str.format and string.Formatter subscript behaviors diverge

[tekknolagi]

BPO	39985
Nosy	@ericvsmith, @tekknolagi
PRs	bpo-39985: Make string.Formatter with empty field name default to 0 #19065
Superseder	bpo-27307: string.Formatter does not support key/attribute access on unnumbered fields

^{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 2020-03-19.15:22:33.694>
created_at = <Date 2020-03-17.01:42:20.125>
labels = ['3.7', 'type-bug', 'library']
title = 'str.format and string.Formatter subscript behaviors diverge'
updated_at = <Date 2020-03-19.15:22:33.693>
user = 'https://github.com/tekknolagi'

bugs.python.org fields:

activity = <Date 2020-03-19.15:22:33.693>
actor = 'eric.smith'
assignee = 'none'
closed = True
closed_date = <Date 2020-03-19.15:22:33.694>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2020-03-17.01:42:20.125>
creator = 'tekknolagi'
dependencies = []
files = []
hgrepos = []
issue_num = 39985
keywords = ['patch']
message_count = 3.0
messages = ['364382', '364603', '364606']
nosy_count = 2.0
nosy_names = ['eric.smith', 'tekknolagi']
pr_nums = ['19065']
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '27307'
type = 'behavior'
url = 'https://bugs.python.org/issue39985'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7']

[tekknolagi]

As I understand it, str.format and string.Formatter are supposed to behave the
same, with string.Formatter being a pluggable variant. While poking at
string.Formatter, I noticed that they do not behave the same when formatting a
nameless subscript:

import string
str.format("{[0]}", "hello")  # => "h"
string.Formatter().format("{[0]}", "hello")  # => KeyError("")

They seem to work the same in the case where the arg is either indexed by
number or by name:

import string
str.format("{0[0]}", "hello")  # => "h"
string.Formatter().format("{0[0]}", "hello")  # => "h"
str.format("{a[0]}", a="hello")  # => "h"
string.Formatter().format("{a[0]}", a="hello")  # => "h"

After some digging, I have come up with a couple ideas:

• Change _string.formatter_field_name_split to treat an empty string field name
  as 0, so that string.Formatter.get_value looks up the arg in args, instead of
  kwargs
• Change string.Formatter.get_value to treat empty string key as 0, and look up
  the arg in args, instead of kwargs

I'm happy to submit a PR if people find one of these two solutions palatable or
have some solutions of their own.

(Note: this may appear in other versions, but I don't have them on my machine
to test.)

[ericvsmith]

Can you see if the patch in bpo-27307 solves your problem?

[ericvsmith]

In fact, this is a duplicate of bpo-27307, so I'm going to close this.