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py.magic.greenlet: Lightweight concurrent programming
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.. contents::
.. sectnum::

Motivation
==========

The "greenlet" package is a spin-off of `Stackless`_, a version of CPython
that supports micro-threads called "tasklets".  Tasklets run
pseudo-concurrently (typically in a single or a few OS-level threads) and
are synchronized with data exchanges on "channels".

A "greenlet", on the other hand, is a still more primitive notion of
micro-thread with no implicit scheduling; coroutines, in other words.  
This is useful when you want to
control exactly when your code runs.  You can build custom scheduled
micro-threads on top of greenlet; however, it seems that greenlets are
useful on their own as a way to make advanced control flow structures.  
For example, we can recreate generators; the difference with Python's own
generators is that our generators can call nested functions and the nested
functions can yield values too.  (Additionally, you don't need a "yield"
keyword.  See the example in :source:`py/c-extension/greenlet/test_generator.py`). 

Greenlets are provided as a C extension module for the regular unmodified
interpreter.

.. _`Stackless`: http://www.stackless.com

Example
-------

Let's consider a system controlled by a terminal-like console, where the user
types commands.  Assume that the input comes character by character.  In such
a system, there will typically be a loop like the following one::

    def process_commands(*args):
        while True:
            line = ''
            while not line.endswith('\n'):
                line += read_next_char()
            if line == 'quit\n':
                print "are you sure?"
                if read_next_char() != 'y':
                    continue    # ignore the command
            process_command(line)

Now assume that you want to plug this program into a GUI.  Most GUI toolkits
are event-based.  They will invoke a call-back for each character the user
presses.  [Replace "GUI" with "XML expat parser" if that rings more bells to
you ``:-)``]  In this setting, it is difficult to implement the
read_next_char() function needed by the code above.  We have two incompatible
functions::

    def event_keydown(key):
        ?? 

    def read_next_char():
        ?? should wait for the next event_keydown() call

You might consider doing that with threads.  Greenlets are an alternate
solution that don't have the related locking and shutdown problems.  You
start the process_commands() function in its own, separate greenlet, and
then you exchange the keypresses with it as follows::

    def event_keydown(key):
             # jump into g_processor, sending it the key
        g_processor.switch(key)

    def read_next_char():
            # g_self is g_processor in this simple example
        g_self = greenlet.getcurrent()
            # jump to the parent (main) greenlet, waiting for the next key
        next_char = g_self.parent.switch()
        return next_char

    g_processor = greenlet(process_commands)
    g_processor.switch(*args)   # input arguments to process_commands()

    gui.mainloop()

In this example, the execution flow is: when read_next_char() is called, it
is part of the g_processor greenlet, so when it switches to its parent
greenlet, it resumes execution in the top-level main loop (the GUI).  When
the GUI calls event_keydown(), it switches to g_processor, which means that
the execution jumps back wherever it was suspended in that greenlet -- in
this case, to the switch() instruction in read_next_char() -- and the ``key``
argument in event_keydown() is passed as the return value of the switch() in
read_next_char().

Note that read_next_char() will be suspended and resumed with its call stack
preserved, so that it will itself return to different positions in
process_commands() depending on where it was originally called from.  This
allows the logic of the program to be kept in a nice control-flow way; we
don't have to completely rewrite process_commands() to turn it into a state
machine.


Usage
=====

Introduction
------------

A "greenlet" is a small independent pseudo-thread.  Think about it as a
small stack of frames; the outermost (bottom) frame is the initial
function you called, and the innermost frame is the one in which the
greenlet is currently paused.  You work with greenlets by creating a
number of such stacks and jumping execution between them.  Jumps are never
implicit: a greenlet must choose to jump to another greenlet, which will
cause the former to suspend and the latter to resume where it was
suspended.  Jumping between greenlets is called "switching".

When you create a greenlet, it gets an initially empty stack; when you
first switch to it, it starts the run a specified function, which may call
other functions, switch out of the greenlet, etc.  When eventually the
outermost function finishes its execution, the greenlet's stack becomes
empty again and the greenlet is "dead".  Greenlets can also die of an
uncaught exception.

For example::

    from py.magic import greenlet

    def test1():
        print 12
        gr2.switch()
        print 34

    def test2():
        print 56
        gr1.switch()
        print 78

    gr1 = greenlet(test1)
    gr2 = greenlet(test2)
    gr1.switch()

The last line jumps to test1, which prints 12, jumps to test2, prints 56,
jumps back into test1, prints 34; and then test1 finishes and gr1 dies.  
At this point, the execution comes back to the original ``gr1.switch()``
call.  Note that 78 is never printed.

Parents
-------

Let's see where execution goes when a greenlet dies.  Every greenlet has a
"parent" greenlet.  The parent greenlet is initially the one in which the
greenlet was created (this can be changed at any time).  The parent is
where execution continues when a greenlet dies.  This way, greenlets are
organized in a tree.  Top-level code that doesn't run in a user-created
greenlet runs in the implicit "main" greenlet, which is the root of the
tree.

In the above example, both gr1 and gr2 have the main greenlet as a parent.  
Whenever one of them dies, the execution comes back to "main".

Uncaught exceptions are propagated into the parent, too.  For example, if
the above test2() contained a typo, it would generate a NameError that
would kill gr2, and the exception would go back directly into "main".  
The traceback would show test2, but not test1.  Remember, switches are not
calls, but transfer of execution between parallel "stack containers", and
the "parent" defines which stack logically comes "below" the current one.

