[issue44124] Unhelpful SyntaxError message with mis-ordering of f-string specifiers

[Angus L'Herrou]

New submission from Angus L'Herrou <piraka at brandeis.edu>:

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 "<stdin>", line 1, in <module>
ValueError: Invalid format specifier
>>> f'{foo=!r}'    # correct ordering of = and !
'foo=12.345'
>>> f'{foo!r=}'    # incorrect ordering of ! and =
  File "<stdin>", 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 "<stdin>", line 1, in <module>
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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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44124>
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