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

You need to detect both cases, for example using the type of the first argument, and accordingly return either the wrapper (when used without parameter) or a decorator (when used with arguments).

from functools import wraps
import inspect

def redirect_output(fn_or_output):
    def decorator(fn):
        @wraps(fn)
        def wrapper(*args, **args):
            # Redirect output
            try:
                return fn(*args, **args)
            finally:
                # Restore output
        return wrapper

    if inspect.isfunction(fn_or_output):
        # Called with no parameter
        return decorator(fn_or_output)
    else:
        # Called with a parameter
        return decorator

When using the @redirect_output("output.log") syntax, redirect_output is called with a single argument "output.log", and it must return a decorator accepting the function to be decorated as an argument. When used as @redirect_output, it is called directly with the function to be decorated as an argument.

Or in other words: the @ syntax must be followed by an expression whose result is a function accepting a function to be decorated as its sole argument, and returning the decorated function. The expression itself can be a function call, which is the case with @redirect_output("output.log"). Convoluted, but true :-)