Broken f-string formatting after ruff 0.9

[AlexElvers]

Ruff 0.9.0/1 are introducing syntax syntax errors in f-strings with braces.

While f-strings with simple sets and dicts do not remove the necessary spaces:

$ echo 'print(f"{ {1, 2, 3} }")\nprint(f"{ {1: 2} }")' > main.py

$ ruff format --diff main.py
1 file already formatted

the spaces around the braces are removed in version 0.9.0/1, which leads to syntax errors:

$ echo 'print(f"{ {1, 2, 3} - {2} }")\nprint(f"{ {1: 2}.keys() }")' > main.py

$ ruff format --diff main.py
--- main.py
+++ main.py
@@ -1,2 +1,2 @@
-print(f"{ {1, 2, 3} - {2} }")
-print(f"{ {1: 2}.keys() }")
+print(f"{{1, 2, 3} - {2}}")
+print(f"{{1: 2}.keys()}")

1 file would be reformatted

$ ruff format main.py
1 file reformatted

$ ruff check main.py
main.py:1:18: SyntaxError: f-string: single '}' is not allowed
  |
1 | print(f"{{1, 2, 3} - {2}}")
  |                  ^
2 | print(f"{{1: 2}.keys()}")
  |

main.py:1:25: SyntaxError: f-string: single '}' is not allowed
  |
1 | print(f"{{1, 2, 3} - {2}}")
  |                         ^
2 | print(f"{{1: 2}.keys()}")
  |

main.py:2:15: SyntaxError: f-string: single '}' is not allowed
  |
1 | print(f"{{1, 2, 3} - {2}}")
2 | print(f"{{1: 2}.keys()}")
  |               ^
  |

main.py:2:23: SyntaxError: f-string: single '}' is not allowed
  |
1 | print(f"{{1, 2, 3} - {2}}")
2 | print(f"{{1: 2}.keys()}")
  |                       ^
  |

Found 4 errors.

$ python main.py
  File "/tmp/foo/main.py", line 1
    print(f"{{1, 2, 3} - {2}}")
                              ^
SyntaxError: f-string: single '}' is not allowed

Ruff 0.8.6 kept these f-strings unchanged:

$ uvx [email protected] format --diff main.py
1 file already formatted

[MichaReiser]

Thanks for reporting this. We do have code handling dictionary and other expressions that use curly braces but it only tests the expression itself rather than the left most or right most sub-expression.