What causes "Error parsing {path}: {e}" in pypa/pip?

Run a TOML linter/validator on the file (e.g. 'python -c "import tomllib,sys; tomllib.load(open(sys.argv[1],\"rb\"))" ./pyproject.toml') to surface the exact location.

pypa/pip · error · InstallationError

Error parsing {path}: {e}

Error message

Error parsing {path}: {e}

What it means

Raised when the pyproject.toml referenced by '--group' exists but fails to parse as TOML (tomllib.TOMLDecodeError). pip wraps the underlying decoder error so the user sees the offending file and the parser's detail. Dependency-group resolution cannot proceed without valid TOML.

Source

Thrown at src/pip/_internal/req/req_dependency_group.py:84

                f"[dependency-groups] data was invalid in {path}: {'; '.join(messages)}"
            ) from eg

    return resolvers


def _load_pyproject(path: str) -> dict[str, Any]:
    """
    This helper loads a pyproject.toml as TOML.

    It raises an InstallationError if the operation fails.
    """
    try:
        with open(path, "rb") as fp:
            return tomllib.load(fp)
    except FileNotFoundError:
        raise InstallationError(f"{path} not found. Cannot resolve '--group' option.")
    except tomllib.TOMLDecodeError as e:
        raise InstallationError(f"Error parsing {path}: {e}") from e
    except OSError as e:
        raise InstallationError(f"Error reading {path}: {e}") from e

View on GitHub (pinned to d7d0d0a394)

Solutions

Run a TOML linter/validator on the file (e.g. 'python -c "import tomllib,sys; tomllib.load(open(sys.argv[1],\"rb\"))" ./pyproject.toml') to surface the exact location.
Open pyproject.toml at the line/column reported in {e} and fix the syntax.
Remove any git conflict markers ('<<<<<<<', '=======', '>>>>>>>') left from an unresolved merge.
Re-validate after editing by re-running the tomllib load before retrying pip.

Example fix

# before — pyproject.toml
[dependency-groups]
dev = ["pytest  "ruff"]   # missing comma

# after
[dependency-groups]
dev = ["pytest", "ruff"]

Defensive patterns

Strategy: validation

Validate before calling

import tomllib, sys
path = sys.argv[1]
try:
    with open(path, 'rb') as f:
        tomllib.load(f)
except tomllib.TOMLDecodeError as e:
    raise SystemExit(f'fix TOML first: {e}')

Type guard

def is_valid_toml(path: str) -> bool:
    import tomllib
    try:
        with open(path, 'rb') as f:
            tomllib.load(f)
        return True
    except (tomllib.TOMLDecodeError, OSError):
        return False

Try / catch

import tomllib
try:
    with open(path, 'rb') as f:
        tomllib.load(f)
    run_pip(['install', '--group', f'{path}:dev'])
except tomllib.TOMLDecodeError as e:
    print(f'TOML invalid: {e}; edit then retry')

Prevention

Validate pyproject.toml in a pre-commit hook with a TOML linter.
Never commit a file with git conflict markers.
Run 'python -m tomllib' load in CI before pip.

When it happens

Trigger: Calling 'pip install --group ./pyproject.toml:dev' where pyproject.toml has a syntax error: unterminated string, duplicate keys, a bare value where a table is expected, mixed tabs/spaces in a multiline array, or trailing characters after a value.

Common situations: Hand-editing pyproject.toml and forgetting a quote or comma; merging a branch that left a conflict marker ('<<<<<<<') in the file; tooling that writes partial TOML; copy-pasting a snippet that uses '=' inside an inline table incorrectly.

Related errors

AI-assisted analysis of pypa/pip@d7d0d0a394 (2026-08-04). Data as JSON: /data/errors/54109bd34aeab893.json. Report an issue: GitHub.