If you run a type checker like ty or pyright they're not decorative — you'll get clear diagnostics for that particular example [1], and any other type errors you might have. You can set up CI so that e.g. blocks PRs from being merged, just like any other test failure.

If you mean types not being checked at runtime, the consensus is that most users don't want to pay the cost of the checks every time the program is run. It's more cost-effective to do those checks at development/test/CI time using a type checker, as described above. But if you _do_ want that, you can opt in to that using something like beartype [2].

[1] https://play.ty.dev/905db656-e271-4a3a-b27d-18a4dd45f5da

[2] https://github.com/beartype/beartype/

    int x = "thing"

No type system will allow for the dynamism that Python supports. It's not a question of how you annotate types, it's about how you resolve types.

https://github.com/patrick-kidger/jaxtyping

From https://news.ycombinator.com/item?id=14246095 (2017) :

> PyContracts supports runtime type-checking and value constraints/assertions (as @contract decorators, annotations, and docstrings).

> Unfortunately, there's yet no unifying syntax between PyContracts and the newer python type annotations which MyPy checks at compile-type.

Or beartype.

Pycontracts has: https://andreacensi.github.io/contracts/ :

  @contract
  def my_function(a : 'int,>0', b : 'list[N],N>0') -> 'list[N]':
  
  @contract(image='array[HxWx3](uint8),H>10,W>10')
  def recolor(image):

parquery/icontract: https://github.com/Parquery/icontract :

> There exist a couple of contract libraries. However, at the time of this writing (September 2018), they all required the programmer either to learn a new syntax (PyContracts) or to write redundant condition descriptions ( e.g., contracts, covenant, deal, dpcontracts, pyadbc and pcd).

  @icontract.require(lambda x: x > 3, "x must not be small")
  def some_func(x: int, y: int = 5) -> None:

  @icontract.require(lambda arr: isinstance(arr, np.ndarray))
  @icontract.require(lambda arr: arr.shape == (3, 3))
  @icontract.require(lambda arr: np.all(arr >= 0), "All elements must be non-negative")
  def process_matrix(arr: np.ndarray):
      return np.sum(arr)

  invalid_matrix = np.array([[1, -2, 3], [4, 5, 6], [7, 8, 9]])
  process_matrix(invalid_matrix)
  # Raises icontract.ViolationError