According to the specification of format string syntax [1] (I meant
str.format(), not f-strings), both argument name and attribute name must
be Python identifiers.
But the current implementation is more lenient and allow arbitrary
sequences of characters while they don't contain '.', '[', ']', '{',
'}', ':', '!'.
>>> '{#}'.format_map({'#': 42})
'42'
>>> import types
>>> '{0.#}'.format(types.SimpleNamespace(**{'#': 42}))
'42'
This can be confusing due to similarity with the format string syntaxes
in str.format() and f-strings.
>> name = 'abc'
>>> f'{name.upper()}'
'ABC'
>>> '{name.upper()}'.format(name='abc')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'upper()'
If accept only identifiers, we could produce more specific error message.
Is there a bug in the documentation or in the implementation?
[1] https://docs.python.org/3/library/string.html#format-string-syntax
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