pylint doesn't understand returns from functions using match case

[wangoland]

Bug description

Pylint displays the following error code:
Error message: 'Attempting to unpack a non-sequence'
Details:

[{
	"resource": "/c:/[REDACTED]/main.py",
	"owner": "python",
	"code": "unpacking-non-sequence",
	"severity": 8,
	"message": "Attempting to unpack a non-sequence",
	"source": "pylint",
	"startLineNumber": 124,
	"startColumn": 5,
	"endLineNumber": 124,
	"endColumn": 5
}]

When the following code snippet is used:

def check_user_can_add_friend(username):
    # Get subscription level
    subscription_level = user_get_subscription_level(username)
    number_friend_requests = user_count_friend_requests(username)
    number_friends = number_friend_requests + user_count_friends(username)

    match subscription_level:
        case "BASIC":
            if number_friends < 5:
                return True, " "
            else:
                return False, "'" + username + "' can only have a maximum of 5 friends"
        case "GOLD":
            if number_friends < 20:
                return True, " "
            else:
                return False, "'" + username + "' can only have a maximum of 20 friends"
        case _:
            return True, " "

If the above match statement is changed to an if and elseif statement then the error reported by pylance for un-packing will go away

Configuration

No response

Command used

Automatically within VS Code via PyLance, the pylance team advised me to raise this with yourselves:
https://github.com/microsoft/pylance-release/issues/2060

Pylint output

Error message: 'Attempting to unpack a non-sequence'
Details:

[{
	"resource": "/c:/[REDACTED]/main.py",
	"owner": "python",
	"code": "unpacking-non-sequence",
	"severity": 8,
	"message": "Attempting to unpack a non-sequence",
	"source": "pylint",
	"startLineNumber": 124,
	"startColumn": 5,
	"endLineNumber": 124,
	"endColumn": 5
}]

Expected behavior

Pylance should not show any error for this line as it can be correctly unpacked.

Pylint version

pylint 2.11.1
astroid 2.8.4
Python 3.10.0 (tags/v3.10.0:b494f59, Oct  4 2021, 19:00:18) [MSC v.1929 64 bit (AMD64)]

OS / Environment

Windows 10 Enterprise (10.0.18363 Build 18363)

Additional dependencies

No response

[Sasszem]

Maybe a bit simpler code to reproduce:

def foo() -> tuple[int, int]:
    match 1:
        case int(a):
            return a, 5
        case None:
            return 0, 0

x, y = foo()

Adding a return None, None or similar after the match statement makes the false positive disappear, but raising an exception does not.

[Pierre-Sassoulas]

I can reproduce with @Sasszem example.

[jacobtylerwalls]

We need astroid to understand returns from match statements:

>>> import astroid
>>> code = """
... def check_user_can_add_friend(username):
...     # Get subscription level
...     subscription_level = user_get_subscription_level(username)
...     number_friend_requests = user_count_friend_requests(username)
...     number_friends = number_friend_requests + user_count_friends(username)
... 
...     match subscription_level:
...         case "BASIC":
...             if number_friends < 5:
...                 return True, " "
...             else:
...                 return False, "'" + username + "' can only have a maximum of 5 friends"
...         case "GOLD":
...             if number_friends < 20:
...                 return True, " "
...             else:
...                 return False, "'" + username + "' can only have a maximum of 20 friends"
...         case _:
...             return True, " "
...  
... z = check_user_can_add_friend('uu')  #@
... """
>>> n = astroid.extract_node(code)
>>> n.targets[0].inferred()
[<Const.NoneType l.None at 0x1105ff090>]

[jacobtylerwalls]

Closing as I don't have immediate plans to work on this.

[jacobtylerwalls]

Whoops, wrong tab. I definitely have plans to work on this. I just put up a PR on astroid: pylint-dev/astroid#2042