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Phantom types for Python

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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


There are a few extras available that can be used to either enable a feature or install a compatible version of a third-party library.

Extra name Feature
[dateutil] Installs python-dateutil. Required for parsing strings with TZAware and TZNaive.
[phonenumbers] Installs phonenumbers. Required to use phantom.ext.phonenumbers.
[pydantic] Installs pydantic.
[hypothesis] Installs hypothesis.
[all] Installs all of the above.
$  python3 -m pip install phantom-types[all]


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.


... and so will this.

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

But this will yield a static type checking error.


To be clear, the reason the first example passes is not because the type checker somehow magically knows about our predicate, but because we provided the type checker with proof through the assert. All the type checker cares about is that runtime cannot continue executing past the assertion, unless the variable is a Name. If we move the calls around like in the example below, the type checker would give an error for the greet() call.

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

Runtime type checking

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

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.

Pydantic support

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"})):
    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 .[all,test]

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

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