Based Static Typing for Python

Basedmypy is a Python type checker that is built on top of the work done by the mypy project. It adds based functionality and breaks compatibility with the cringe parts of pep 484.

Based features include:

• Typesafe by default (optional and dynamic typing still supported)
• Baseline functionality
• Support for Intersection types
• Default return type of None instead of Any
• Generic TypeVar bounds
• Based type-guards
• Infer parameter type from default value
• Infer overload types
• Bare literals
• Tuple literal types

See the features for more information, or the docs for a comprehensive list.

BasedPyright

Also, take a look at BasedPyright, a based type checker based on Pyright!

Usage

Installation

Basedmypy can be installed using pip from PyPI or from this GitHub repo:

python -m pip install -U basedmypy

Running

Basedmypy is installed as an alternative to, and in place of, the mypy installation:

mypy test.py

python -m mypy test.py

Integrations

If you are using IntelliJ IDEA/PyCharm, we recommend the basedtyping plugin

If you are using vscode, we recommend the mypy extension

Features

Have you ever tried to use Python's type system and thought to yourself "This doesn't seem based"?

Well fret no longer, as basedmypy has got you covered!

You can find a comprehensive list in the docs.

Baseline

Basedmypy has baseline, baseline is based! It allows you to adopt new strictness or features without the burden of refactoring and fixing every new error, just save all current errors to the baseline file and resolve them at what ever pace you want. Only new code will report new errors.

Read more and see examples in the docs

Intersection Types

Using the & operator or basedtyping.Intersection you can denote intersection types:

class Growable(ABC, Generic[T]):
    @abstractmethod
    def add(self, item: T): ...


class Resettable(ABC):
    @abstractmethod
    def reset(self): ...


def f(x: Resettable & Growable[str]):
    x.reset()
    x.add("first")

Type Joins

Mypy joins types to their common base type:

a: int
b: str
reveal_type(a if bool() else b)  # Revealed type is "builtins.object"

Basedmypy joins types into unions instead:

a: int
b: str
reveal_type(a if bool() else b)  # Revealed type is "int | str"

Bare Literals

Literal is so cumbersome! just use a bare literal instead.

class Color(Enum):
    RED = auto()


a: 1 | 2
b: True | Color.RED

Default Return Type

The default return type of functions is None instead of Any: (configurable with the default_return option.)

def f(name: str):
    print(f"Hello, {name}!")


reveal_type(f)  # (str) -> None

Generic TypeVar Bounds

Basedmpy allows the bounds of TypeVars to be generic.

So you are able to have functions with polymorphic generic parameters:

E = TypeVar("E")
I = TypeVar("I", bound=Iterable[E])


def foo(i: I, e: E) -> I:
    assert e not in i
    return i


reveal_type(foo(["based"], "mypy"))  # N: Revealed type is "list[str]"
reveal_type(foo({1, 2}, 3))  # N: Revealed type is "set[int]"

Based type-guards

Type-guards have been re-designed from the ground up:

# The target parameter of the typeguard can be specified
def guard(name: str, x: object) -> x is int: ...

# impossible type-guards show an error
def bad(x: str) -> x is int: ...  # error: A type-guard's type must be assignable to its parameter's type. (guard has type "int", parameter has type "str")

class A: ...
class B: ...
def is_b(x: object) -> x is B: ...

x = A()
assert is_b(x)
# type-guards narrow instead of resetting the type
reveal_type(x)  # A & B

# type-guards work on instance parameters
class Foo:
    def guard(self) -> self is int: ...

f = Foo()
assert f.guard()
reveal_type(f)  # Foo & int

Overload Implementation Inference

The types in overload implementations (including properties) can be inferred:

@overload
def f(a: int) -> str: ...


@overload
def f(a: str) -> int: ...


def f(a):
    reveal_type(a)  # int | str
    return None  # error: expected str | int


class A:
    @property
    def foo(self) -> int: ...

    @foo.setter
    def foo(self, value): ...  # no need for annotations

Infer Function Parameters

Infer the type of a function parameter from its default value:

def f(a=1, b=True):
    reveal_type((a, b))  # (int, bool)

Tuple Literal Types

Basedmypy allows denotation of tuple types with tuple literals:

a: (int, str) = (1, "a")

Types in Messages

Basedmypy makes significant changes to error and info messages, consider:

T = TypeVar("T", bound=int)


def f(a: T, b: list[str | 1 | 2]) -> Never:
    reveal_type((a, b))


reveal_type(f)

Mypy shows:

Revealed type is "Tuple[T`-1, Union[builtins.str, Literal[1], Literal[2]]]"
Revealed type is "def [T <: builtins.int] (a: T`-1, b: Union[builtins.str, Literal[1], Literal[2]]) -> <nothing>"

Basedmypy shows:

Revealed type is "(T@f, str | 1 | 2)"
Revealed type is "def [T: int] (a: T, b: str | 1 | 2) -> Never"

Got a question or found a bug?

Feel free to start a discussion or raise an issue, we're happy to respond:

• basedmypy tracker for basedmypy issues
• basedtypeshed tracker for issues with specific modules
• basedtyping tracker for issues with the 'basedtyping' package (runtime functionality).