A codemod to make your implicit optional type hints PEP 484 compliant.

This tool will make changes to your code. Make sure you're using version control, like git.

pipx run no_implicit_optional <path>

Alternatively, and perhaps more familiarly:

pip install no_implicit_optional
no_implicit_optional <path>

To make this tool use PEP 604 X | None syntax instead of Optional[X], use the --use-union-or flag. Note that this syntax is only fully supported on Python 3.10 and newer.

By default, mypy 0.982 and earlier allowed eliding Optional for arguments with default values of None. From experience, this was found to be a source of confusion and bugs.

In 2018, PEP 484 was updated to require the explicit use of Optional (or a Union with None) and mypy enforces this when run using mypy --strict or mypy --no-implicit-optional. Similarly, other type checkers like pyright do not recognise implicit Optional at all.

Here's what this looks like in practice:

def bad(x: int = None):
    ...
def good(x: Optional[int] = None):
    ...
def good(x: Union[int, None] = None):
    ...
# PEP 604 syntax, requires Python 3.10+ or `from __future__ import annotations`
def good(x: int | None = None):
    ...

Anyway, mypy is changing its default to match PEP 484 and disallow implicit Optional. In order to make the transition easier, this tool will try to automatically fix your code, building off of libcst to do so.

Also refer to:

• https://peps.python.org/pep-0484/#union-types
• python/mypy#9091
• python/mypy#13401

Use mypy --implicit-optional or set implicit_optional = True in your mypy config.