Confusing error messages in str.format()

[serhiy-storchaka]

BPO	28146
Nosy	@pitrou, @vstinner, @ericvsmith, @ezio-melotti, @serhiy-storchaka, @miss-islington, @iritkatriel
PRs	bpo-28146: Fix a confusing error message in str.format() #24213 [3.8] bpo-28146: Fix a confusing error message in str.format() (GH-24213) #26110 [3.9] bpo-28146: Fix a confusing error message in str.format() (GH-24213) #26111 [3.10] bpo-28146: Fix a confusing error message in str.format() (GH-24213) #26112

^{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 = <Date 2021-05-13.21:39:16.746>
created_at = <Date 2016-09-14.11:12:35.529>
labels = ['interpreter-core', 'type-bug', '3.9', '3.10', '3.11', 'expert-unicode']
title = 'Confusing error messages in str.format()'
updated_at = <Date 2021-05-14.08:08:15.534>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2021-05-14.08:08:15.534>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2021-05-13.21:39:16.746>
closer = 'pitrou'
components = ['Interpreter Core', 'Unicode']
creation = <Date 2016-09-14.11:12:35.529>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 28146
keywords = ['patch']
message_count = 7.0
messages = ['276415', '276417', '385052', '393608', '393610', '393612', '393631']
nosy_count = 7.0
nosy_names = ['pitrou', 'vstinner', 'eric.smith', 'ezio.melotti', 'serhiy.storchaka', 'miss-islington', 'iritkatriel']
pr_nums = ['24213', '26110', '26111', '26112']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28146'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[serhiy-storchaka]

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

[ericvsmith]

See also bpo-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".

[iritkatriel]

My PR fixes the second case ('{: }') and Serhiy's PR on bpo-27772 fixes the first case ('{:04}').

[miss-islington]

New changeset 2d78023 by Miss Islington (bot) in branch '3.10':
bpo-28146: Fix a confusing error message in str.format() (GH-24213)
2d78023

[miss-islington]

New changeset 133013e by Miss Islington (bot) in branch '3.9':
bpo-28146: Fix a confusing error message in str.format() (GH-24213)
133013e

[pitrou]

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

[vstinner]

commit 4aeee0b
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