Python tricks: making sure a function is only called once

Sometimes I want a function to be called only a single time (there are many use-cases, but specifically this time I wanted to change the way the Python garbage collection worked when dealing with Qt and threads).

Anyways, usually I did that setting some flag so that the second time a function is called, it'd check that flag and would skip the function in the second time.


After tinkering a bit about it, I came with a shorter version changing the function code which I thought is pretty nice:

 def func():  
   print 'Calling func only this time'  
   func.func_code = (lambda:None).func_code  

Now, calling it a second time will actually execute the lamda:None code.

Yes, I know you'll say it's cryptic, so, I'm including a decorator version below which does the same checking with a variable:

 @call_only_once  
 def func():  
   print 'Calling func only this time'  
 def call_only_once(func):  
   def new_func(*args, **kwargs):  
     if not new_func._called:  
       try:  
         return func(*args, **kwargs)  
       finally:  
         new_func._called = True  
   new_func._called = False  
   return new_func