Making decorators with optional arguments [duplicate]

Answer 1

A generic decorator for decorating decorator definitions, expressing that decorated decorator accepts default arguments, which are set if none are explicitly given.

from functools import wraps

def default_arguments(*default_args, **default_kwargs):
  def _dwrapper(decorator):
    @wraps(decorator)
    def _fwrapper(*args, **kwargs):
      if callable(args[0]) and len(args) == 1 and not kwargs:
        return decorator(*default_args, **default_kwargs)(args[0])
      return decorator(*args, **kwargs)
    return _fwrapper
  return _dwrapper

It can be used in either of ways.

from functools import lru_cache   # memoization decorator from Python 3

# apply decorator to decorator post definition
lru_cache = (default_arguments(maxsize=100)) (lru_cache)  
# could also be:
#   @default_arguments(maxsize=100)
#   class lru_cache(object):
#     def __init__(self, maxsize):
#       ...
#     def __call__(self, wrapped_function):
#       ...


@lru_cache   # this works
def fibonacci(n):
  ...

@lru_cache(200)   # this also works
def fibonacci(n):
  ...

Answer 2

Here is an other solution that work also if the optional argument is a callable:

def test_equal(func=None, optional_value=None):
    if func is not None and optional_value is not None:
        # prevent user to set func parameter manually
        raise ValueError("Don't set 'func' parameter manually")
    if optional_value is None:
        optional_value = 10  # The default value (if needed)

    def inner(function):
        def func_wrapper(*args, **kwargs):
            # do something
            return function(*args, **kwargs) == optional_value

        return func_wrapper

    if not func:
        return inner
    return inner(func)

This way both syntax will work:

@test_equal
def does_return_10():
    return 10

@test_equal(optional_value=20)
def does_return_20():
    return 20

# does_return_10() return True
# does_return_20() return True