Counting python method calls within another method

Question

I'm actually trying doing this in Java, but I'm in the process of teaching myself python and it made me wonder if there was an easy/clever way to do this with wrappers or something.

I want to know how many times a specific method was called inside another method. For example:

def foo(z):
    #do something
    return result

def bar(x,y):
    #complicated algorithm/logic involving foo
    return foobar

So for each call to bar with various parameters, I'd like to know how many times foo was called, perhaps with output like this:

>>> print bar('xyz',3)
foo was called 15 times
[results here]
>>> print bar('stuv',6)
foo was called 23 times
[other results here]

edit: I realize I could just slap a counter inside bar and dump it when I return, but it would be cool if there was some magic you could do with wrappers to accomplish the same thing. It would also mean I could reuse the same wrappers somewhere else without having to modify any code inside the method.

Answer 1

Sounds like almost the textbook example for decorators!

def counted(fn):
    def wrapper(*args, **kwargs):
        wrapper.called += 1
        return fn(*args, **kwargs)
    wrapper.called = 0
    wrapper.__name__ = fn.__name__
    return wrapper

@counted
def foo():
    return

>>> foo()
>>> foo.called
1

You could even use another decorator to automate the recording of how many times a function is called inside another function:

def counting(other):
    def decorator(fn):
        def wrapper(*args, **kwargs):
            other.called = 0
            try:
                return fn(*args, **kwargs)
            finally:
                print '%s was called %i times' % (other.__name__, other.called)
        wrapper.__name__ = fn.__name__
        return wrapper
    return decorator

@counting(foo)
def bar():
    foo()
    foo()

>>> bar()
foo was called 2 times

If foo or bar can end up calling themselves, though, you'd need a more complicated solution involving stacks to cope with the recursion. Then you're heading towards a full-on profiler...

Possibly this wrapped decorator stuff, which tends to be used for magic, isn't the ideal place to be looking if you're still ‘teaching yourself Python’!