New submission from Carl Friedrich Bolz-Tereick <cfb...@gmx.de>:
The following behaviour of %-formatting changed between Python3.6 and
Python3.7, and is in my opinion a bug that was introduced.
So far, it has been possible to add conversion flags to a conversion specifier
in %-formatting, even if the conversion is '%' (meaning a literal % is emitted
and no argument consumed).
Eg this works in Python3.6:
>>>> "%+%abc% %" % ()
'%abc%'
The conversion flags '+' and ' ' are ignored.
Was it discussed and documented anywhere that this is now an error? Because
Python3.7 has the following strange behaviour instead:
>>> "%+%abc% %" % ()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: not enough arguments for format string
That error message is just confusing, because the amount of arguments is not
the problem here. If I pass a dict (thus making the number of arguments
irrelevant) I get instead:
>>> "%+%abc% %" % {}
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: unsupported format character '%' (0x25) at index 2
(also a confusing message, because '%' is a perfectly fine format character)
In my opinion this behaviour should either be reverted to how Python3.6 worked,
or the new restrictions should be documented and the error messages improved.
----------
messages: 360965
nosy: Carl.Friedrich.Bolz
priority: normal
severity: normal
status: open
title: bug in %-formatting in Python, related to escaped %-characters
versions: Python 3.7
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue39486>
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