Approaching Python 3000: function annotations

Python 3000 introduces function annotations, regulated by PEP-3107. This one is of particular interest to me as I've previously written one of the method signature checking decorators which this PEP is supposed to replace.

PEP-3107's has two major points:

1. Annotations can be anything. Any Python expression can be attached to a function argument or result value. For example, it is possible to write

def max(a: int, b: int) -> int:
def max(a: "first", b: "second") -> "maximum":
def foo(x: { ("no", "reason"): lambda x: x**2 }):

2. Annotations have no semantics and are not enforced, they are purely syntactical. For example, it's ok to write

def max(a: int, b: int) -> int:
...
max("foo", "bar") # nothing happens

The interpretation of annotations is left to 3rd party libraries. The language thus offers an unprecedented semantical freedom to the developers, but let's see what are the implications.

One problem is that you will have to choose your one true annotations. Specific interpretation of function annotations depends upon external module, library or application, and you acknowledge this dependence explicitly by either modifying your code or having it processed by external application.

For example, you could have chosen to use function annotations for method signature type checking, using typecheck decorator (resembling my type checking decorators)

@typecheck
def foo(a: int) -> int:

but once you have chosen the @typecheck implementation you have to stick with it and treat all your function annotations as type checks. Stacking multiple annotations is technically possible, but practically it is not, because the standard does not specify how multiple annotations should be multiplexed. Consider that you have

@typecheck
def foo(i: int):

in place and want to add a docstring kind of annotation to foo's first argument:

def foo(i: "comment"):

Should it be

def foo(i: (int, "comment")):

or

def foo(i: {"typecheck": int, "docstring": "comment"}):

?

No matter which way you choose, both typecheck and docstring must be prepared to extract their annotations from the actually encountered multiplexed construct. This means that two independent implementations must understand the same multiplexing format. Since such multiplexing is not standardized, it is impossible.

One seemingly reasonable way of such multiplexing could have been an iterable with instances of classes descended from some base class (Annotation?) for example

def foo(i: (typecheck(int), docstring("comment"))):

This example may be correct but it perfectly illustrates how having multiple semantically different annotations seriously hamper the visual quality and readability of the code.

Which opens a final question of whether the gains of the only particular annotations that you choose outweigh the loss of syntactical brevity.