pandas-dev/pandas · error · KeyError

Invalid parser '{parser}' passed, valid parsers are {PARSERS

Error message

Invalid parser '{parser}' passed, valid parsers are {PARSERS.keys()}

What it means

Raised by _check_parser (pandas/core/computation/eval.py:97) as a KeyError when the `parser` argument to pd.eval / DataFrame.eval is not one of the keys in PARSERS (typically 'pandas' and 'python'). The parser determines how the expression string is tokenized (e.g. whether @-prefix locals or special syntax is allowed), so an unknown parser is rejected before parsing.

Source

Thrown at pandas/core/computation/eval.py:97

    return engine


def _check_parser(parser: str) -> None:
    """
    Make sure a valid parser is passed.

    Parameters
    ----------
    parser : str

    Raises
    ------
    KeyError
      * If an invalid parser is passed
    """
    if parser not in PARSERS:
        raise KeyError(
            f"Invalid parser '{parser}' passed, valid parsers are {PARSERS.keys()}"
        )


def _check_resolvers(resolvers) -> None:
    if resolvers is not None:
        for resolver in resolvers:
            if not hasattr(resolver, "__getitem__"):
                name = type(resolver).__name__
                raise TypeError(
                    f"Resolver of type '{name}' does not "
                    "implement the __getitem__ method"
                )


def _check_expression(expr) -> None:
    """
    Make sure an expression is not an empty string

View on GitHub (pinned to 71959b8cb9)

Solutions

1. Use a valid parser: 'pandas' or 'python'.
2. Omit parser to use the default ('pandas' for DataFrame.eval).
3. If you need @-local variable access, use parser='pandas' (the '@' prefix is only supported by the pandas parser).

Example fix

# before
pd.eval('a + b', parser='pyparsing')

# after
pd.eval('a + b', parser='pandas')

from pandas.core.computation.expr import PARSERS

def validate_parser(parser):
    if parser not in PARSERS:
        raise KeyError(f"Invalid parser '{parser}', valid: {list(PARSERS)}")
    return parser

from pandas.core.computation.expr import PARSERS

def is_valid_parser(parser) -> bool:
    return parser in PARSERS

try:
    result = pd.eval('a + b', parser=parser)
except KeyError as e:
    if 'Invalid parser' in str(e):
        result = pd.eval('a + b', parser='pandas')
    else:
        raise

When it happens

Trigger: `pd.eval('a + b', parser='pyparsing')`, `df.eval('a + b', parser='custom')`, a typo, or passing a parser name from a different pandas version.

Common situations: Typos, guessing parser names, code migrated across versions where parser naming changed, or dynamic config feeding an unvalidated value.

Related errors

• Invalid engine '{engine}' passed, valid engines are {valid_e
• Label(s) {list(cols)} do not exist
• expr cannot be an empty string
• The '@' prefix is only supported by the pandas parser
• only a single expression is allowed

AI-assisted analysis of pandas-dev/pandas@71959b8cb9 (2026-08-07). Data as JSON: /api/errors/3fbe914ca7d24f2e. Report an issue: GitHub.