[Python-Dev] The syntax of replacement fields in format strings (original) (raw)

Eric V. Smith eric at trueblade.com
Tue Oct 31 06:52:23 EDT 2017

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 '.', '[', ']', '{', '}', ':', '!'. >>> '{#}'.formatmap({'#': 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 "", line 1, in 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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