Confusing error messages for %-formatting, related to escaped %-characters

[cfbolz]

BPO	39486
Nosy	@cfbolz, @serhiy-storchaka, @iritkatriel

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-01-29.14:49:09.257>
labels = ['interpreter-core', 'type-bug', '3.11']
title = 'Confusing error messages for %-formatting, related to escaped %-characters'
updated_at = <Date 2022-01-28.20:04:26.600>
user = 'https://github.com/cfbolz'

bugs.python.org fields:

activity = <Date 2022-01-28.20:04:26.600>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2020-01-29.14:49:09.257>
creator = 'Carl.Friedrich.Bolz'
dependencies = []
files = []
hgrepos = []
issue_num = 39486
keywords = []
message_count = 4.0
messages = ['360965', '360974', '360977', '412029']
nosy_count = 4.0
nosy_names = ['Carl.Friedrich.Bolz', 'SilentGhost', 'serhiy.storchaka', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39486'
versions = ['Python 3.11']

[cfbolz]

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.

[SilentGhost]

This seemed to be a consequence of bpo-29568.

[cfbolz]

Ok, that means it's intentional. I still think it's missing a documentation change and consistent error messages.

[iritkatriel]

On 3.11:

>>> "%+%abc% %" % ()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: not enough arguments for format string

>>> "%+%abc% %" % {}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: unsupported format character '%' (0x25) at index 2