Raw strings break the FLY002 fix

[dscorbett]

Summary

When the argument to str.join contains only string literals and the first is raw, the fix for static-join-to-f-string (FLY002) can introduce a syntax error or change the program’s behavior.

If a subsequent string isn’t raw, its contents will be inserted into the fix’s string, regardless of syntax errors. Example:

$ cat >fly002_1.py <<'# EOF'
"".join((r"", '"'))
# EOF

$ ruff check --isolated fly002_1.py --select FLY002 --unsafe-fixes --diff 2>&1 | grep error:
error: Fix introduced a syntax error. Reverting all changes.

Another example:

$ cat >fly002_2.py <<'# EOF'
"".join((r"", "\\"))
# EOF

$ ruff check --isolated fly002_2.py --select FLY002 --unsafe-fixes --diff 2>&1 | grep error:
error: Fix introduced a syntax error. Reverting all changes.

Another example:

$ cat >fly002_3.py <<'# EOF'
"".join((r"", "\0"))
# EOF

$ ruff check --isolated fly002_3.py --select FLY002 --unsafe-fixes --fix
Found 1 error (1 fixed, 0 remaining).

$ xxd fly002_3.py
00000000: 7222 0022 0a                             r".".

$ python fly002_3.py 2>&1 | tail -n 1
SyntaxError: source code cannot contain null bytes

It can also change the program’s behavior. Example:

$ cat >fly002_4.py <<'# EOF'
print("".join((r"", r'".__class__.__name__, 0xA, sep="~')))
# EOF

$ python fly002_4.py
".__class__.__name__, 0xA, sep="~

$ ruff check --isolated fly002_4.py --select FLY002 --unsafe-fixes --fix
Found 1 error (1 fixed, 0 remaining).

$ cat fly002_4.py
print(r"".__class__.__name__, 0xA, sep="~")

$ python fly002_4.py
str~10

If any string contains a carriage return via an escape sequence, the fix emits a multiline string containing a literal carriage return. Literal carriage returns are interpreted as line feeds, so this changes the program’s behavior. Example:

$ cat >fly002_5.py <<'# EOF'
print(repr("".join((r"", "\r"))))
# EOF

$ python fly002_5.py
'\r'

$ ruff check --isolated fly002_5.py --select FLY002 --unsafe-fixes --fix
Found 1 error (1 fixed, 0 remaining).

$ xxd fly002_5.py
00000000: 7072 696e 7428 7265 7072 2872 2222 220d  print(repr(r""".
00000010: 2222 2229 290a                           """)).

$ python fly002_5.py
'\n'

Version

ruff 0.14.2 (83a3bc4 2025-10-23)