Error codes unique to basedmypy¶

Check that a decorated function isn’t dynamic [no-any-decorated]¶

From disallow_any_decorated

Ban use of `Any` [no-any-explicit]¶

From disallow_any_explicit

Ban any expression that contains `Any` [no-any-expr]¶

From disallow_any_expr

Don’t subclass `Any` [no-subclass-any]¶

From disallow_subclassing_any

Don’t use anything that contains `Any` [no-untyped-usage]¶

From no_untyped_usage

Check that type variables are used in accordance with their variance [unsafe-variance]¶

inT = TypeVar("inT", contravariant=True)
outT = TypeVar("outT", covariant=True)

class X(Generic[inT, outT]):
    def foo(self,
        t: outT  # This usage of this covariant type variable is unsafe as a return type.  [unsafe-variance]
    ) -> inT:  # This usage of this contravariant type variable is unsafe as a return type.  [unsafe-variance]
        pass

Unsupported usages of typeguards [typeguard-limitation]¶

Mypy does not yet support typeguarding a star argument:

def guard(x: object) -> x is int: ...

x: object
xs = x,
assert guard(*xs)  # Type guard on star argument is not yet supported  [typeguard-limitation]
reveal_type(x)  # object

Check that typeguard definitions are valid [typeguard-subtype]¶

def guard(x: str) -> x is int:  # A type-guard's type must be assignable to its parameter's type. (guard has type "int", parameter has type "str")  [typeguard-subtype]
    ...

Check for regex issues [regex]¶

re.compile("(foo).*)")  # error: unbalanced parenthesis at position 7  [regex]
m = re.match("(?P<name>\w+)", s)
assert m
m.group("nane")  # error: no such group: 'nane'  [regex]

Check that strings are useful [helpful-string]¶

Catching unintentional values in f-strings:

def make_id(i: int | None) -> str:
    return f"id-{i}"  # The string for "None" isn't helpful for a user-facing message  [helpful-string]

Check that casts are valid [bad-cast]¶

Casting between two non-overlapping types is often a mistake:

a: int
cast(str, a)  # Conversion of type "int" to type "str" may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to "object" first.  [bad-cast]

Check for bad usages of callabes and function types [callable-functiontype]¶

Because functions can create methods when accessed in certain ways, basedmypy has checks regarding their usages:

class A:
    a: "(int) -> object" = lambda i: i + 1  # Assigning a "FunctionType" on the class will become a "MethodType"  [callable-functiontype]

A().a(1)  # runtime TypeError: <lambda>() takes 1 positional argument but 2 were given

Check that callables are valid [possible-function]¶

Because a function can create methods when accessed in certain ways, basedmypy has lint rules regarding their validity as Callables:

c: "(int) -> object" = lambda i: i + 1
class A:
    a: "(int) -> object" = c  # This "CallableType" could be a "FunctionType", which when assigned via the class, would produce a "MethodType"  [possible-function]

A().a(1)  # runtime TypeError: <lambda>() takes 1 positional argument but 2 were given

A scenario isn’t being handled correctly by basedmypy itself¶

Sometimes, the implementation of a the type system is not completely sound and basedmypy needs to be updated.