type mappings¶

a type parameter written with in ranges over a type mapping — the set of types it is allowed to be — instead of sitting beneath an upper bound:

def f[T in (int, str)](x: T) -> T: ...

transpiles to:

def f[T: (int, str)](x: T) -> T: ...

python has only the one spelling: after a :, a tuple means constraints. that leaves no way to write a tuple type as a bound, so basedpython gives the constraint set its own keyword and lets : keep meaning what it says

syntax¶

def f[T in (int, str)](x: T) -> T: ...

class Container[T in (int, str, bytes)]: ...

type Alias[T in (int, str)] = list[T]

in follows the parameter's name, so it composes with the modifiers written ahead of it — variance, reified:

class Container[in out T in (int, str)]: ...

semantics¶

T in (int, str) declares a constrained typevar — T can only be specialized to exactly int or str, never a subtype and never a union:

def f[T in (int, str)](x: T) -> T:
    return x

reveal_type(f(1))    # int
reveal_type(f("a"))  # str

T: (int, str) is something else entirely: a typevar whose upper bound is the type tuple[int, str]

# a mapping: T is int or str
def constrained[T in (int, str)](x: T) -> T: ...

# a bound: T must be a subtype of tuple[int, str]
def bounded[T: (int, str)](x: T) -> T: ...

at least two members¶

a mapping of one leaves nothing to choose between, and python rejects it too:

# error: [invalid-type-variable-constraints] TypeVar must have at least two constrained types
def f[T in (int,)](): ...

`in` is not a range¶

a mapping is an unordered set, so it has no top to hang a bound range from. the two forms are alternatives, not combinable

polyfill¶

on targets below python 3.12 the mapping becomes a legacy TypeVar with positional constraint arguments:

# generated for python < 3.12
from typing import TypeVar
_T = TypeVar("_T", int, str)
def f(x: _T) -> _T: ...

on 3.12+ the mapping keeps pep 695 syntax and becomes python's constraint tuple:

# generated for python 3.12+
def f[T: (int, str)](x: T) -> T: ...

that rewrite is the pep 695 polyfill at work, not this feature