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For example:
>>> '{:04}'.format('abc')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: '=' alignment not allowed in string format specifier
There is no any '=' in the format string.
>>> '{: }'.format('abc')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: Sign not allowed in string format specifier
There is no any sign ('+' or '-') in the format string.
There also an inconsistency between the wording of "something not allowed in string format specifier" and:
>>> '{:x}'.format('abc')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: Unknown format code 'x' for object of type 'str'
>>> '{:xx}'.format('abc')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier

See also issue 27772.
The difference in the error messages is due to the first ones looking for specific invalid combinations (in this case things the string formatter does not understand), and the last one which is "I have no idea what you're asking for".

My PR fixes the second case ('{: }') and Serhiy's PR on Issue27772 fixes the first case ('{:04}').

New changeset 2d780237d95cd3d95401f52be2edeac8b458eb68 by Miss Islington (bot) in branch '3.10':
bpo-28146: Fix a confusing error message in str.format() (GH-24213)
https://github.com/python/cpython/commit/2d780237d95cd3d95401f52be2edeac8b458eb68

New changeset 133013e8a1ecd570266de766e2df9745a24343be by Miss Islington (bot) in branch '3.9':
bpo-28146: Fix a confusing error message in str.format() (GH-24213)
https://github.com/python/cpython/commit/133013e8a1ecd570266de766e2df9745a24343be

It seems like this issue is entirely fixed now, closing.

commit 4aeee0b47b3a2b604bbac37040320ffc88c291f2
Author: Irit Katriel <iritkatriel@yahoo.com>
Date: Thu May 13 21:55:55 2021 +0100
 bpo-28146: Fix a confusing error message in str.format() (GH-24213)
 
 
 
 Automerge-Triggered-By: GH:pitrou