keyword arguments in subscripts¶

basedpython allows keyword arguments inside subscriptions:

result = x[a, z=1]

transpiles to:

result = x.__getitem__(a, z=1)

python's subscript grammar doesn't accept keyword args (PEP 637 was rejected), so basedpython lowers the call to the explicit __getitem__ method. positional and keyword args are forwarded in source order

type subscripts¶

when the value is a known generic class, kw subscripts lower to a positional type subscript instead of a __getitem__ call. unbound typevars fall back to their declared defaults:

class A[T = int, R = str]: ...

a: A[T=bool]    # → a: A[bool, str]
b: A[R=int]     # → b: A[int, int]
c: A[R=int, T=bool]    # → c: A[bool, int]

ty's type-checking sees the reordered positional form, so type errors point at the declared typevar order

single-arg form¶

A[T=int] (no surrounding tuple) is also accepted for single- and multi-typevar classes. For multi-typevar classes the same defaults rule applies; for single-typevar classes the keyword name is dropped:

class B[T]: ...
b: B[T=int]     # → b: B[int]

scope¶

the rewrite fires for any subscription containing at least one keyword binding. all-positional subscripts are untouched

see also¶

• type parameter separators — / and * restrict which type parameters may be given by name
• tuple member access (expr.N) — dot-indexing companion form

inlay hints¶

a positional type argument of a generic with more than one type parameter gets that parameter's name as an inlay hint, in the keyword form the subscript would take:

class Cache[Key, Value]: ...

def f(c: Cache[⟨Key=⟩str, ⟨Value=⟩int]) -> None: ...

a subscript that already binds by keyword is left alone, as is a single-typevar generic (whose keyword is dropped anyway) and a variadic one

a specialization a hint renders rather than annotates is named the same way, so both sides of an assignment read alike:

class Pair[Key, Value]:
    init(key: Key, value: Value)

a⟨: Pair[Key=int, Value=str]⟩ = Pair⟨[Key=1, Value="x"]⟩(⟨key=⟩1, ⟨value=⟩"x")

this is .by only — python's subscript grammar has no keyword form, so a hint there would spell something the file cannot be written as