted) name, given the possible ambiguity with a capture pattern. It looks like Color.RED and only matches values equal to the corresponding value. It never binds.

4.6.1. Problem

>>> from enum import Enum
>>>
>>>
>>> class Color(Enum):
...     RED = 'r'
...     GREEN = 'g'
...     BLUE = 'b'
>>>
>>> color = Color.RED
>>>
>>> if color is Color.RED:
...     print('red')
... elif color is Color.GREEN:
...     print('green')
... elif color is Color.BLUE:
...     print('blue')
...
red

4.6.2. Solution

>>> from enum import Enum
>>>
>>>
>>> class Color(Enum):
...     RED = 'r'
...     GREEN = 'g'
...     BLUE = 'b'
>>>
>>> color = Color.RED
>>>
>>> match color:
...     case Color.RED: print('red')
...     case Color.GREEN: print('green')
...     case Color.BLUE: print('blue')
...
red

4.6.3. Case Study

import requests


DATA = 'https://python3.info'
resp = requests.get(DATA)

if resp.status_code == 200:
    html = resp.text
elif resp.status_code == 301:
    raise ConnectionResetError('Redirect')
elif resp.status_code == 403:
    raise PermissionError('Forbidden')
else:
    raise ConnectionError('Invalid status code')


print(html[:110])
# <!DOCTYPE html>
# <html class="writer-html5" lang="en" data-content_root="./">
# <head>
#   <meta charset="utf-8" />

from enum import Enum
import requests


class HTTPStatus(Enum):
    OK = 200
    MOVED_PERMANENTLY = 301
    FORBIDDEN = 403


DATA = 'https://python3.info'
resp = requests.get(DATA)
status_code = HTTPStatus(resp.status_code)

if status_code is HTTPStatus.OK:
    html = resp.text
elif status_code is HTTPStatus.MOVED_PERMANENTLY:
    raise ConnectionResetError('Redirect')
elif status_code is HTTPStatus.FORBIDDEN:
    raise PermissionError('Forbidden')
else:
    raise ConnectionError('Invalid status code')


print(html[:110])
# <!DOCTYPE html>
# <html class="writer-html5" lang="en" data-content_root="./">
# <head>
#   <meta charset="utf-8" />

from enum import Enum
import requests


class HTTPStatus(Enum):
    OK = 200
    MOVED_PERMANENTLY = 301
    FORBIDDEN = 403


DATA = 'https://python3.info'
resp = requests.get(DATA)

match HTTPStatus(resp.status_code):
    case HTTPStatus.OK: html = resp.text
    case HTTPStatus.MOVED_PERMANENTLY: raise ConnectionResetError('Redirect')
    case HTTPStatus.FORBIDDEN: raise PermissionError('Forbidden')
    case _: raise ConnectionError('Invalid status code')


print(html[:110])
# <!DOCTYPE html>
# <html class="writer-html5" lang="en" data-content_root="./">
# <head>
#   <meta charset="utf-8" />

from http import HTTPStatus
import requests


DATA = 'https://python3.info'
resp = requests.get(DATA)

match HTTPStatus(resp.status_code):
    case HTTPStatus.OK: html = resp.text
    case HTTPStatus.MOVED_PERMANENTLY: raise ConnectionResetError('Redirect')
    case HTTPStatus.FORBIDDEN: raise PermissionError('Forbidden')
    case _: raise ConnectionError('Invalid status code')


print(html[:110])
# <!DOCTYPE html>
# <html class="writer-html5" lang="en" data-content_root="./">
# <head>
#   <meta charset="utf-8" />

4.6.4. Use Case - 1

SetUp:

>>> from enum import IntEnum, StrEnum
>>> import requests

Usage:

>>> class Color(StrEnum):
...     RED = '#FF0000'
...     GREEN = '#00FF00'
...     BLUE = '#0000FF'
>>>
>>>
>>> mycolor = '#FF0000'
>>>
>>> match mycolor:
...     case Color.RED:   print('red')
...     case Color.BLUE:  print('blue')
...     case Color.GREEN: print('green')
...
red

4.6.5. Use Case - 2

>>> class HTTPStatus(IntEnum):
...     OK = 200
...     REDIRECT = 300
...     SERVER_ERROR = 500
>>>
>>>
>>> resp = requests.get('https://python3.info')
>>>
>>> match resp.status_code:
...     case HTTPStatus.OK:                print('ok')
...     case HTTPStatus.MOVED_PERMANENTLY: print('redirect')
...     case HTTPStatus.SERVER_ERROR:      print('error')
...
ok

4.6.6. Use Case - 3

>>> from http import HTTPStatus
>>> import requests
>>>
>>>
>>> resp = requests.get('https://python3.info')
>>>
>>> match resp.status_code:
...     case HTTPStatus.OK:                print('ok')
...     case HTTPStatus.MOVED_PERMANENTLY: print('redirect')
...     case HTTPStatus.SERVER_ERROR:      print('error')
...
ok

4.6.7. Use Case - 4

Test Setup:

>>> class Keyboard:
...     def on_key_press(self): ...
>>>
>>> keyboard = Keyboard()

>>> class Game:
...     def quit(self): ...
...     def move_left(self): ...
...     def move_up(self): ...
...     def move_right(self): ...
...     def move_down(self): ...
>>>
>>> game = Game()

Use Case:

>>> from enum import Enum
>>>
>>>
>>> class Key(Enum):
...     ESC = 27
...     ARROW_LEFT = 37
...     ARROW_UP = 38
...     ARROW_RIGHT = 39
...     ARROW_DOWN = 40
>>>
>>> match keyboard.on_key_press():
...     case Key.ESC:          game.quit()
...     case Key.ARROW_LEFT:   game.move_left()
...     case Key.ARROW_UP:     game.move_up()
...     case Key.ARROW_RIGHT:  game.move_right()
...     case Key.ARROW_DOWN:   game.move_down()
...     case _: raise ValueError(f'Unrecognized key')
Traceback (most recent call last):
ValueError: Unrecognized key

4.6.8. Assignments