New submission from Angus L'Herrou :
The f-string grammar clearly specifies the correct order of f-string =, !, and
: specifiers:
replacement_field ::= "{" f_expression ["="] ["!" conversion] [":"
format_spec] "}"
However, when these components are used in the wrong order, the error messages,
while understandable if you know the grammar, are not exactly helpful for users
of all knowledge levels.
>>> foo = 12.345
>>> f'{foo=:.2f}' # correct ordering of = and :
'foo=12.35'
>>> f'{foo:.2f=}' # incorrect ordering of : and =
Traceback (most recent call last):
File "", line 1, in
ValueError: Invalid format specifier
>>> f'{foo=!r}'# correct ordering of = and !
'foo=12.345'
>>> f'{foo!r=}'# incorrect ordering of ! and =
File "", line 1
SyntaxError: f-string: expecting '}'
>>> bar = 'abcd'
>>> f'{bar!r:.2s}' # correct ordering of ! and :
"'a"
>>> f'{bar:.2s!r}' # incorrect ordering of : and !
Traceback (most recent call last):
File "", line 1, in
ValueError: Invalid format specifier
It would be more helpful to have more descriptive error messages specifying the
correct order of these features. f-string format specifiers, especially ! and
=, are in my experience fairly poorly known features, and more descriptive
feedback when they are used incorrectly would avoid discouraging users from
using them at all upon encountering a cryptic error.
Since __format__ can have an arbitrary implementation for different data types,
and therefore there might be some user-defined class that accepts :.2f!r as a
valid format specifier, the ValueErrors here might have to stay, but at least
the SyntaxError from f'{foo!r=}' could be clearer.
--
components: Parser
messages: 393587
nosy: angus-lherrou, lys.nikolaou, pablogsal
priority: normal
severity: normal
status: open
title: Unhelpful SyntaxError message with mis-ordering of f-string specifiers
type: enhancement
versions: Python 3.10, Python 3.11, Python 3.8, Python 3.9
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<https://bugs.python.org/issue44124>
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