Fixes error message for f string and str-bytes-safe

[sobolevn]

It does not mention .format() anymore.

Closes #10979

[JelleZijlstra]

The new wording is harder to understand. I'd suggest something closer to the original wording but using an f-string, like this:

Suggested change

	'On Python 3 formatting string with "{}" and bytes argument '
	"On Python 3 f'{b"abc"}' produces \"b'abc'\", not 'abc'; "

However, it looks like the same code path is used for f-strings and .format(). Is it easy to use separate messages for each of these?

[sobolevn]

Is it easy to use separate messages for each of these?

Currently mypy uses the same exact ast to represent both .format and f strings. I can add new metadata to CallExpr (which is used for representation). But, I've decided that simply changing an error message is the way to go.

Maybe you can come up with better wording?

[JelleZijlstra]

Then I suggest we just always use the f-string version of the message if we're type checking Python 3, since I would assume that's the more likely case. Mypy doesn't support 3.5 any more. If this message is triggered when we're checking Python 2 code (can it?) we should still mention .format().

[sobolevn]

Then I suggest we just always use the f-string version of the message if we're type checking Python 3, since I would assume that's the more likely case.

I cannot agree, for example in my projects f string is not allowed by our styleguide: https://wemake-python-stylegui.de/en/latest/pages/usage/violations/consistency.html#wemake_python_styleguide.violations.consistency.FormattedStringViolation So, I know that a lot of people still use .format() 🙂

But, we can use the common part like {} and {!r}. Maybe something like

On Python 3 '{b"abc"}' produces \"b'abc'\", not 'abc'; use "{b"abc"!r}" if this is desired behavior

[sobolevn]

@JelleZijlstra please, check out new error message. What do you think?

[Olaktal]

Hello

I encounter this error also ...

poetry run mypy --version        
mypy 0.931

poetry run mypy helpers/aws/s3.py
helpers/aws/s3.py:490: error: On Python 3 formatting "b'abc'" with "{}" produces "b'abc'", not "abc"; use "{!r}" if this is desired behavior

helpers/aws/s3.py

                        raise S3Exception(
                            f"cannot convert following line to json: {line}"
                        ) from json_convert_error

Any idea why ? My version seem pretty recent so I don't understand why it is not fixed 🤔