pretzel 1.0.10
Pretzel - asynchronous python framework
License: Apache License 2.0

Pretzel
-------

|Build Status| |Coverage Status|

Is an asynchronous application framework for python

Features
--------

-  C# like async/await(async/yield) paradigm for asynchronous
   programming (monad base)
-  Cool asynchronous I/O loop implementation
-  Uniform asynchronous stream implementation for sockets and pipes
-  Interact with subprocesses asynchronously
-  Greenlet support (but not required)
-  Remote code executing over ssh or in child process (with only
   requirements python and ssh)
-  Python 2/3, PyPy (starting from 2.0) compatible
-  Asynchronous python shell ``python -mpretzel.apps.shell`` (requires
   greenlet)

Installation
------------

As git submodule:
``git submodule add git://github.com/aslpavel/pretzel.git <path_to_submodule>``
Pip from git:
``pip install git+git://github.com/aslpavel/pretzel-pkg.git`` Pip from
PyPI ``pip install pretzel``

Approach
--------

Usage of asynchronous functions is similar to C# async/await but instead
of ``async`` attribute it uses ``@async`` decorator and instead of
``await`` keyword, ``yield`` is used. Internally unit of asynchrony is
implemented as continuation monad ``Cont`` with embedded ``Result``
monad (similar to Haskell's ``Cont`` and ``Either`` monads) as its
value. One important difference of ``Cont`` monad from C# ``Task``
object, is that ``Task`` represents already running asynchronous
operation, but continuation monad is a sequence of computations, and
this computations are not started. ``.future()`` method on instance of
``Cont`` can be used to create ``Task`` like object. To use this library
you don't have to understand notion of the monad. Simple asynchronous
function would look like this.

.. code:: python

    from pretzel.monad import async
    from pretzel.core import sleep

    @async
    def print_after(delay, *args, **kwargs):
      """Calls print function after the lapse of `delay` sedonds.
      """
      yield sleep(delay)  # execution will be resumed in delay seconds
      print(*args, **kwargs)

To return something meaningful in python3 you can just use ``return``
keyword, but in python2 you have to use ``do_return`` function (it will
also work in python3) as ``return`` with value cannot be used inside a
generator function. Result of such asynchronous function is again a
continuation monad, if exception is thrown during execution of its body,
it is marshaled to receiver of the result and can be processed
correctly. For example.

.. code:: python

    @async
    def process_error():
      @async
      def throw_after(delay, error):
        yield sleep(delay)
        raise error

      try:
        yield throw_after(1, ValueError('test error'))
      except ValueError as error:
        # process error in a meaningfull way
      do_return('done')  # exectly equivalent to: return 'done'

Asynchronous values (continuation monads) can be composed with two
helper functions ``async_all`` and ``async_any``.

.. code:: python

    @async
    def composition_example():
      yield async_all([sleep(1), sleep(2)])  # will be resumed in 2 seconds
      yield async_any([sleep(1), sleep(2)])  # will be resumed in 1 sedond

      result_all = yield async_all([func1(), func2()])  # = (result1, result2)
      reuslt_any = yield async_any([func1(), func2()])  # = result1 | result2

``Cont`` monad can also be called with callback function as its
argument, in this case, on completion of asynchronous operation,
callback will be called with ``Result`` monad. If callback function is
not specified default, then default continuation callback will be used
which only reports errors if any.

.. code:: python

    >>> sleep(1)(print)
    Result(val:1374307530.015137)
    >>> sleep(None)()
    [continuation] error in coroutine started from
      File "<console>", line 1, in <module>
    `-------------------------------------------------------------------------------
    Host   : fiend
    Process: 13492
    Error  : TypeError("unsupported operand type(s) for +: 'float' and 'NoneType'")

    Traceback (most recent call last):
      File "./pretzel/monad/do.py", line 26, in do_block
        return value(block(*args, **kwargs))
      File "./pretzel/core/core.py", line 118, in sleep
        do_done(self.time_queue.on(time() + delay))
    TypeError: unsupported operand type(s) for +: 'float' and 'NoneType'

Inside body of asynchronous function you can ``yield`` not only ``Cont``
monad directly, but any object implementing ``.__monad__()`` method
which returns ``Cont`` monad. There are many such types in this library,
for example ``Event``

.. code:: python

    @async
    def func():
      print(1)
      yield event
      print(2)
      print((yield event))
    event = Event()
    func()()     # 1 is printed
    event('e0')  # 2 is printed
    event('e1')  # 'e1' is printed

Main loop
---------

``Core`` class implements I/O loop, and it is used internally to
implement asynchronous streams, timers and more. Previously used
``sleep`` function will work correctly only in presence of running I/O
loop. Simplest way to intialize and use ``Core`` object is to use
``@app`` decorator.

