Mailman 3 Support for sealed classes - Typing-sig

20 May 2021

      Hello typing-sig,

I wanted to suggest adding support for sealed classes to the python type
system.

What are sealed classes?

Sealed classes are classes of which all subclasses are known statically.
This is usually accomplished by requiring all subclasses to be created
in the same file/module as the parent class.

What languages support sealed classes?

Sealed classes are supported by C#, Kotlin and starting with Java 15
even by Java.

What can sealed classes be used for?

Sealed classes are useful for checking the exhaustiveness of if/else
towers and (python 3.10) match statements for example when working with
abstract syntax trees.
Take a look at this example:

```
class Node(ABC): ...
class Value(Node): ...
class BinOp(Node): ...
class UnOp(Node): ...

n: Node

match n:
    case Value(v): ...
    case BinOp(n1, op, n2): ...
    case UnOp(op, n1): ...
    # Can we statically check if all cases are handled?
```

Sealed classes in python:

My suggestion would be to add a new class "Sealed" to typing and
typing_extensions that can be inherited from:

class Node(Sealed): ...

Type checkers should make sure that no subclasses of Node are created in
modules other than the module Node was created in and should treat
sealed classes as abstract classes. Additionally they can keep track of
all the subclasses of Node do implement exhaustiveness checking.

The runtime implementation of sealed could also prevent non-typechecked
code from creating additional subclasses:

```
class Sealed(ABC):
    def __init_subclass__(cls):
        for c in cls.__mro__:
            if Sealed in c.__bases__ and c.__module__ != cls.__module__:
                raise RuntimeError('Sealed class can\'t be subclassed
from another module. Sealed in module "{}".'.format(c.__module__))
```

Final example:

a.py:

```
from typing import Sealed

class Base(Sealed):
    pass

class A(Base):
    pass

class B(A):
    pass
```

b.py:

```
from a import Base, A

class C(Base):  # raises an exception
    pass

class D(A):  # also raises an exception
    pass
```

Adrian Freund

Support for sealed classes

Adrian Freund

Brandt Bucher

Guido van Rossum

asafspades＠gmail.com

Arun Sharma

Arun Sharma

Arun Sharma

Arun Sharma

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