Recommend reveal_type in PEP 484

When #136 will be fixed, it will be nice to have something that could be used in place of issubclass for "experimentation" with types. Mypy already provides such a function -- reveal_type. I think it works very well and maybe it is a good idea to add a recommendation to PEP 484 that static type checkers should provide a function to reveal the inferred types? Or maybe even add a function

def reveal_type(obj):
    print('Runtime type is:', getattr(obj, '__orig_class__',  type(obj)))
    return obj

to typing.py so that it can be used for this purpose by other type checkers.

I wish reveal_type() was a Python builtin. Often when I test code, I have to temporarily comment out reveal_type calls, which is a bit annoying.

I use this hack:

from typing import TYPE_CHECKING

if not TYPE_CHECKING:
   globals()['reveal_type'] = print

I support making it a builtin. File a bpo issue and write a PR and I will add it.

@ilevkivskyi

One idea I have in addition to @ilevkivskyi's code above is to use the warnings module instead of just printing the output. This allows filtering, adds special handling by test runners etc.

If reveal_type is added as an actual builtin call, its full type signature will need to be formalized.

Would you be open to reveal_type returning a str (the actual revealed type) rather than returning the value passed as an input argument?

Pyright currently implements reveal_type such that the return type of the call is a literal string value of the revealed type.

This allows us to use it in test cases, like this:

def func(x: int, y: str):
   z = x if x > 3 else y
   t1: Literal['int | str'] = reveal_type(z)

We use this in thousands of places within pyright's test suites. It would be significant work to change this.

Hm. In mypy IIRC reveal_type(x) returns x. This makes it possible to wrap arbitrary subexpressions in it without changing their meaning. I would regret losing this.

You could introduce a new helper with the desired meaning in a pyright-specific header.

This makes it possible to wrap arbitrary subexpressions in it without changing their meaning

Oh, that's clever. I hadn't considered that.

OK, we can introduce a new helper.

I am implementing this by adding typing.reveal_type:

This will be in Python 3.11 and the next release of typing-extensions.

https://docs.python.org/3.11/library/typing.html#typing.reveal_type

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Recommend reveal_type in PEP 484 #277

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Recommend reveal_type in PEP 484 #277

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