[issue40246] Different error messages for same error - invalid string prefixes

[Serhiy Storchaka]

Serhiy Storchaka <storchaka+cpython at gmail.com> added the comment:

> It's no different from other edge cases where for some reason the space seems optional, like `1and 2`.

The parser is not consistent here. `0or[]` is an error, while `0and[]` and `1or[]` are valid. See https://mail.python.org/archives/list/python-dev@python.org/message/D2WPCITHG2LBQAP7DBTC6CY26WQUBAKP/

> A possible solution would be to only emit a SyntaxError if the NAME directly preceding a STRING token contains one of the valid string prefixes (either one of 'f', 'r', 'u', 'b').

This would not help for `if'a'<=x<='z'`. And we will get more breakage when add more string prefixes.

We should either require a whitespace between an identifier and a string literal (with SyntaxWarning in meantime) or allow to omit it.

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<https://bugs.python.org/issue40246>
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