string.Formatter.parse does not handle auto-numbered positional fields

[SDesch]

BPO	45704
Nosy	@ericvsmith

^{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 2021-11-03.11:55:47.784>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.11', '3.7']
title = 'string.Formatter.parse does not handle auto-numbered positional fields'
updated_at = <Date 2021-11-05.18:06:11.745>
user = 'https://bugs.python.org/SDesch'

bugs.python.org fields:

activity = <Date 2021-11-05.18:06:11.745>
actor = 'eric.smith'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-11-03.11:55:47.784>
creator = 'SDesch'
dependencies = []
files = []
hgrepos = []
issue_num = 45704
keywords = []
message_count = 9.0
messages = ['405610', '405619', '405624', '405627', '405636', '405757', '405796', '405802', '405814']
nosy_count = 2.0
nosy_names = ['eric.smith', 'SDesch']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45704'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10', 'Python 3.11']

Linked PRs

• gh-89867: string.Formatter auto numbering doc updates #129617

[SDesch]

It appears when adding auto-numbered positional fields in python 3.1 Formatter.parse was not updated to handle them and currently returns an empty string as the field name.

list(Formatter().parse('hello {}'))  # [('hello ', '', '', None)]

This does not align with Formatter.get_field which according to the docs: "Given field_name as returned by parse() (see above), convert it to an object to be formatted."

When supplying an empty string to .get_field() you get a KeyError

Formatter().get_field("", [1, 2, 3], {}). # raises KeyError

[ericvsmith]

For reference, the documentation is at https://docs.python.org/3/library/string.html#custom-string-formatting

I guess in your example it should return:
[('hello ', '0', '', None)]

[SDesch]

Yes it should return a string containing the index of the positional argument i.e. "0" so that it is compatible with .get_field(). Side note: It's a somewhat weird that .get_field expects a string while .get_value expects an int for positional arguments.