On 4/29/05, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote: > > Message: 2 > Date: Thu, 28 Apr 2005 21:56:42 -0600 > From: Shane Hathaway <[EMAIL PROTECTED]> > Subject: Re: [Python-Dev] Re: anonymous blocks > To: [EMAIL PROTECTED] > Cc: Ka-Ping Yee <[EMAIL PROTECTED]>, Python Developers List > <python-dev@python.org> > Message-ID: <[EMAIL PROTECTED]> > Content-Type: text/plain; charset=ISO-8859-1 > > > I think this concept can be explained clearly. I'd like to try > explaining PEP 340 to someone new to Python but not new to programming. > I'll use the term "block iterator" to refer to the new type of > iterator. This is according to my limited understanding. > > "Good programmers move commonly used code into reusable functions. > Sometimes, however, patterns arise in the structure of the functions > rather than the actual sequence of statements. For example, many > functions acquire a lock, execute some code specific to that function, > and unconditionally release the lock. Repeating the locking code in > every function that uses it is error prone and makes refactoring difficult. > > "Block statements provide a mechanism for encapsulating patterns of > structure. Code inside the block statement runs under the control of an > object called a block iterator. Simple block iterators execute code > before and after the code inside the block statement. Block iterators > also have the opportunity to execute the controlled code more than once > (or not at all), catch exceptions, or receive data from the body of the > block statement. > > "A convenient way to write block iterators is to write a generator. A > generator looks a lot like a Python function, but instead of returning a > value immediately, generators pause their execution at "yield" > statements. When a generator is used as a block iterator, the yield > statement tells the Python interpreter to suspend the block iterator, > execute the block statement body, and resume the block iterator when the > body has executed. > > "The Python interpreter behaves as follows when it encounters a block > statement based on a generator. First, the interpreter instantiates the > generator and begins executing it. The generator does setup work > appropriate to the pattern it encapsulates, such as acquiring a lock, > opening a file, starting a database transaction, or starting a loop. > Then the generator yields execution to the body of the block statement > using a yield statement. When the block statement body completes, > raises an uncaught exception, or sends data back to the generator using > a continue statement, the generator resumes. At this point, the > generator can either clean up and stop or yield again, causing the block > statement body to execute again. When the generator finishes, the > interpreter leaves the block statement." > > Is it understandable so far? >
I've been skipping most of the anonymous block discussion and thus, I only had a very vague idea of what it was about until I read this explanation. Yes, it is understandable -- assuming it's correct :-) Mind you though, I'm not new to python and I've been writing system software for 20+ years. -- Luis P Caamano Atlanta, GA USA _______________________________________________ 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
