Syntax errors related to indentation after backslashes #19301
Description
Summary
Ruff has false positives and false negatives when detecting syntax errors related to indentation after backslashes. Python’s documentation says “the whitespace up to the first backslash determines the indentation”, so the leading white space of the next line shouldn’t cause a syntax error, though this principle has been interpreted inconsistently in different Python versions. See python/cpython#90249.
False positive (unexpected indentation) (in all Python versions):
$ cat >test_1.py <<'# EOF'
if True:
pass
\
print("1")
# EOF
$ python3.13 test_1.py
1
$ ruff --isolated check test_1.py --output-format concise -q --target-version py313
test_1.py:3:1: SyntaxError: Unexpected indentation
test_1.py:5:1: SyntaxError: Expected a statementFalse positive (missing indentation) (except in Python 3.9 only, where it is a true positive):
$ cat >test_2.py <<'# EOF'
if True:
\
print("2")
# EOF
$ python3.13 test_2.py
2
$ ruff --isolated check test_2.py --output-format concise -q --target-version py313
test_2.py:3:1: SyntaxError: Expected an indented block after `if` statement
$ python3.9 test_2.py 2>&1 | tail -n 1
IndentationError: expected an indented block
$ ruff --isolated check test_2.py --output-format concise -q --target-version py39
test_2.py:3:1: SyntaxError: Expected an indented block after `if` statementFalse negative (unexpected indentation) (except in Python 3.9 and earlier, where it is a true negative):
$ cat >test_3.py <<'# EOF'
\
\
print("3")
# EOF
$ python3.13 test_3.py 2>&1 | tail -n 1
IndentationError: unexpected indent
$ ruff --isolated check test_3.py --target-version py313
All checks passed!
$ python3.9 test_3.py
3
$ ruff --isolated check test_3.py --target-version py39
All checks passed!False positive (unexpected indentation) (except in Python 3.10 and later, where it is a true positive):
$ cat >test_4.py <<'# EOF'
\
print("4")
# EOF
$ python3.9 test_4.py
4
$ ruff --isolated check test_4.py --output-format concise -q --target-version py39
test_4.py:1:1: SyntaxError: Unexpected indentation
test_4.py:3:1: SyntaxError: Expected a statement
$ python3.10 test_4.py 2>&1 | tail -n 1
IndentationError: unexpected indent
$ ruff --isolated check test_4.py --output-format concise -q --target-version py310
test_4.py:1:1: SyntaxError: Unexpected indentation
test_4.py:3:1: SyntaxError: Expected a statementFalse positive (no matching indentation) (in all Python versions):
$ cat >test_5.py <<'# EOF'
if True:
pass
\
print("5")
# EOF
$ python3.13 test_5.py
5
$ ruff --isolated check test_5.py --output-format concise -q --target-version py313
test_5.py:3:1: SyntaxError: unindent does not match any outer indentation level
test_5.py:5:1: SyntaxError: Expected dedent, found end of fileVersion
ruff 0.12.3 (5bc81f2 2025-07-11)