pytest-dev/pytest · error · SyntaxError

escaping with "\" not supported in marker expression

Error message

escaping with "\" not supported in marker expression

What it means

A SyntaxError raised by the marker-expression tokenizer when a quoted string value contains a backslash. pytest's expression grammar deliberately rejects escape sequences; the offending backslash position is reported via the SyntaxError offset tuple. It fires only after a successful close-quote match, while scanning the captured value.

Solutions

1. Remove all backslashes from the marker value, or replace them with forward slashes / character classes.
2. Pass complex data via a fixture or environment variable instead of the marker expression string.
3. Define the marker with the value in code (not via -m) so escaping is handled by Python, not the expression grammar.

Example fix

# before
pytest -m "my_mark(path='C:\\temp')"
# after
pytest -m "my_mark(path='C:/temp')"

def is_safe_marker_value(value: str) -> bool:
    return '\\' not in value

try:
    Expression.compile(expr)
except SyntaxError as e:
    if 'escaping' in e.msg:
        value = value.replace('\\', '/')
        ...

When it happens

Trigger: Using `-m "my_mark(path='C:\\temp')"`, `-m "my_mark(regex='a\\b')"`, or any marker kwarg value containing a backslash. Even a single backslash before the closing quote triggers it because value.find('\\') is non-negative.

Common situations: Windows file paths or regex patterns passed as marker kwarg values through -m; copying a Python string literal (with escapes) into a CLI marker expression.

Related errors

• closing quote " " is missing
• unexpected character
• : : at column
• expected ; got
• not a valid python identifier

AI-assisted analysis of pytest-dev/pytest@0d6fbdeffa (2026-08-11). Data as JSON: /api/errors/f389a931ee3d64df. Report an issue: GitHub.

Appendix: source

Thrown at src/_pytest/mark/expression.py:106

            elif input[pos] == ")":
                yield Token(TokenType.RPAREN, ")", pos)
                pos += 1
            elif input[pos] == "=":
                yield Token(TokenType.EQUAL, "=", pos)
                pos += 1
            elif input[pos] == ",":
                yield Token(TokenType.COMMA, ",", pos)
                pos += 1
            elif (quote_char := input[pos]) in ("'", '"'):
                end_quote_pos = input.find(quote_char, pos + 1)
                if end_quote_pos == -1:
                    raise SyntaxError(
                        f'closing quote "{quote_char}" is missing',
                        (FILE_NAME, 1, pos + 1, input),
                    )
                value = input[pos : end_quote_pos + 1]
                if (backslash_pos := value.find("\\")) != -1:
                    raise SyntaxError(
                        r'escaping with "\" not supported in marker expression',
                        (FILE_NAME, 1, pos + backslash_pos + 1, input),
                    )
                yield Token(TokenType.STRING, value, pos)
                pos += len(value)
            else:
                match = re.match(r"(:?\w|:|\+|-|\.|\[|\]|\\|/)+", input[pos:])
                if match:
                    value = match.group(0)
                    if value == "or":
                        yield Token(TokenType.OR, value, pos)
                    elif value == "and":
                        yield Token(TokenType.AND, value, pos)
                    elif value == "not":
                        yield Token(TokenType.NOT, value, pos)
                    else:
                        yield Token(TokenType.IDENT, value, pos)
                    pos += len(value)

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