py_annotation_switch

A hack for writing switch statements in type annotations for Python.

Why should I use this?

You most definitely should not use this in any real capacity, for any project, ever. It is fun as a proof-of-concept, however. This is done solely for entertainment purposes.

How do I use this?

Switch is the switch-case implementation, used as a context manager. Annotating the designated keyword (defaulting to case) defines cases.

Cases are in the form:

[keyword]: (*case identifier[s], (
  *statements to evaluate,
))

The return value of the case is what the last item in the statements tuple evaluates to.

Simplest case (ha):

from __future__ import annotations
from annotation_switch import __annotations__, Switch

switch_case = Switch(3)
with switch_case:
  case: (0, 1, 2, (
    print("Zero, One, or Two."),
    3 < 3
  ))
  case: (3, (
    print("Three."),
    3 == 3
  ))

print(switch_case.output)  # True

Case with default:

3 )) print(switch_case.output) # True ">

from __future__ import annotations
from annotation_switch import __annotations__, Switch, default
# Importing default is not necessary for the code to work as intended,
# but it's included here to quiet intellisense of common IDEs.

switch_case = Switch(5)
with switch_case:
  case: (0, 1, 2, (
    print("Zero, One, or Two."),
    5 < 3
  ))
  case: (3, (
    print("Three."),
    5 == 3
  ))
  case: ("default", (
    print("What comes after 3?"),
    5 > 3
  ))

print(switch_case.output)  # True

Case with error being raised when case is not resolved:

from __future__ import annotations
from annotation_switch import __annotations__, Switch, CONFIG

CONFIG.defaults_to_none = False

switch_case = Switch("pineapple")
with switch_case:
  case: ("apple", (
    print("fruit"),
    "fruit"
  ))
  case: ("pine", (
    print("tree"),
    "tree"
  ))

#  raises SwitchCaseNotValidError("pineapple")

Note: After the switch statement is exited at the resolution of the context manager, the Switch object is converted to an OutputWrapper holding only an output member. This is to encourage the user to not reuse the Switch object, which might have unintended effects.