how to do a conditional decorator in python

A decorator is simply a function applied to another function. You can apply it manually:

def foo():
   # whatever
   time.sleep(2)

if doing_performance_analysis:
    foo = timeit(foo)

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.