* Mike Kent:
On Mar 4, 8:04 pm, Robert Kern <robert.k...@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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