How to create a Python decorator that can be used either with or without parameters?

Question

I\'d like to create a Python decorator that can be used either with parameters:

@redirect_output(\"somewhere.log\")
def foo():
    ....

or

Answer 1

Using keyword arguments with default values (as suggested by kquinn) is a good idea, but will require you to include the parenthesis:

@redirect_output()
def foo():
    ...

If you would like a version that works without the parenthesis on the decorator you will have to account both scenarios in your decorator code.

If you were using Python 3.0 you could use keyword only arguments for this:

def redirect_output(fn=None,*,destination=None):
  destination = sys.stderr if destination is None else destination
  def wrapper(*args, **kwargs):
    ... # your code here
  if fn is None:
    def decorator(fn):
      return functools.update_wrapper(wrapper, fn)
    return decorator
  else:
    return functools.update_wrapper(wrapper, fn)

In Python 2.x this can be emulated with varargs tricks:

def redirected_output(*fn,**options):
  destination = options.pop('destination', sys.stderr)
  if options:
    raise TypeError("unsupported keyword arguments: %s" % 
                    ",".join(options.keys()))
  def wrapper(*args, **kwargs):
    ... # your code here
  if fn:
    return functools.update_wrapper(wrapper, fn[0])
  else:
    def decorator(fn):
      return functools.update_wrapper(wrapper, fn)
    return decorator

Any of these versions would allow you to write code like this:

@redirected_output
def foo():
    ...

@redirected_output(destination="somewhere.log")
def bar():
    ...