.. code:: python

    """Minimal pretzel application

    Sleeps for one second, then prints 'done' and exits.
    """
    from pretzel.app import app
    from pretzel.core import sleep

    @app
    def main():
      yield sleep(1)
      print('done')

    if __name__ == '__main__':
      main()

See `Core's init
module <https://github.com/aslpavel/pretzel/blob/master/core/__init__.py>`__
for full list of available functions and their descriptions.

Remoting
--------

Main reason for creation of this framework was to execute code on a set
of machines via ssh connection. It is achieved by usage of
``SSHConnection`` class. ``SSHConnection`` is a callable object which
returns proxy object for its argument. You can call proxy object, get
its attributes or items ``proxy[item]``, result of such operations is
again a proxy object with this embedded operations. Proxy implements
monad interface, and to get result of embedded chain of operations you
can yield it inside asynchronous function. In this example we create
proxy for ``os.getpid`` function, call it and then execute on remote
process by yielding it. There is no need for pretzel to be installed on
remote machine.

.. code:: python

    import os
    from pretzel.app import app
    from pretzel.remoting import SSHConnection

    @app
    def main():
      """Connect to localhost via ssh and print remote process's pid

      Note:
        You have to be able to login to the remote host without
        entering any password (by means of ssh keys) otherwise
        connecition will fail.
      """
      with (yield SSHConnection('localhost')) as conn:
        print((yield conn(os.getpid)()))

    if __name__ == '__main__':
      main()

Connection can marshal any pickle-able object, or ``Sender`` object plus
any object which is reducible to set of pickle-able and ``Sender``
objects. ``Proxy`` and ``Connection`` itself are examples of such
objects. You can also create proxy object from any arbitrary object with
``proxify`` or ``proxify_func``.

.. code:: python

    import os
    from pretzel.app import app
    from pretzel.remoting import SSHConnection, proxify

    class Remote(object):
      """Object which will be used remotely
      """
      def __init__(self):
        self.value = 0

      def next(self):
        self.value += 1
        return self.value

      def getpid(self):
        return os.getpid()

    @app
    def main():
      with (yield SSHConnection('localhost')) as conn:
        with (yield proxify(conn(Remote)())) as o:  # remote object proxy
          print(os.getpid(), (yield o.getpid()))    # prints two different pids
          print((yield o.next()))  # prints 1
          print((yield o.next()))  # prints 2

    if __name__ == '__main__':
      main()

But ``Cont`` monad is not marshallable, that is why there is special
operation on proxy object ``~`` which is equivalent to ``yield`` inside
asynchronous function. Here is an example of remote execution of
asynchronous function.

.. code:: python

    from pretzel.app import app
    from pretzel.process import process_call
    from pretzel.remoting import SSHConnection

    @app
    def main():
      """Execute 'ls' on remote machine and show result of the execution
      """
      with (yield SSHConnection('localhost')) as conn:
        out, err, code = yield ~conn(process_call)('ls')
        print(out.decode())

    if __name__ == '__main__':
      main()

There is also a way to work with multiple connections as if it one, by
means of ``composite_ssh_conn``. It accepts list of hosts and returns
composite connection, which behaves as ordinary connection but returns
set of results.

.. code:: python

    import os
    from pretzel.app import app
    from pretzel.remoting import composite_ssh_conn

    @app
    def main():
      hosts = ['localhost', 'localhost']
      with (yield composite_ssh_conn(hosts)) as conns:
        result = yield conns(os.getpid)()
        print(result)  # List(25163, 25162) - iterable object of pids

    if __name__ == '__main__':
      main()

Remoting submodule can be used as workaround for python's GIL, in a
similar fashion to ``multiprocessing`` module. You can use
``ForkConnection`` (or ``composite_fork_conn``) which behaves as
``SSHConnection`` but instead of connecting via ssh, it just spawns new
process.

.. code:: python

    import time
    from pretzel.app import app
    from pretzel.remoting import composite_fork_conn

    def computation_heavy_task():
      """Some computation intensive task
      """
      start_time = time.time()
      time.sleep(10)
      stop_time = time.time()
      return int(stop_time - start_time)

    @app
    def main():
      with (yield composite_fork_conn(10)) as conns:  # create 10 connections
        result = yield conns(computation_heavy_task)()
        print(result)  # prints List(10, 10, 10, 10, 10, 10, 10, 10, 10, 10)

    if __name__ == '__main__':
      main()

Examples
--------

-  `Simple echo server <https://gist.github.com/aslpavel/5635559>`__
-  `Cat remote file over
   ssh <https://gist.github.com/aslpavel/5635610>`__

.. |Build Status| image:: https://api.travis-ci.org/aslpavel/pretzel.png
   :target: https://travis-ci.org/aslpavel/pretzel
.. |Coverage Status| image:: https://coveralls.io/repos/aslpavel/pretzel/badge.png?branch=master
   :target: https://coveralls.io/r/aslpavel/pretzel?branch=master