How do __enter__ and __exit__ work in Python decorator classes?

Question

I'm trying to create a decorator class that counts how many times a function is called, but I'm getting an error message that says:

    "TypeError: __exit__() takes exactly 1 argument (4 given)" 

and I really don't know how I'm giving it four arguments. My code looks like this:

class fcount2(object):     __instances = {}     def __init__(self, f):         self.__f = f         self.__numcalls = 0         fcount2.__instances[f] = self      def __call__(self, *args, **kwargs):         self.__numcalls += 1         return self.__f(*args, **kwargs)      def __enter__(self):         return self      def __exit__(self):         return self      @staticmethod     def count(f):         return fcount2.__instances[self.__f].__numcalls   @fcount2 def f(n):     return n+2  for n in range(5):     print f(n)    print 'f count =',f.count  def foo(n):     return n*n  with fcount2(foo) as g:     print g(1)     print g(2) print 'g count =',g.count print 'f count =',f.count  with fcount2(f) as g:     print g(1)     print g(2) print 'g count =',g.count print 'f count =',f.count  with f:     print f(1)     print g(2) print 'g count =',g.count print 'f count =',f.count 

Are there some other parameters I should (or shouldn't) be passing into the def exit function? Any tips or ideas would be appreciated.

As an aside, my line of code that says "print 'f count =',f.count" appears to be outputting the memory address rather than the value, but that's a whole different problem.

Answer 1

the __exit__() method should accept information about exceptions that come up in the with: block. See here.

The following modification of your code works:

def __exit__(self, exc_type, exc_value, tb):     if exc_type is not None:         traceback.print_exception(exc_type, exc_value, tb)         # return False # uncomment to pass exception through      return True 

Then you can try raising an exception in one of your with: blocks and it'll be caught in __exit__().