type projections¶

basedpython adds X[*] to represent the top materialization of a generic class. useful as a concise way to denote "any value of X, with safety guaranteed"

def f(data: list[*]):
    reveal_type(data[0])  # object
    data[0] = 1  # error

this can also be understood as list[out object]. the type parameters are taken to their bounds

semantics¶

the upper-bound materialization of T — every Any is replaced by the maximal concrete type allowed by its variance position. for an invariant generic like list[Any], the materialization stays as Top[list[Any]] (no single concrete type subsumes every list[T]). for covariant positions, Any collapses to object; for contravariant, to Never

reveal_type shows the original surface form in .by files:

def _(a: list[*]):
    reveal_type(a)               # revealed: list[*]

scope¶

X[*] is basedpython-only: a .py file using list[*] produces a parse error ("bare * in subscription is not valid in .py files")

polyfill¶

X[*, *, ...] lowers to ty_extensions.Top[X[Any, ..., Any]]. Top is a type-checker-only construct evaluated by ty