Issue #22952: improve multiprocessing doc introduction and defer notes until appropriate.

Patch by Davin Potts.

Doc/library/multiprocessing.rst

@@ -18,41 +18,27 @@ to this, the :mod:`multiprocessing` module allows the programmer to fully
 leverage multiple processors on a given machine.  It runs on both Unix and
 Windows.
 
-.. warning::
+The :mod:`multiprocessing` module also introduces APIs which do not have
+analogs in the :mod:`threading` module.  A prime example of this is the
+:class:`Pool` object which offers a convenient means of parallelizing the
+execution of a function across multiple input values, distributing the
+input data across processes (data parallelism).  The following example
+demonstrates the common practice of defining such functions in a module so
+that child processes can successfully import that module.  This basic example
+of data parallelism using :class:`Pool`, ::
 
-    Some of this package's functionality requires a functioning shared semaphore
-    implementation on the host operating system. Without one, the
-    :mod:`multiprocessing.synchronize` module will be disabled, and attempts to
-    import it will result in an :exc:`ImportError`. See
-    :issue:`3770` for additional information.
+   from multiprocessing import Pool
 
-.. note::
+   def f(x):
+       return x*x
+
+   if __name__ == '__main__':
+       p = Pool(5)
+       print(p.map(f, [1, 2, 3]))
+
+will print to standard output ::
 
-    Functionality within this package requires that the ``__main__`` module be
-    importable by the children. This is covered in :ref:`multiprocessing-programming`
-    however it is worth pointing out here. This means that some examples, such
-    as the :class:`multiprocessing.Pool` examples will not work in the
-    interactive interpreter. For example::
-
-        >>> from multiprocessing import Pool
-        >>> p = Pool(5)
-        >>> def f(x):
-        ...     return x*x
-        ...
-        >>> p.map(f, [1,2,3])
-        Process PoolWorker-1:
-        Process PoolWorker-2:
-        Process PoolWorker-3:
-        Traceback (most recent call last):
-        Traceback (most recent call last):
-        Traceback (most recent call last):
-        AttributeError: 'module' object has no attribute 'f'
-        AttributeError: 'module' object has no attribute 'f'
-        AttributeError: 'module' object has no attribute 'f'
-
-    (If you try this it will actually output three full tracebacks
-    interleaved in a semi-random fashion, and then you may have to
-    stop the master process somehow.)
+   [1, 4, 9]
 
 
 The :class:`Process` class
@@ -99,7 +85,6 @@ For an explanation of why (on Windows) the ``if __name__ == '__main__'`` part is
 necessary, see :ref:`multiprocessing-programming`.
 
 
-
 Exchanging objects between processes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -175,6 +160,14 @@ that only one process prints to standard output at a time::
 Without using the lock output from the different processes is liable to get all
 mixed up.
 
+.. warning::
+
+   Some of this package's functionality requires a functioning shared semaphore
+   implementation on the host operating system. Without one, the
+   :mod:`multiprocessing.synchronize` module will be disabled, and attempts to
+   import it will result in an :exc:`ImportError`. See
+   :issue:`3770` for additional information.
+
 
 Sharing state between processes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -290,6 +283,34 @@ For example::
 Note that the methods of a pool should only ever be used by the
 process which created it.
 
+.. note::
+
+   Functionality within this package requires that the ``__main__`` module be
+   importable by the children. This is covered in :ref:`multiprocessing-programming`
+   however it is worth pointing out here. This means that some examples, such
+   as the :class:`Pool` examples will not work in the interactive interpreter.
+   For example::
+
+      >>> from multiprocessing import Pool
+      >>> p = Pool(5)
+      >>> def f(x):
+      ...     return x*x
+      ...
+      >>> p.map(f, [1,2,3])
+      Process PoolWorker-1:
+      Process PoolWorker-2:
+      Process PoolWorker-3:
+      Traceback (most recent call last):
+      Traceback (most recent call last):
+      Traceback (most recent call last):
+      AttributeError: 'module' object has no attribute 'f'
+      AttributeError: 'module' object has no attribute 'f'
+      AttributeError: 'module' object has no attribute 'f'
+
+   (If you try this it will actually output three full tracebacks
+   interleaved in a semi-random fashion, and then you may have to
+   stop the master process somehow.)
+
 
 Reference
 ---------