mypy doesn't understand partial unpacking with star arguments

[ArniDagur]

Bug Report

While working on qutebrowser/qutebrowser#5317 I think I may have found a bug in mypy. The following code looks correct, and runs fine on my machine.

To Reproduce

1. Save the following code to file /tmp/reproducer.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# vim: fenc=utf-8:et:ts=4:sts=4:sw=4:fdm=marker
from typing import Optional, Sequence, Mapping
import collections

class ModuleInfo:
    def __init__(
        self,
        name: str,
        version_attributes: Sequence[str],
        min_version: Optional[str] = None
    ):
        self.name = name
        self._version_attributes = version_attributes
        self.min_version = min_version

MODULE_INFO: Mapping[str, ModuleInfo] = collections.OrderedDict([
    (name, ModuleInfo(name, *args))
    for (name, *args) in
    [
        ('sip', ['SIP_VERSION_STR']),
        ('colorama', ['VERSION', '__version__']),
        ('pypeg2', ['__version__']),
        ('jinja2', ['__version__']),
        ('pygments', ['__version__']),
        ('yaml', ['__version__']),
        ('adblock', ['__version__'], "0.3.2"),
        ('cssutils', ['__version__']),
        ('attr', ['__version__']),
        ('PyQt5.QtWebEngineWidgets', []),
        ('PyQt5.QtWebEngine', ['PYQT_WEBENGINE_VERSION_STR']),
        ('PyQt5.QtWebKitWidgets', []),
    ]
])

2. Run mypy /tmp/reproducer.py.

Expected Behavior

There should be no type errors.

Actual Behavior

The output of the command in step 2 returns:

reproducer.py:19: error: List comprehension has incompatible type List[Tuple[Sequence[str], ModuleInfo]]; expected List[Tuple[str, ModuleInfo]]
reproducer.py:19: error: Argument 1 to "ModuleInfo" has incompatible type "Sequence[str]"; expected "str"
reproducer.py:19: error: Argument 2 to "ModuleInfo" has incompatible type "*List[Sequence[str]]"; expected "Optional[str]"
Found 3 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 0.790
• Mypy command-line flags: mypy /tmp/reproducer.py
• Mypy configuration options from mypy.ini (and other config files): None, I think
• Python version used: Python 3.8.6
• Operating system and version: Gentoo Linux

[sobolevn]

This happens because your iterable type is not infered correctly:

from typing import Optional, Sequence, Mapping, Tuple, Union

items = [
    ('sip', ['SIP_VERSION_STR']),
    ('colorama', ['VERSION', '__version__']),
    ('pypeg2', ['__version__']),
    ('jinja2', ['__version__']),
    ('pygments', ['__version__']),
    ('yaml', ['__version__']),
    ('adblock', ['__version__'], "0.3.2"),
    ('cssutils', ['__version__']),
    ('attr', ['__version__']),
    ('PyQt5.QtWebEngineWidgets', []),
    ('PyQt5.QtWebEngine', ['PYQT_WEBENGINE_VERSION_STR']),
    ('PyQt5.QtWebKitWidgets', []),
]

reveal_type(items)
# note: Revealed type is 'builtins.list[builtins.tuple*[typing.Sequence[builtins.str]]]'

And:

from typing import Optional, Sequence, Mapping, Tuple, Union

items: Sequence[Union[
    Tuple[str, Sequence[str]],
    Tuple[str, Sequence[str], str],
]] = [
    ('sip', ['SIP_VERSION_STR']),
    ('colorama', ['VERSION', '__version__']),
    ('pypeg2', ['__version__']),
    ('jinja2', ['__version__']),
    ('pygments', ['__version__']),
    ('yaml', ['__version__']),
    ('adblock', ['__version__'], "0.3.2"),
    ('cssutils', ['__version__']),
    ('attr', ['__version__']),
    ('PyQt5.QtWebEngineWidgets', []),
    ('PyQt5.QtWebEngine', ['PYQT_WEBENGINE_VERSION_STR']),
    ('PyQt5.QtWebKitWidgets', []),
]

reveal_type(items)
# note: Revealed type is 'typing.Sequence[Union[Tuple[builtins.str, typing.Sequence[builtins.str]], Tuple[builtins.str, typing.Sequence[builtins.str], builtins.str]]]'

[ArniDagur]

When I try the following

MODULE_INFO: Mapping[str, ModuleInfo] = collections.OrderedDict([
    (name, ModuleInfo(name, *args))
    for (name, *args) in cast(
        Sequence[Union[Tuple[str, Sequence[str]], Tuple[str, Sequence[str], str]]],
        [
            ('sip', ['SIP_VERSION_STR']),
            ('colorama', ['VERSION', '__version__']),
            ('pypeg2', ['__version__']),
            ('jinja2', ['__version__']),
            ('pygments', ['__version__']),
            ('yaml', ['__version__']),
            ('adblock', ['__version__'], "0.3.2"),
            ('cssutils', ['__version__']),
            ('attr', ['__version__']),
            ('PyQt5.QtWebEngineWidgets', []),
            ('PyQt5.QtWebEngine', ['PYQT_WEBENGINE_VERSION_STR']),
            ('PyQt5.QtWebKitWidgets', []),
        ]
    )
])

I still get the error

reproducer.py:19: error: Argument 2 to "ModuleInfo" has incompatible type "*List[Sequence[str]]"; expected "Optional[str]"
Found 1 error in 1 file (checked 1 source file)

[sobolevn]

Ok, I followed all the way down to the rabbit hole.

This happens because is_subtype_ignoring_tvars returns that str is a subtype of Sequence. And [Sequence[str], str] gets joined into Sequence[str]. This is happing in the check_multi_assignment_from_union method of Checker

In contrast to

from typing import Optional, Sequence, Mapping, Tuple, Union

items: Sequence[Union[
    Tuple[str, Sequence[str]],
    Tuple[str, Sequence[str], int],
]]

for a, *args in items:
    reveal_type(a)
    reveal_type(args)

# ex.py:25: note: Revealed type is 'builtins.str'
# ex.py:26: note: Revealed type is 'Union[builtins.list[typing.Sequence*[builtins.str]], builtins.list[builtins.object*]]'