Skip Montanaro wrote: > After seeing so many messages about "with" statements my eyes began to glaze > over, so I stopped following that thread. Then I saw mention of "context > manager" with no reference to any PEPs or to the with statement to provide > context.
The main outcome of the PEP 343 terminology discussion was some proposed documentation I put on the Sourceforge patch tracker ([1]). The patch is currently assigned to Raymond (since he started the terminology discussion) but any other reviews would be welcome. Since SF currently doesn't want to play, and the proposed documentation isn't that long, I've included the latest version below for anyone who wants to read it. > None of the context-providing messages seemed to have been indexed > by Google when I checked, so searching for "Python context manager" failed > to return anything useful. Hence the post. Google appears to have spidered the list archives some time today, so anyone else doing the same search should get some relevant hits. Cheers, Nick. [1] http://www.python.org/sf/1234057 ========================================== With Statements and Context Management A frequent need in programming is to ensure a particular action is taken after a specific section of code has been executed (such as closing a file or releasing a lock). Traditionally, this is handled using 'try'/'finally' statements. However, that approach can lead to the reproduction of non-trivial amounts of boilerplate whenever the action needs to be invoked. A simpler way to achieve this in Python is to use the 'with' statement along with the appropriate context manager. Context managers define an action which is taken to enter the context and a second action to exit the context (usually restoring the environment that existed before the context was entered). The 'with' statement ensures that the context is entered and exited at the appropriate times (that is, before and after the execution of the suite contained in the 'with' statement). The precise behaviour of the 'with' statement is governed by the supplied context manager - an object which supports the context management protocol. This protocol consists of two methods: __enter__(self): Context managers use this method to enter the desired context before the execution of the contained suite. This method is called without arguments before execution of the contained suite starts. If the 'as' clause of the 'with' statement is used, the value returned from this method is assigned to the specified target. Many context managers will return self from this method, but returning a different object may make sense for some managers (for example, see the 'closing' suite manager described below). __exit__(self, exc_type, exc_value, exc_traceback): Context managers use this method to exit the context after execution of the contained suite. This method is called after execution of the contained suite is completed. If execution completed due to an exception, the details of that exception are passed as arguments. Otherwise, all three arguments are set to None. If exception details are passed in, and this method returns without incident, then the original exception continues to propagate. Otherwise, the exception raised by this method will replace the original exception. Using Contexts to Manage Resources The simplest use of context management is to strictly control the handling of key resources (such as files, generators, database connections, synchronisation locks). These resource managers will generally acquire the resource in their __enter__ method, although some resource managers may accept the resource to be managed as an argument to the constructor or acquire it during construction. Resource managers will then release the resource in their __exit__ method. For example, the following context manager allows prompt closure of any resource with a 'close' method (e.g. a generator or file): class closing(object): def __init__(self, resource): self.resource = resource def __enter__(self): return self.resource def __exit__(self, *exc_info): self.resource.close() with closing(my_generator()) as g: # my_generator() is assigned to g via call to __enter__() for item in g: print item # g is closed as the with statement ends Some resources (such as threading.Lock) support the context management protocol natively, allowing them to be used directly in 'with' statements. The meaning of the established context will depend on the specific resource. In the case of threading.Lock, the lock is acquired by the __enter__ method, and released by the __exit__ method. with the_lock: # Suite is executed with the_lock held # the_lock is released as the with statement ends More Context Management Examples While resource management may be the most obvious use of the context management protocol, many more uses are possible (otherwise it would have been called the resource management protocol!). For example, when used as a context manager, a decimal.Context object will set itself as the current Decimal arithmetic context in the __enter__ method, and then automatically revert back to the previous Decimal arithmetic context in the __exit__ method. This allows the code in the contained suite to manipulate the Decimal arithmetic context freely, without needing to worry about manually undoing any changes. with decimal.getcontext() as ctx: ctx.prec = 48 # Perform high precision calculations within the context # Precision reverts to default here Another example is the use of contexts to handle insertion of the appropriate start and end tags when generating HTML: class tag(object): def __init__(self,name): self.name = name def __enter__(self): print "<%s>" % self.name def __exit__(self, exc_type, *exc_details): if exc_type is None: print "</%s>" % self.name with tag('html'): with tag('body'): with tag('h1'): print "Some heading" with tag('p'): print "This is paragraph 1" with tag('p'): print "This is paragraph 2" with tag('h2'): print "Another heading" Some other possibilities for context management include automatic exception logging and handling of database transactions. Using Generators to Define Context Managers In conjunction with the 'contextmanager' decorator, Python's generators provide a convenient way to implement the context management protocol, and share state between the __enter__ and __exit__ methods. The generator must yield exactly once during normal execution. The context manager's __enter__ method executes the generator up to that point, and the value yielded is returned. The remainder of the generator is executed by the context manager's __exit__ method. Any exceptions that occur in the managed context will be injected into the generator at the location of the yield statement. For example, here are the 'closing' and 'tag' context manager examples written using generators: @contextmanager def closing(resource): try: yield resource finally: resource.close() @contextmanager def tag(name): print "<%s>" % name yield None print "</%s>" % name The operation of the contextmanager decorator is described by the following Python equivalent (although the exact error messages may differ): class ContextWrapper(object): def __init__(self, gen): self.gen = gen def __enter__(self): try: return self.gen.next() except StopIteration: raise RuntimeError("generator didn't yield") def __exit__(self, type, value, traceback): if type is None: try: self.gen.next() except StopIteration: return else: raise RuntimeError("generator didn't stop") else: try: self.gen.throw(type, value, traceback) except (type, StopIteration): return else: raise RuntimeError("generator didn't stop") def contextmanager(func): def factory(*args, **kwds): return ContextWrapper(func(*args, **kwds)) factory.__name__ = func.__name__ factory.__doc__ = func.__doc__ factory.__dict__ = func.__dict__ return factory ========================================== -- Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia --------------------------------------------------------------- http://boredomandlaziness.blogspot.com _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
