maybe-missing

maybe-missing is a tiny typed helper for one annoyingly real distinction:

• None means “the caller explicitly sent null”
• MISSING means “the caller did not send this value at all”

That difference matters in patch APIs, dataclasses, settings layers, and anywhere a database column is nullable but an omitted field should mean “leave it alone”.

Why this exists

Python’s Optional[T] answers only one question: can the value be None?

It does not answer the other question: was there a value in the first place?

That becomes awkward fast:

• JSON null is a valid payload value
• a nullable Postgres column may need to be set to NULL intentionally
• omitting a field in a PATCH request should usually mean “don’t update it”
• a dataclass default should sometimes mean “missing”, not “defaulted to null”

So this package gives you exactly two public names:

• Maybe[T]
• MISSING

And then gets out of your way.

Installation

pip install maybe-missing

Usage

from maybe_missing import Maybe, MISSING


def update_display_name(display_name: Maybe[str | None] = MISSING) -> None:
    if display_name is MISSING:
        print("leave the existing value alone")
    elif display_name is None:
        print("explicitly store NULL")
    else:
        print(f"store {display_name!r}")

Dataclass example

from dataclasses import dataclass

from maybe_missing import Maybe, MISSING


@dataclass(slots=True)
class UserPatch:
    nickname: Maybe[str | None] = MISSING
    bio: Maybe[str | None] = MISSING


def apply_patch(patch: UserPatch) -> None:
    if patch.nickname is not MISSING:
        # update nickname to a string or to NULL
        ...

    if patch.bio is not MISSING:
        # update bio to a string or to NULL
        ...

API example

Imagine a request body for partial updates:

{
  "nickname": null
}

This should mean:

• nickname was provided
• its value is explicitly null
• the server should write NULL

While this body:

{}

should mean:

• nickname was not provided
• keep the current value as is

Maybe[T] helps model that cleanly in Python code.

There are only a couple of lines though

Yes.

There are only a couple of lines.

And that’s exactly why it’s more pleasant not to duplicate them across your projects and just do:

from maybe_missing import Maybe, MISSING

Isn’t it?

Typing

The package includes py.typed, so type checkers and IDEs can treat it as a typed distribution.

Compatibility

This package currently targets Python 3.10+.