*type and **type is not parsed properly for Python 2 type hints

[skasch]

Environment data

• Language Server version: 2021.4.2
• OS and version: Ubuntu 16.04
• Python version (& distribution if applicable, e.g. Anaconda): 2.7.12

Expected behaviour

*type and **type type hints are parsed properly by Pylance.

Actual behaviour

*type and **type type hints raise an error.

Logs

I did not manage to get any relevant log, even with Trace-level logging enabled.

Code Snippet / Additional information

def test_fun(*args, **kwargs):
    # type: (*int, **float) -> int
    return sum(args) + sum(round(kwarg) for kwarg in kwargs.values())

According to PEP-484, this should be the standard way to type hint * and ** arguments in older versions of Python. However, writing the type hint that way raises errors:

In the code, args is considered to have type Tuple[Any, ...] and kwargs is considered to have type Dict[str, Any].

However, it looks like using line-by-line type hints work:

def test_fun(
    *args,  # type: int
    **kwargs,  # type: float
):
    # type: (...) -> int
    return sum(args) + sum(round(kwarg) for kwarg in kwargs.values())

Here, args is considered to have type Tuple[int, ...], and kwargs is considered to have type Dict[str, float].

I also tried removing the * and **:

def test_fun(*args, **kwargs):
    # type: (int, float) -> int
    return sum(args) + sum(round(kwarg) for kwarg in kwargs.values())

In that case, Pylance does find the right types, args is considered to have type Tuple[int, ...] and kwargs to have type Dict[str, float], but this is inconsistent with the expected format (and I find it also harder to read).

[erictraut]

Pylance is designed for Python 3 and doesn't generally support Python 2. It does have minimal support for function annotation comments with the goal of supporting type stubs for libraries that work with Python 2 and 3, but this support has many limitations.

I'm curious why you are still using function annotation comments in your code. Is it not feasible to switch to Python 3 syntax?

We don't plan to make any further significant investments in Python 2 function annotation comments. However, in this particular case, it's simple to eliminate the parsing error that you're seeing (just a one-line change), so I'll make that change in the parser. I'm not adding full semantic validation (e.g. verifying that * and ** parameter annotations line up with the corresponding *args and **kwargs parameters in the signature). That would require a much bigger investment.

The parser change will be included in the next release of pylance.

[skasch]

Thank you very much for taking the time to fix that, despite it being a Python 2-specific issue!

Unfortunately, I'm currently working on a pretty large codebase running in Python 2, which is why I cannot use the Python 3 syntax. We are working on a transition to Python 3, but in the meantime, we try as much as possible to write Python 2 code that can also be interpreted as Python 3 code. Thanks to that, I've found that Pylance is really useful for me to analyze the code, despite it being Python 2 code. There are of course some limitations, and having to use type comments instead of type annotations is one of them.

[jakebailey]

This issue has been fixed in version 2021.4.3, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202143-29-april-2021