Instantiation
-------------

``py.magic.greenlet`` is the greenlet type, which supports the following
operations:

``greenlet(run=None, parent=None)``
    Create a new greenlet object (without running it).  ``run`` is the
    callable to invoke, and ``parent`` is the parent greenlet, which
    defaults to the current greenlet.

``greenlet.getcurrent()``
    Returns the current greenlet (i.e. the one which called this
    function).

``greenlet.GreenletExit``
    This special exception does not propagate to the parent greenlet; it
    can be used to kill a single greenlet.

The ``greenlet`` type can be subclassed, too.  A greenlet runs by calling
its ``run`` attribute, which is normally set when the greenlet is
created; but for subclasses it also makes sense to define a ``run`` method
instead of giving a ``run`` argument to the constructor.

Switching
---------

Switches between greenlets occur when the method switch() of a greenlet is
called, in which case execution jumps to the greenlet whose switch() is
called, or when a greenlet dies, in which case execution jumps to the
parent greenlet.  During a switch, an object or an exception is "sent" to
the target greenlet; this can be used as a convenient way to pass
information between greenlets.  For example::

    def test1(x, y):
        z = gr2.switch(x+y)
        print z

    def test2(u):
        print u
        gr1.switch(42)

    gr1 = greenlet(test1)
    gr2 = greenlet(test2)
    gr1.switch("hello", " world")

This prints "hello world" and 42, with the same order of execution as the
previous example.  Note that the arguments of test1() and test2() are not
provided when the greenlet is created, but only the first time someone
switches to it.

Here are the precise rules for sending objects around:

``g.switch(obj=None or *args)``
    Switches execution to the greenlet ``g``, sending it the given
    ``obj``.  As a special case, if ``g`` did not start yet, then it will
    start to run now; in this case, any number of arguments can be
    provided, and ``g.run(*args)`` is called.

Dying greenlet
    If a greenlet's ``run()`` finishes, its return value is the object 
    sent to its parent.  If ``run()`` terminates with an exception, the
    exception is propagated to its parent (unless it is a
    ``greenlet.GreenletExit`` exception, in which case the exception
    object is caught and *returned* to the parent).

Apart from the cases described above, the target greenlet normally
receives the object as the return value of the call to ``switch()`` in
which it was previously suspended.  Indeed, although a call to
``switch()`` does not return immediately, it will still return at some
point in the future, when some other greenlet switches back.  When this
occurs, then execution resumes just after the ``switch()`` where it was
suspended, and the ``switch()`` itself appears to return the object that
was just sent.  This means that ``x = g.switch(y)`` will send the object
``y`` to ``g``, and will later put the (unrelated) object that some
(unrelated) greenlet passes back to us into ``x``.

Note that any attempt to switch to a dead greenlet actually goes to the
dead greenlet's parent, or its parent's parent, and so on.  (The final
parent is the "main" greenlet, which is never dead.)

Methods and attributes of greenlets
-----------------------------------

``g.switch(obj=None or *args)``
    Switches execution to the greenlet ``g``.  See above.

``g.run``
    The callable that ``g`` will run when it starts.  After ``g`` started,
    this attribute no longer exists.

``g.parent``
    The parent greenlet.  This is writeable, but it is not allowed to
    create cycles of parents.

``g.gr_frame``
    The current top frame, or None.

``g.dead``
    True if ``g`` is dead (i.e. it finished its execution).

``bool(g)``
    True if ``g`` is active, False if it is dead or not yet started.

``g.throw([typ, [val, [tb]]])``
    Switches execution to the greenlet ``g``, but immediately raises the
    given exception in ``g``.  If no argument is provided, the exception
    defaults to ``greenlet.GreenletExit``.  The normal exception
    propagation rules apply, as described above.  Note that calling this
    method is almost equivalent to the following::

        def raiser():
            raise typ, val, tb
        g_raiser = greenlet(raiser, parent=g)
        g_raiser.switch()

    except that this trick does not work for the
    ``greenlet.GreenletExit`` exception, which would not propagate
    from ``g_raiser`` to ``g``.

Greenlets and Python threads
----------------------------

Greenlets can be combined with Python threads; in this case, each thread
contains an independent "main" greenlet with a tree of sub-greenlets.  It
is not possible to mix or switch between greenlets belonging to different
threads.

Garbage-collecting live greenlets
---------------------------------

If all the references to a greenlet object go away (including the
references from the parent attribute of other greenlets), then there is no
way to ever switch back to this greenlet.  In this case, a GreenletExit
exception is generated into the greenlet.  This is the only case where a
greenlet receives the execution asynchronously.  This gives
``try:finally:`` blocks a chance to clean up resources held by the
greenlet.  This feature also enables a programming style in which
greenlets are infinite loops waiting for data and processing it.  Such
loops are automatically interrupted when the last reference to the
greenlet goes away.

The greenlet is expected to either die or be resurrected by having a new
reference to it stored somewhere; just catching and ignoring the
GreenletExit is likely to lead to an infinite loop.

Greenlets do not participate in garbage collection; cycles involving data
that is present in a greenlet's frames will not be detected.  Storing
references to other greenlets cyclically may lead to leaks.
