Add `warn_unused_ignores = True` to our mypy config

[pradyunsg]

We should add warn_unused_ignores = True in our mypy global configuration in a follow up PR.

Originally posted by @pradyunsg in #7932 (comment)

This would catch "unused" / redundant # type: ignore comments, and allow us to remove them when they're no longer necessary.

[deveshks]

I added warn_unused_ignores = True in setup.cfg's mypy section and on running tox -e lint, the following warnings were generated.

mypy errors

mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

src/pip/_internal/models/direct_url.py:41: error:
unused
'type:
ignore'
comment
            expected_type = six.string_types ...
            ^
src/pip/_internal/utils/compat.py:46: error:
unused
'type:
ignore'
comment
            cache_from_source = imp.cache_fro...
            ^
src/pip/_internal/utils/unpacking.py:186: error:
unused
'type:
ignore'
comment
                    fn = split_leading_dir(fn...
                    ^
src/pip/_internal/operations/install/wheel.py:68: error:
unused
'type:
ignore'
comment
        return (digest, str(length))  # type:...
        ^
src/pip/_internal/req/req_file.py:412: error:
unused
'type:
ignore'
comment
                shlex.split(options_str), def...
                ^
src/pip/_internal/req/req_file.py:434: error:
unused
'type:
ignore'
comment
        return ' '.join(args), ' '.join(optio...
        ^
src/pip/_internal/cli/main_parser.py:74: error:
unused
'type:
ignore'
comment
            sys.stdout.write(parser.version) ...
            ^
src/pip/_internal/commands/debug.py:41: error:
unused
'type:
ignore'
comment
            implementation = sys.implementati...
            ^
Found 8 errors in 7 files (checked 135 source files)

mypy, for Python 2.......................................................Failed
- hook id: mypy
- exit code: 1

src/pip/_internal/req/req_file.py:408: error:
unused
'type:
ignore'
comment
                options_str = options_str.enc...
                ^
Found 1 error in 1 file (checked 132 source files)

So in the PR for this, do we also remove the unused #type: ignore comments to get rid of these warnings? If yes, I will go ahead and create a PR adding the warn_unused_ignores option and remove the unnecessary type: ignore: (I guess we would also need to fix other mypy errors which come up after removing them)

[deveshks]

After getting rid of the type: ignore from the lines for which we were warned in the above comment, we got new errors from mypy as follows. Most of them were from mypy 2, only one is from mypy 3.

mypy errors are removing unnecessary type: ignore

mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

src/pip/_internal/req/req_file.py:408: error:
Incompatible
types
in
assignment
(expression
has
type
"bytes",
variable
has
type
"str")
    ...      options_str = options_str.encode('u...
                           ^
Found 1 error in 1 file (checked 135 source files)

mypy, for Python 2.......................................................Failed
- hook id: mypy
- exit code: 1

src/pip/_internal/utils/compat.py:46: error:
Module
has
no
attribute
"cache_from_source"
    ...cache_from_source = imp.cache_from_source
                           ^
src/pip/_internal/models/direct_url.py:41: error:
Incompatible
types
in
assignment
(expression
has
type
"Tuple[Type[str], Type[unicode]]",
variable
has
type
"Type[T]")
            expected_type = six.string_types
                            ^
src/pip/_internal/req/req_file.py:412: error:
Argument
1
to
"split"
has
incompatible
type
"unicode";
expected
"Optional[str]"
    ...        shlex.split(options_str), default...
                           ^
src/pip/_internal/req/req_file.py:434: error:
Incompatible
return
value
type
(got
"Tuple[unicode, unicode]",
expected
"Tuple[str, unicode]")
        return ' '.join(args), ' '.join(optio...
               ^
src/pip/_internal/utils/unpacking.py:186: error:
Incompatible
types
in
assignment
(expression
has
type
"Union[str, unicode]",
variable
has
type
"str")
    ...               fn = split_leading_dir(fn)...
                           ^
src/pip/_internal/operations/install/wheel.py:68: error:
Incompatible
return
value
type
(got
"Tuple[unicode, str]",
expected
"Tuple[str, str]")
        return (digest, str(length))
                ^
src/pip/_internal/cli/main_parser.py:74: error:
Argument
1
to
"write"
of
"IO"
has
incompatible
type
"unicode";
expected
"str"
            sys.stdout.write(parser.version)
                             ^
src/pip/_internal/commands/debug.py:41: error:
Module
has
no
attribute
"implementation"
    ...      implementation = sys.implementation
                              ^
Found 8 errors in 7 files (checked 132 source files)

[pradyunsg]

Gah, looks like I should remove the --pretty that was added to our mypy runs.

[pradyunsg]

Okay, looks like we gotta keep this until we kill Python 2 support. Until we remove that, we can't remove some of those comments. We're triggering 2 mypy runs, that hit 2 separate "bunches" of # type: ignore comments. :)

[deveshks]

Okay, looks like we gotta keep this until we kill Python 2 support. :)

Agreed, the effort we will put to correct the types after removing type: ignore just for Python 2 is not worth it, given we are going to remove Python 2 support after a few days.

[deveshks]

Gah, looks like I should remove the --pretty that was added to our mypy runs.

Do you want me to create a PR to do that. The difference in displaying mypy errors is pretty obvious

• with --pretty

src/pip/_internal/req/req_file.py:408: error:
Incompatible
types
in
assignment
(expression
has
type
"bytes",
variable
has
type
"str")
    ...      options_str = options_str.encode('u...
                           ^
Found 1 error in 1 file (checked 135 source files)

• without --pretty

src/pip/_internal/req/req_file.py:408: error: Incompatible types in assignment (expression has type "bytes", variable has type "str")
Found 1 error in 1 file (checked 135 source files)

[pradyunsg]

Do you want me to create a PR to do that. The difference in displaying mypy errors is pretty obvious

Yea, a PR removing --pretty + a bug report on mypy that --pretty doesn’t work with pre-commit on Windows, is the best thing to do here IMO.