how to do a conditional decorator in python

Question

Is it possible to decorator a function conditionally. For example, I want to decorate the function foo() with a timer function (timeit) only doing_performance_analysis is True (see the psuedo-code below).

if doing_performance_analysis:
  @timeit
  def foo():
    """
    do something, timeit function will return the time it takes
    """
    time.sleep(2)
else:
  def foo():
    time.sleep(2)  

Answer 1

Decorators are simply callables that return a replacement, optionally the same function, a wrapper, or something completely different. As such, you could create a conditional decorator:

def conditional_decorator(dec, condition):
    def decorator(func):
        if not condition:
            # Return the function unchanged, not decorated.
            return func
        return dec(func)
    return decorator

Now you can use it like this:

@conditional_decorator(timeit, doing_performance_analysis)
def foo():
    time.sleep(2)  

The decorator could also be a class:

class conditional_decorator(object):
    def __init__(self, dec, condition):
        self.decorator = dec
        self.condition = condition

    def __call__(self, func):
        if not self.condition:
            # Return the function unchanged, not decorated.
            return func
        return self.decorator(func)

Here the __call__ method plays the same role as the returned decorator() nested function in the first example, and the closed-over dec and condition parameters here are stored as arguments on the instance until the decorator is applied.