string.Formatter class not allowing {.field}

[avdwoude]

BPO	44496
Nosy	@ericvsmith, @corona10

^{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 2021-06-23.07:26:04.217>
labels = ['3.8', 'type-bug', 'library']
title = 'string.Formatter class not allowing {.field}'
updated_at = <Date 2021-11-03.13:39:08.555>
user = 'https://bugs.python.org/avdwoude'

bugs.python.org fields:

activity = <Date 2021-11-03.13:39:08.555>
actor = 'eric.smith'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-06-23.07:26:04.217>
creator = 'avdwoude'
dependencies = []
files = []
hgrepos = []
issue_num = 44496
keywords = []
message_count = 2.0
messages = ['396383', '405615']
nosy_count = 3.0
nosy_names = ['eric.smith', 'corona10', 'avdwoude']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44496'
versions = ['Python 3.8']

[avdwoude]

I expected the custom Formatter to behave the same as the normal "".format() function, but unnamed args or not supported.
Below is an example, which fails at a KeyError.

from string import Formatter

class test():
    def __init__(self):
        self.msg = "OK"
t = test()

print("Normal format() is {.msg}".format(t))

f = Formatter()
print(f.format("Formatter.format() is {.msg}", t))

[ericvsmith]

I think this is a dupe of bpo-27307.

[encukou]

Fixed by: #21767