sealed classes¶

a sealed class declares a closed hierarchy: it may be subclassed freely from anywhere within its own workspace, but not from a dependency. the type checker knows the full set of subclasses, and every sealed class exposes a __sealed_members__ tuple of those subclasses at runtime

sealed is a class modifier, written ahead of class like final or abstract:

sealed class Shape
class Circle(Shape)
class Square(Shape)

transpiles to:

class Shape: ...
class Circle(Shape): ...
class Square(Shape): ...
Shape.__sealed_members__ = (Circle, Square)

__sealed_members__¶

each sealed class gains a __sealed_members__ attribute: a tuple of its same-module direct subclasses, in source order. the transpiler materializes the tuple at runtime by emitting an assignment after the last subclass, as above

sealed class Shape
class Circle(Shape)
class Square(Shape)

reveal_type(Shape.__sealed_members__)  # (type[Circle], type[Square])

only direct, same-module subclasses are members — a subclass of a subclass, or a subclass defined in another module, is not listed. this matches what the runtime transform can see (it only ever sees one file)

workspace boundary¶

a sealed class may be subclassed from anywhere in the workspace that defines it, including a different module:

# shapes.by
sealed class Shape

# circle.by
from shapes import Shape

class Circle(Shape): ...  # fine — same workspace

subclassing a sealed class that lives in a dependency is an error, since the dependency owns the closed hierarchy:

from some_dependency import Shape

class Circle(Shape): ...  # error: [subclass-of-sealed-class]

transpilation¶

sealed is a forward-only transform: the modifier keyword is stripped and the __sealed_members__ assignment is inserted. there is no reverse transform, so round-tripping python back to basedpython does not recover the sealed keyword