Classes | Functions
tornado::gen Namespace Reference

Classes

class  _NullYieldPoint
class  Arguments
class  BadYieldError
class  Callback
class  KeyReuseError
class  LeakedCallbackError
class  Multi
class  Runner
class  Task
class  UnknownKeyError
class  Wait
class  WaitAll
class  YieldPoint

Functions

def engine

Detailed Description

``tornado.gen`` is a generator-based interface to make it easier to
work in an asynchronous environment.  Code using the ``gen`` module
is technically asynchronous, but it is written as a single generator
instead of a collection of separate functions.

For example, the following asynchronous handler::

class AsyncHandler(RequestHandler):
    @asynchronous
    def get(self):
        http_client = AsyncHTTPClient()
        http_client.fetch("http://example.com",
                          callback=self.on_fetch)

    def on_fetch(self, response):
        do_something_with_response(response)
        self.render("template.html")

could be written with ``gen`` as::

class GenAsyncHandler(RequestHandler):
    @asynchronous
    @gen.engine
    def get(self):
        http_client = AsyncHTTPClient()
        response = yield gen.Task(http_client.fetch, "http://example.com")
        do_something_with_response(response)
        self.render("template.html")

`Task` works with any function that takes a ``callback`` keyword
argument.  You can also yield a list of ``Tasks``, which will be
started at the same time and run in parallel; a list of results will
be returned when they are all finished::

def get(self):
    http_client = AsyncHTTPClient()
    response1, response2 = yield [gen.Task(http_client.fetch, url1),
                                  gen.Task(http_client.fetch, url2)]

For more complicated interfaces, `Task` can be split into two parts:
`Callback` and `Wait`::

class GenAsyncHandler2(RequestHandler):
    @asynchronous
    @gen.engine
    def get(self):
        http_client = AsyncHTTPClient()
        http_client.fetch("http://example.com",
                          callback=(yield gen.Callback("key"))
        response = yield gen.Wait("key")
        do_something_with_response(response)
        self.render("template.html")

The ``key`` argument to `Callback` and `Wait` allows for multiple
asynchronous operations to be started at different times and proceed
in parallel: yield several callbacks with different keys, then wait
for them once all the async operations have started.

The result of a `Wait` or `Task` yield expression depends on how the callback
was run.  If it was called with no arguments, the result is ``None``.  If
it was called with one argument, the result is that argument.  If it was
called with more than one argument or any keyword arguments, the result
is an `Arguments` object, which is a named tuple ``(args, kwargs)``.

Function Documentation

def tornado.gen.engine (   func)
Decorator for asynchronous generators.

Any generator that yields objects from this module must be wrapped
in this decorator.  The decorator only works on functions that are
already asynchronous.  For `~tornado.web.RequestHandler`
``get``/``post``/etc methods, this means that both the
`tornado.web.asynchronous` and `tornado.gen.engine` decorators
must be used (for proper exception handling, ``asynchronous``
should come before ``gen.engine``).  In most other cases, it means
that it doesn't make sense to use ``gen.engine`` on functions that
don't already take a callback argument.

Definition at line 91 of file gen.py.



rosbridge_server
Author(s): Jonathan Mace
autogenerated on Mon Oct 6 2014 06:58:14