Core Python Programming (2nd Edition)

● tHReading.BoundedSemaphore
Since files are first on the list and the simplest example, here is a code snippet of what it looks like to
use a with statement:
with open('/etc/passwd', 'r') as f:
for eachLine in f:
# ...do stuff with eachLine or f...
What this code snippet will do is... well, this is Python, so you can probably already guess. It will do
some preliminary work, such as attempt to open the file, and if all goes well, assign the file object to f.
Then it iterates over each line in the file and does whatever processing you need to do. Once the file has
been exhausted, it is closed. If an exception occurs either at the beginning, middle, or end of the block,
then some cleanup code must be done, but the file will still be closed automatically.
Now, because a lot of the details have been pushed down and away from you, there are really two levels
of processing that need to occur: First, the stuff at the user levelas in, the things you need to take care
of as the user of the objectand second, at the object level. Since this object supports the context
management protocol, it has to do some "context management."
10.4.2. *Context Management Protocol
Unless you will be designing objects for users of the with statement, i.e., programmers who will be using
your objects to design their applications with, most Python programmers are going to be just users of
the with statement and can skip this optional section.
We are not going into a full and deep discussion about context management here, but we will explain
the types of objects and the functionality that are necessary to be protocol-compliant and thus be
eligible to be used with the with statement.
Previously, we described a little of how the protocol works in our example with the file object. Let us
elaborate some more here.
Context Expression (context_expr), Context Manager
When the with statement is executed, the context expression is evaluated to obtain what is called a
context manager. The job of the context manager is to provide a context object. It does this by invoking
its required __context__() special method. The return value of this method is the context object that will
be used for this particular execution of the with_suite. One side note is that a context object itself can
be its own manager, so context_expr can really be either a real context manager or a context object
serving as its own manager. In the latter case, the context object also has a __context__() method,
which returns self, as expected.
Context Object, with_suite
Once we have a context object, its __enter__() special method is invoked. This does all the preliminary
stuff before the with_suite executes. You will notice in the syntax above that there is an optional as var
piece following context_expr on the with statement line. If var is provided, it is assigned the return value
of __enter__(). If not, the return value is thrown away. So for our file object example, its context
object's __enter__() returns the file object so it can be assigned to f.