Add a Context Types section to parallel the Iterator Types section (uses the…

Add a Context Types section to parallel the Iterator Types section (uses the same terminology as the 2.5a1 implementation)

Doc/lib/libstdtypes.tex

@@ -1753,6 +1753,102 @@ implemented in C will have to provide a writable
 \end{memberdesc}
 
 
+\subsection{Context Types \label{typecontext}}
+
+\versionadded{2.5}
+\index{context protocol}
+\index{context management protocol}
+\index{protocol!context}
+\index{protocol!context management}
+
+Python's \keyword{with} statement supports the concept of a runtime
+context defined by a context object.  This is implemented using three
+distinct methods; these are used to allow user-defined classes to
+define a context.
+
+The \dfn{context protocol} consists of a single method that needs
+to be provided for an object to define a runtime context:
+
+\begin{methoddesc}[context]{__context__}{}
+  Return a context manager object.  The object is required to support
+  the context management protocol described below.  If an object
+  supports different kinds of runtime context, additional methods can
+  be provided to specifically request context managers for those
+  kinds of context.  (An example of an object supporting multiple kinds
+  of context would be a synchronisation object which supported both
+  a locked context for normal thread synchronisation and an unlocked
+  context to temporarily release a held lock while performing a
+  potentially long running operation)
+\end{methoddesc}
+
+The context manager objects themselves are required to support the
+following three methods, which together form the
+\dfn{context management protocol}:
+
+\begin{methoddesc}[context manager]{__context__}{}
+  Return the context manager object itself.  This is required to
+  allow both contexts and context managers to be used with the
+  \keyword{with} statement.
+\end{methoddesc}
+
+\begin{methoddesc}[context manager]{__enter__}{}
+  Set up the runtime context and return either the defining context
+  object or another object related to the runtime context. The value
+  returned by this method is bound to the identifier in the
+  \keyword{as} clause of \keyword{with} statements using this context.
+  (An example of a context with a context manager that returns the
+  original context object is file objects, which are returned from
+  __enter__() to allow \function{open()} to be used directly in a with
+  statement. An example of a context manager that returns a related
+  object is \code{decimal.Context} which returns a copy of the original
+  context to allow changes to be made to the current decimal context
+  without affecting code outside the \keyword{with} statement).
+\end{methoddesc}
+
+\begin{methoddesc}[context manager]{__exit__}{exc_type, exc_val, exc_tb}
+  Tear down the runtime context and return a Boolean flag indicating if
+  an expection that occurred should be suppressed. If an exception
+  occurred while executing the body of the \keyword{with} statement, the
+  arguments contain the exception type, value and traceback information.
+  Otherwise, all three arguments are \var{None}.
+  Returning a true value from this method will cause the \keyword{with}
+  statement to suppress the exception and continue execution with the
+  statement immediately following the \keyword{with} statement. Otherwise
+  the exception continues propagating after this method has finished
+  executing. Exceptions that occur during execution of this method will
+  replace any exception that occurred in the body of the \keyword{with}
+  statement.
+  The exception passed in should never be reraised explicitly - instead,
+  this method should return a false value to indicate that the method
+  completed successfully and does not want to suppress the raised
+  exception. This allows context management code (such as
+  \code{contextlib.nested}) to easily detect whether or not an
+  \method{__exit__()} method has actually failed.
+\end{methoddesc}
+
+Python defines several context objects to support easy thread
+synchronisation, prompt closure of files or other objects, and
+thread-safe manipulation of the decimal arithmetic context. The
+specific types are not important beyond their implementation of
+the context protocol.
+
+Python's generators and the \code{contextlib.contextmanager}
+decorator provide a convenient way to implement the context
+and context management protocols.  If a context object's
+\method{__context__()} method is implemented as a generator decorated
+with the \code{contextlib.contextmanager} decorator, it will
+automatically return a context manager object supplying the
+necessary \method{__context__()}, \method{__enter__()} and
+\method{__exit__()} methods.
+
+Note that there is no specific slot for any of these methods in the
+type structure for Python objects in the Python/C API. Extension
+types wanting to define these methods must provide them as a normal
+Python accessible method. Compared to the overhead of setting up the
+runtime context, the overhead of a single class dictionary lookup
+is negligible.
+
+
 \subsection{Other Built-in Types \label{typesother}}
 
 The interpreter supports several other kinds of objects.