Phantom types for Python will help you make illegal states unrepresentable and avoid shotgun parsing by enabling you to practice "Parse, don't validate".

Checkout the complete documentation on Read the Docs →

$  python3 -m pip install phantom-types

By introducing a phantom type we can define a pre-condition for a function argument.

from phantom import Phantom
from phantom.predicates.collection import contained


class Name(str, Phantom, predicate=contained({"Jane", "Joe"})):
    ...


def greet(name: Name):
    print(f"Hello {name}!")

Now this will be a valid call.

greet(Name.parse("Jane"))

... and so will this.

joe = "Joe"
assert isinstance(joe, Name)
greet(joe)

But this will yield a static type checking error.

greet("bird")

By combining phantom types with a runtime type-checker like beartype or typeguard, we can achieve the same level of security as you'd gain from using contracts.

import datetime
from beartype import beartype
from phantom.datetime import TZAware


@beartype
def soon(dt: TZAware) -> TZAware:
    return dt + datetime.timedelta(seconds=10)

The soon function will now validate that both its argument and return value is timezone aware, e.g. pre- and post conditions.

Phantom types are ready to use with pydantic and have integrated support out-of-the-box. Subclasses of Phantom work with both pydantic's validation and its schema generation.

class Name(str, Phantom, predicate=contained({"Jane", "Joe"})):
    @classmethod
    def __schema__(cls) -> Schema:
        return super().__schema__() | {
            "description": "Either Jane or Joe",
            "format": "custom-name",
        }


class Person(BaseModel):
    name: Name
    created: TZAware


print(json.dumps(Person.schema(), indent=2))

The code above outputs the following JSONSchema.

{
  "title": "Person",
  "type": "object",
  "properties": {
    "name": {
      "title": "Name",
      "description": "Either Jane or Joe",
      "format": "custom-name",
      "type": "string"
    },
    "created": {
      "title": "TZAware",
      "description": "A date-time with timezone data.",
      "type": "string",
      "format": "date-time"
    }
  },
  "required": ["name", "created"]
}

Install development requirements, preferably in a virtualenv:

$ python3 -m pip install .[test,pydantic,phonenumbers]

Run tests:

$ pytest
# or
$ make test

Linting and static type checking is setup with pre-commit, after installing it you can setup hooks with the following command, so that checks run before you push changes.

# configure hooks to run when pushing
$ pre-commit install -t pre-push
# or when committing
$ pre-commit install -t pre-commit
# run all checks
$ pre-commit run --all-files
# or just a single hook
$ pre-commit run mypy --all-files

In addition to static type checking, the project is setup with pytest-mypy-plugins to test that exposed mypy types work as expected, these checks will run together with the rest of the test suite, but you can single them out with the following command.

$ make test-typing