Comment by oivey

How does that work for partial forwarding? Back when I last checked there was no imagining a DRY solution for that even for linear cases, let alone diamond inheritance.

  # Simple case, any halfway-decent type system should be able to handle this.
  def inner(*, i): pass
  def middle(*, m, **kwargs): inner(**kwargs)
  def outer(*, o, **kwargs): middle(**kwargs)
  kwargs = ...; outer(**kwargs)

  # More complicated case, but fairly common in Python code.
  class A:
    def __init__(self, *, a):
      pass
  class B(A):
    def __init__(self, *, b, **kwargs):
      super().__init__(**kwargs)
  class C(A):
    def __init__(self, *, c, **kwargs):
      super().__init__(**kwargs)
  class D(B, C):
    def __init__(self, *, d, **kwargs):
      super().__init__(**kwargs)
  kwargs = ...; D(**kwargs)

  # An even more complicated case involves `kwargs.pop()`, forwarding without `**`.

Can the above be typechecked yet?

`interface` in TS allows extra keys, `type` does do not. Usually best to use `type` and add an intersection with some other type if you want extras (`Record<string,unknown>` is all right for arbitrary extra keys)

The current situation is worse, IMO. Lots of code that could be checked cannot, and lots of code that could have a more clearly defined interface does not.

Go is a terrible point of comparison. Python’s type system should not pointlessly match what other languages have chosen due to some myopic dogma.

Python is a dynamic language where one of its major features is structural subtyping, aka duck typing. It’s effectively an alternative to inheritance. Features have been added to help support this, like TypedDicts and Protocols already. They don’t go far enough.

I want to be able to say “this function argument is a dictionary that has at least the keys a, b, and c.” It gives the contract that the function only accesses those keys, and others will be ignored. The type checker can check that the function doesn’t access undeclared keys and the annotation helps communicate to client code what the interface is supposed to be.

Lots of Python’s bolted on type checking seems to be straight jacketed to match what’s in C, Java, Go, etc. Those languages don’t contain the only possible type systems or static checkers. There’s a serious lack of imagination. Python’s type system should be designed around enabling and making safer what the language is already good at.