type parameter separators¶

a type parameter list is written the way a value parameter list is, so the same / and bare * separators divide it into positional-only, positional-or-keyword, and keyword-only sections:

class C[A, /, B, *, D]: ...

A can only be given positionally, D only by name, and B either way. this mirrors how def f(a, /, b, *, d) restricts its value arguments.

enforcement¶

the restriction is checked at every specialization site — a subscript on the class, or an annotation naming it:

class C[A, /, B]: ...

C[A=int, B=str]   # error: `A` is positional-only
C[int, str]       # ok

class C[A, *, B]: ...

C[int, str]       # error: `B` is keyword-only
C[int, B=str]     # ok

a variadic is not a separator¶

a *Ts type variable tuple is a parameter, not a separator. unlike *args, it does not make what follows it keyword-only, because a type argument list resolves a variadic from both ends:

class C[A, *Ts, B]: ...

C[int, str, bytes, bool]   # A=int, Ts=(str, bytes), B=bool

rules¶

the separators follow the value-parameter rules:

at least one parameter must precede /
at least one parameter must follow *
/ must come before *
neither separator may appear twice

a violation is a syntax error.

scope¶

separators are a .by reading only. python's own type-parameter grammar has no separators, so a / in a .py type parameter list is a syntax error.

lowering¶

python type parameter lists cannot carry separators, so they are erased. the positional-only / keyword-only meaning is checked against the .by source, not the emitted python:

class C[A, /, B, *, D]: ...

transpiles to:

class C[A, B, D]: ...