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Dependent types for Python

Project description

dependent-types

Dependent types for Python.

Installation

python3 -m pip install dependent-types

Abstract

  • Make illegal states unrepresentable.
  • Parse, don't validate
  • Abuse __instancecheck__ and type-guards.
  • Values are checked at runtime but no extra instances/subclasses are instantiated.

Usage

Builtin types

dept.datetime

  • TZAware
  • TZNaive

dept.numeric

  • Natural
  • NegativeInt
  • Portion

dept.re

  • Match

dept.sized

  • NonEmpty
  • Empty

Creating dependent types

To create new dependent types, subclass dept.base.Dependent and define an __instancecheck__ method:

from typing import Any
from typing import TYPE_CHECKING

from dept.base import Dependent


class Greeting(str, Dependent):
    @classmethod
    def __instancecheck__(cls, instance: Any) -> bool:
        return (
            isinstance(instance, str)
            and instance.startswith(("Hello", "Hi"))
        )


hello = "Hello there"
# We can narrow types using mypy's type guards
assert isinstance(hello, Greeting)
# or explicitly when we need to
hi = Greeting.from_instance("Hi there")

# The runtime types are unchanged and will still be str for our greetings
assert type(hello) is str
assert type(hi) is str

# But their static types will be Greeting, retaining the information that our
# strings are not just any strs
if TYPE_CHECKING:
    reveal_type(hello)
    reveal_type(hi)

# As this string doesn't fulfill our __instancecheck__, it will not be an
# instance of Greeting.
assert not isinstance("Goodbye", Greeting)

Checkout out the dacite example for how to create dataclasses with rich dependently typed fields without duplicating type definitions or losing parsed information.

Project details


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