Support flag-only arguments

[MaxG87]

I would like to have arguments that do not accept any value. Thus,

./my-script.py --flag

should work, but

./my-script.py --flag SomeValue

should not. In argparse this can be achieved by

parser.add_argument("--flag", action="store_true")

but I do not know how to achieve that in python-fire.

[dbieber]

Current status

./my-script.py --flag

should work already,
however

./my-script.py --flag SomeValue

will also be accepted by Fire.

---

Possible workaround

You can raise a FireError in your function if you receive an invalid input with a message for the user to indicate an input error.

Using this strategy you can write decorators for yourself to satisfy your needs.

E.g.

import fire

def bool_only(fn):
  def fn2(arg):
    if arg not in [True, False]:
      raise fire.core.FireError('Invalid input, bool required.')
    return fn(arg)
  return fn2

@bool_only
def my_script(value):
  return not value

fire.Fire(my_script)

---

Possible Future Solution

We're currently discussing adding a "strict" mode to fire, which uses type annotations or default values to do type checking. If this were enabled, then you could annotate an argument as a boolean and only a boolean could be passed. --flag=True would still work, but --flag=SomeValue would not.