A "scopeguard" for Python

Alf P. Steinbach alfps at start.no
Fri Mar 5 20:13:23 CET 2010


* Mike Kent:
> On Mar 4, 8:04 pm, Robert Kern <robert.k... at gmail.com> wrote:
> 
>> No, the try: finally: is not implicit. See the source for
>> contextlib.GeneratorContextManager. When __exit__() gets an exception from the
>> with: block, it will push it into the generator using its .throw() method. This
>> raises the exception inside the generator at the yield statement.
> 
> Wow, I just learned something new.  My understanding of context
> managers was that the __exit__ method was guaranteed to be executed
> regardless of how the context was left.  I have often written my own
> context manager classes, giving them the __enter__ and __exit__
> methods.  I had mistakenly assumed that the @contextmanager decorator
> turned a generator function into a context manager with the same
> behavior as the equivalent context manager class.  Now I learn that,
> no, in order to have the 'undo' code executed in the presence of an
> exception, you must write your own try/finally block in the generator
> function.
> 
> This raises the question in my mind: What's the use case for using
> @contextmanager rather than wrapping your code in a context manager
> class that defines __enter__ and __exit__, if you still have to
> manager your own try/finally block?

Robert Kern and Steve Howell have already given given good answers.

As it happened this was news to me also, because I'm not that well-versed in 
Python and it seems contrary to the purpose of providing a simpler way to write 
a simple init-cleanup wrapper.

But additionally, if you want that, then you can define it, e.g.


<code>
# Py3

def simplecleanup( generator_func ):
     class SimpleCleanup:
         def __init__( self, *args, **kwargs ):
             self.generator = generator_func( *args, **kwargs )

         def __enter__( self ):
             self.generator.send( None )
             return self

         def __exit__( self, x_type, x_obj, x_traceback ):
             try:
                 self.generator.send( x_obj )    # x_obj is None if no exception
             except StopIteration:
                 pass                            # Expected

     return SimpleCleanup


@simplecleanup
def hello_goodbye( name ):
     print( "Hello, {}!".format( name ) )
     yield
     print( "Goodbye {}!".format( name ) )


try:
     with hello_goodbye( "Mary" ):
         print( "Talk talk talk..." )
         raise RuntimeError( "Offense" )
except:
     pass
print()


@simplecleanup
def sensitive_hello_goodbye( name ):
     print( "Hello, {}!".format( name ) )
     x = yield
     if x is not None:
         print( "Uh oh, {}!".format( x ) )
         print( "Good day {}!".format( name ) )
     else:
         print( "C u, {}!".format( name ) )


try:
     with sensitive_hello_goodbye( "Jane" ):
         print( "Talk talk talk..." )
         raise RuntimeError( "Offense" )
except:
     pass
</code>


Cheers,

- Alf



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