tiangolo/fastapi · error · RuntimeError

Form data requires "python-multipart" to be installed. It…

Error message

Form data requires "python-multipart" to be installed. It seems you installed "multipart" instead. 
You can remove "multipart" with: 

pip uninstall multipart

And then install "python-multipart" with: 

pip install python-multipart

What it means

RuntimeError raised by ensure_multipart_is_installed when the wrong package is present: `from python_multipart import __version__` failed (or version is too old) but `from multipart import __version__` succeeded, and then `from multipart.multipart import parse_options_header` failed with ImportError. The `multipart` package on PyPI is a different, incompatible library from `python-multipart`, which is what FastAPI/Starlette need to parse form/multipart bodies.

Solutions

Uninstall the wrong package and install the right one: pip uninstall multipart && pip install python-multipart.
Add python-multipart (not multipart) to requirements.txt / pyproject dependencies.
Install fastapi[standard], which already pulls python-multipart.
After fixing, verify with: python -c "import python_multipart; print(python_multipart.__version__)".

Example fix

# before
pip install multipart   # wrong package
# -> RuntimeError: It seems you installed "multipart" instead.

# after
pip uninstall multipart
pip install python-multipart

Defensive patterns

Strategy: fallback

Validate before calling

# Verify the correct package is importable before using Form/File
import importlib.util, sys

if importlib.util.find_spec("python_multipart") is None:
    print("python-multipart missing or wrong 'multipart' installed")
    sys.exit(1)

# also guard against the wrong package shadowing the right one
import importlib
try:
    importlib.import_module("multipart.multipart")  # parse_options_header lives here
except ImportError:
    print("the wrong 'multipart' package is installed; install python-multipart")
    sys.exit(1)

Try / catch

try:
    from python_multipart import __version__
    assert __version__ > "0.0.12"
except (ImportError, AssertionError):
    # fallback: refuse to start Form/File routes, or instruct install
    raise RuntimeError("pip install python-multipart")

Prevention

Always install python-multipart, never the bare `multipart` package.
Pin python-multipart in requirements.txt / pyproject explicitly.
Prefer fastapi[standard] so the correct parser is pulled in transitively.
After dependency changes, verify `import python_multipart` succeeds before deploying.

When it happens

Trigger: An endpoint or dependency uses Form() or File() (multipart parsing), so FastAPI calls ensure_multipart_is_installed(), but the environment has `multipart` installed instead of `python-multipart`.

Common situations: A developer ran `pip install multipart` (wrong package, same import name) instead of `pip install python-multipart`; a transitive dependency pulled in the bare `multipart` package; an old requirements file with the wrong name; fastapi[standard] not installed and the correct package missing.

Related errors

AI-assisted analysis of tiangolo/fastapi@3e8d1526d8 (2026-08-11). Data as JSON: /api/errors/93df0e4177ef02d2. Report an issue: GitHub.

Appendix: source

Thrown at fastapi/dependencies/utils.py:126

        assert __version__ > "0.0.12"
    except (ImportError, AssertionError):
        try:
            # __version__ is available in both multiparts, and can be mocked
            from multipart import (  # type: ignore[no-redef,import-untyped]
                __version__,
            )

            assert __version__
            try:
                # parse_options_header is only available in the right multipart
                from multipart.multipart import (  # type: ignore[import-untyped]
                    parse_options_header,
                )

                assert parse_options_header
            except ImportError:
                logger.error(multipart_incorrect_install_error)
                raise RuntimeError(multipart_incorrect_install_error) from None
        except ImportError:
            logger.error(multipart_not_installed_error)
            raise RuntimeError(multipart_not_installed_error) from None


def get_parameterless_sub_dependant(*, depends: params.Depends, path: str) -> Dependant:
    assert callable(depends.dependency), (
        "A parameter-less dependency must have a callable dependency"
    )
    own_oauth_scopes: list[str] = []
    if isinstance(depends, params.Security) and depends.scopes:
        own_oauth_scopes.extend(depends.scopes)
    return get_dependant(
        path=path,
        call=depends.dependency,
        scope=depends.scope,
        own_oauth_scopes=own_oauth_scopes,
    )

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