Catching exception in context manager __enter__()

Question

Is it possible to ensure the __exit__() method is called even if there is an exception in __enter__()?

>>> class TstContx(object): ...    def __enter__(self): ...        raise Exception('Oops in __enter__') ... ...    def __exit__(self, e_typ, e_val, trcbak): ...        print "This isn't running" ...  >>> with TstContx(): ...     pass ...  Traceback (most recent call last):   File "<stdin>", line 1, in <module>   File "<stdin>", line 3, in __enter__ Exception: Oops in __enter__ >>>  

Edit

This is as close as I could get...

class TstContx(object):     def __enter__(self):         try:             # __enter__ code         except Exception as e             self.init_exc = e          return self      def __exit__(self, e_typ, e_val, trcbak):         if all((e_typ, e_val, trcbak)):             raise e_typ, e_val, trcbak          # __exit__ code   with TstContx() as tc:     if hasattr(tc, 'init_exc'): raise tc.init_exc      # code in context 

In hind sight, a context manager might have not been the best design decision

Answer 1

Like this:

import sys  class Context(object):     def __enter__(self):         try:             raise Exception("Oops in __enter__")         except:             # Swallow exception if __exit__ returns a True value             if self.__exit__(*sys.exc_info()):                 pass             else:                 raise       def __exit__(self, e_typ, e_val, trcbak):         print "Now it's running"   with Context():     pass 

To let the program continue on its merry way without executing the context block you need to inspect the context object inside the context block and only do the important stuff if __enter__ succeeded.

class Context(object):     def __init__(self):         self.enter_ok = True      def __enter__(self):         try:             raise Exception("Oops in __enter__")         except:             if self.__exit__(*sys.exc_info()):                 self.enter_ok = False             else:                 raise         return self      def __exit__(self, e_typ, e_val, trcbak):         print "Now this runs twice"         return True   with Context() as c:     if c.enter_ok:         print "Only runs if enter succeeded"  print "Execution continues" 

As far as I can determine, you can't skip the with-block entirely. And note that this context now swallows all exceptions in it. If you wish not to swallow exceptions if __enter__ succeeds, check self.enter_ok in __exit__ and return False if it's True.

Answer 2

No. If there is the chance that an exception could occur in __enter__() then you will need to catch it yourself and call a helper function that contains the cleanup code.