Skipping execution of -with- block

Question

I am defining a context manager class and I would like to be able to skip the block of code without raising an exception if certain conditions are met during instantiation. For example,

class My_Context(object):     def __init__(self,mode=0):         """         if mode = 0, proceed as normal         if mode = 1, do not execute block         """         self.mode=mode     def __enter__(self):         if self.mode==1:             print 'Exiting...'             CODE TO EXIT PREMATURELY     def __exit__(self, type, value, traceback):         print 'Exiting...'  with My_Context(mode=1):     print 'Executing block of codes...' 

Answer 1

If you want an ad-hoc solution that uses the ideas from withhacks (specifically from AnonymousBlocksInPython), this will work:

import sys import inspect  class My_Context(object):     def __init__(self,mode=0):         """         if mode = 0, proceed as normal         if mode = 1, do not execute block         """         self.mode=mode     def __enter__(self):         if self.mode==1:             print 'Met block-skipping criterion ...'             # Do some magic             sys.settrace(lambda *args, **keys: None)             frame = inspect.currentframe(1)             frame.f_trace = self.trace     def trace(self, frame, event, arg):         raise     def __exit__(self, type, value, traceback):         print 'Exiting context ...'         return True 

Compare the following:

with My_Context(mode=1):     print 'Executing block of code ...' 

with

with My_Context(mode=0):     print 'Executing block of code ... '