google/python-fire · error · FireError

Missing required flags:

Error message

Missing required flags:

What it means

Python keyword-only parameters without defaults (required_kwonly) must be supplied by the caller. Fire raises FireError('Missing required flags:', missing_kwonly) naming the flags that were not provided when calling the function through the CLI.

Source

Thrown at fire/core.py:743

    """Parses the list of `args` into (varargs, kwargs), remaining_args."""
    kwargs, remaining_kwargs, remaining_args = _ParseKeywordArgs(args, fn_spec)

    # Note: _ParseArgs modifies kwargs.
    parsed_args, kwargs, remaining_args, capacity = _ParseArgs(
        fn_spec.args, fn_spec.defaults, num_required_args, kwargs,
        remaining_args, metadata)

    if fn_spec.varargs or fn_spec.varkw:
      # If we're allowed *varargs or **kwargs, there's always capacity.
      capacity = True

    extra_kw = set(kwargs) - set(fn_spec.kwonlyargs)
    if fn_spec.varkw is None and extra_kw:
      raise FireError('Unexpected kwargs present:', extra_kw)

    missing_kwonly = set(required_kwonly) - set(kwargs)
    if missing_kwonly:
      raise FireError('Missing required flags:', missing_kwonly)

    # If we accept *varargs, then use all remaining arguments for *varargs.
    if fn_spec.varargs is not None:
      varargs, remaining_args = remaining_args, []
    else:
      varargs = []

    for index, value in enumerate(varargs):
      varargs[index] = _ParseValue(value, None, None, metadata)

    varargs = parsed_args + varargs
    remaining_args += remaining_kwargs

    consumed_args = args[:len(args) - len(remaining_args)]
    return (varargs, kwargs), consumed_args, remaining_args, capacity

  return _ParseFn

View on GitHub (pinned to 716bbc23d7)

Solutions

1. Supply the missing flag: mytool run --required_a=value
2. Give the parameter a default value in the signature
3. Wrap with a check in the component to produce a friendlier message

Example fix

// before
def deploy(*, env): ...  # invoked: deploy
// after
$ deploy --env=prod   # or: def deploy(*, env='dev'): ...

spec = inspect.signature(fn)
missing = [n for n, p in spec.parameters.items()
           if p.kind == p.KEYWORD_ONLY and p.default is p.empty and n not in supplied]
if missing:
    raise TypeError(f'missing required flags: {missing}')

try:
    fire.Fire(component)
except fire.core.FireError as e:
    if 'Missing required flags' in str(e):
        print('supply required flags; see -- --help', file=sys.stderr)
        sys.exit(2)
    raise

When it happens

Trigger: Calling fn(*, required_a, required_b=2) without passing --required_a; programmatic Fire usage constructing kwargs that omit a required keyword-only arg.

Common situations: Functions converted to keyword-only args (after the * in signature) that scripts still call positionally; CI invocations missing flags that a developer passes interactively.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• The function received no value for the required argument:
• Given file path does not exist.
• Unable to load module from specified path.
• Fire can only be called on .py files.
• Fire was passed a filename which could not be found.

AI-assisted analysis of google/python-fire@716bbc23d7 (2026-08-28). Data as JSON: /api/errors/d3a49a1b5e177d80. Report an issue: GitHub.