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Tue Oct 31 06:52:23 EDT 2017 · https://mail.python.org/pipermail/python-dev/2017-October/150053.html

If I had it to do over again, I’d implement it more strictly and only allow chars that are valid in identifiers. 

But see https://bugs.python.org/issue31907 for a case that is currently valid and would break if we changed how it worked. 

I’m not sure it’s worth the churn of deprecating this and eventually making it illegal. 

--
Eric.

> On Oct 31, 2017, at 6:37 AM, Serhiy Storchaka <storchaka at gmail.com> wrote:
> 
> 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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[Python-Dev] The syntax of replacement fields in format strings