Improve bytes formatting error #14959
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Fixes python#11806 This error message was last changed in python#11139 to make it more applicable to f-strings. Since users are still getting confused, it makes sense to be more explicit about what the recommended fix is. There was also an extra set of quotes and a mention of Python 3 that didn't help.
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This looks like a big improvement in my opinion!
Should we maybe also improve some of the docs around this error code, like Jukka suggested in #11806 (comment), before closing the issue, though?
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Diff from mypy_primer, showing the effect of this PR on open source code: steam.py (https://github.com/Gobot1234/steam.py)
- steam/client.py:814: error: On Python 3 formatting "b'abc'" with "{}" produces "b'abc'", not "abc"; use "{!r}" if this is desired behavior [str-bytes-safe]
+ steam/client.py:814: error: If x = b'abc' then f"{x}" or "{}".format(x) produces "b'abc'", not "abc". If this is desired behavior, use f"{x!r}" or "{!r}".format(x). Otherwise, decode the bytes [str-bytes-safe]
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I know it's not the usual mypy style for error messages, but I do think this would be much clearer here if backticks were used to indicate the code snippets (the fact that I.e. I'd find this even clearer: |
| @@ -638,8 +638,8 @@ def g() -> int: | |||
| '%d' % 'no' # E: Incompatible types in string interpolation (expression has type "str", placeholder has type "Union[int, float, SupportsInt]") [str-format] | |||
| '%d + %d' % (1, 2, 3) # E: Not all arguments converted during string formatting [str-format] | |||
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| '{}'.format(b'abc') # E: On Python 3 formatting "b'abc'" with "{}" produces "b'abc'", not "abc"; use "{!r}" if this is desired behavior [str-bytes-safe] | |||
| '%s' % b'abc' # E: On Python 3 formatting "b'abc'" with "%s" produces "b'abc'", not "abc"; use "%r" if this is desired behavior [str-bytes-safe] | |||
| '{}'.format(b'abc') # E: If x = b'abc' then f"{x}" or "{}".format(x) produces "b'abc'", not "abc". If this is desired behavior, use f"{x!r}" or "{!r}".format(x). Otherwise, decode the bytes [str-bytes-safe] | |||
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What do you think about injecting real user's values into the error message?
| 'On Python 3 formatting "b\'abc\'" with "{}" ' | ||
| 'produces "b\'abc\'", not "abc"; ' | ||
| 'use "{!r}" if this is desired behavior', | ||
| 'If x = b\'abc\' then f"{x}" or "{}".format(x) produces "b\'abc\'", ' |
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Sadly, we have no way of telling whether it is a .format call or fstring. It would really help with the error message.
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Is that because we desugar f-strings into a CallExpr that looks just like a str.format call? Maybe we can stash some data in the CallExpr to indicate that it derived from an f-string.
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Thanks all!
Opened #14959 for this
I think probably okay as is, backticks aren't used anywhere else in mypy and this is an error message where additional visual noise seems harmful. Not sure how familiar users are with markdown either...
We won't always have the bytes value at hand. I think the example "abc" makes it clearer that the problem isn't with the specific value, just the act of formatting
I agree that this would be nice |
Fixes #11806
This error message was last changed in #11139 to make it more applicable to f-strings. Since users are still getting confused, it makes sense to be more explicit about what the recommended fix is. There was also an extra set of quotes and a mention of Python 3 that didn't help.
I think this is still very much a useful error, it caught a bug I had recently.