Type Union with NoReturn

[jfmend]

Environment data

• Language Server version: 2021.10.3
• OS and version: linux x64
• Python version (and distribution if applicable, e.g. Anaconda):
• python.analysis.indexing: undefined
• python.analysis.typeCheckingMode: basic

Expected behaviour

No type errors reported

Actual behaviour

Error resulting from Union[A, B, ..., NoReturn] not being considered equivalent to Union[A, B, ...] which I believe it should.
To be clear, a [variable, function_arg, return_type] of type A should not accept an expression of type NoReturn (since this is either useless or a bug) in which case a type error is correctly reported. However, a scenario where a type A is expected should accept an expression of type Union[A, NoReturn].

Notice that, in Python, the evaluation of any expression may result in an Exception being raised (ex: MemoryError). This means that an expression of some type A is always implicitly of type Union[A, NoReturn].

Code Snippet / Additional information

from typing import NoReturn


def raises(exception: Exception) -> NoReturn:
    '''Helper function to allow raising an exception in an expression context'''
    raise exception


# Example 1 

n = 1
n_str: str
# incorrect type ERROR (assigning str | NoReturn to var of type str):
n_str = str(n) if n > 0 else raises(
        Exception(f'Must be positive: N = {n}'))
print(n_str)


# Example 2

import os
from pathlib import Path


def find_productFile(product_dir: Path) -> Path:
    dir_files = os.listdir(product_dir)
    # incorrect type ERROR (returning Path | NoReturn where Path is expected):
    return ( product_dir/'product.xml'   if 'product.xml' in dir_files
        else product_dir/'manifest.safe' if 'manifest.safe' in dir_files
        else raises(Exception(
                'Product not identified as one of [Sentinel-1, RadarSat2]')))

[erictraut]

Thanks for reporting the issue. I've created a tracking bug in the pyright issue tracker.

[erictraut]

We already elide NoReturn from unions in several other cases, but this logic was not included for ternary expressions. It's trivial to add this case. This will be included in the next release.

[jfmend]

Thank you for the quick reply.
I am not sure if this should be another issue or not, but type check errors with Unions containing NoReturn also occurs in or expressions:

from typing import NoReturn


def raises(exception: Exception) -> NoReturn:
    '''Helper function to allow raising an exception in an expression context'''
    raise exception


n = 1
n_str: str
# incorrect type ERROR (assigning str | NoReturn to var of type str):
n_str = str(n) or raises(RuntimeError(f'Must be positive: N = {n}'))
print(n_str)

[erictraut]

I've added support for or and and operators as well.

[heejaechang]

fixed in 2021.11.0