Source code for tornado.gen

"""``tornado.gen`` implements generator-based coroutines.

.. note::

   The "decorator and generator" approach in this module is a
   precursor to native coroutines (using ``async def`` and ``await``)
   which were introduced in Python 3.5. Applications that do not
   require compatibility with older versions of Python should use
   native coroutines instead. Some parts of this module are still
   useful with native coroutines, notably `multi`, `sleep`,
   `WaitIterator`, and `with_timeout`. Some of these functions have
   counterparts in the `asyncio` module which may be used as well,
   although the two may not necessarily be 100% compatible.

Coroutines provide an easier way to work in an asynchronous
environment than chaining callbacks. Code using coroutines is
technically asynchronous, but it is written as a single generator
instead of a collection of separate functions.

For example, here's a coroutine-based handler:

.. testcode::

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

.. testoutput::
   :hide:

Most asynchronous functions in Tornado return a `.Future`;
yielding this object returns its ``Future.result``.

You can also yield a list or dict of ``Futures``, which will be
started at the same time and run in parallel; a list or dict of results will
be returned when they are all finished:

.. testcode::

    @gen.coroutine
    def get(self):
        http_client = AsyncHTTPClient()
        response1, response2 = yield [http_client.fetch(url1),
                                      http_client.fetch(url2)]
        response_dict = yield dict(response3=http_client.fetch(url3),
                                   response4=http_client.fetch(url4))
        response3 = response_dict['response3']
        response4 = response_dict['response4']

.. testoutput::
   :hide:

If the `~functools.singledispatch` library is available (standard in
Python 3.4, available via the `singledispatch
<https://pypi.python.org/pypi/singledispatch>`_ package on older
versions), additional types of objects may be yielded. Tornado includes
support for ``asyncio.Future`` and Twisted's ``Deferred`` class when
``tornado.platform.asyncio`` and ``tornado.platform.twisted`` are imported.
See the `convert_yielded` function to extend this mechanism.

.. versionchanged:: 3.2
   Dict support added.

.. versionchanged:: 4.1
   Support added for yielding ``asyncio`` Futures and Twisted Deferreds
   via ``singledispatch``.

"""
import asyncio
import builtins
import collections
from collections.abc import Generator
import functools
from functools import singledispatch
from inspect import isawaitable
import sys

from tornado.concurrent import (Future, is_future, chain_future, future_set_exc_info,
                                future_add_done_callback, future_set_result_unless_cancelled)
from tornado.ioloop import IOLoop
from tornado.log import app_log
from tornado.util import TimeoutError


class KeyReuseError(Exception):
    pass


class UnknownKeyError(Exception):
    pass


class LeakedCallbackError(Exception):
    pass


class BadYieldError(Exception):
    pass


class ReturnValueIgnoredError(Exception):
    pass


def _value_from_stopiteration(e):
    try:
        # StopIteration has a value attribute beginning in py33.
        # So does our Return class.
        return e.value
    except AttributeError:
        pass
    try:
        # Cython backports coroutine functionality by putting the value in
        # e.args[0].
        return e.args[0]
    except (AttributeError, IndexError):
        return None


def _create_future():
    future = Future()
    # Fixup asyncio debug info by removing extraneous stack entries
    source_traceback = getattr(future, "_source_traceback", ())
    while source_traceback:
        # Each traceback entry is equivalent to a
        # (filename, self.lineno, self.name, self.line) tuple
        filename = source_traceback[-1][0]
        if filename == __file__:
            del source_traceback[-1]
        else:
            break
    return future


[docs]def coroutine(func): """Decorator for asynchronous generators. Any generator that yields objects from this module must be wrapped in this decorator (or use ``async def`` and ``await`` for similar functionality). Coroutines may "return" by raising the special exception `Return(value) <Return>`. In Python 3.3+, it is also possible for the function to simply use the ``return value`` statement (prior to Python 3.3 generators were not allowed to also return values). In all versions of Python a coroutine that simply wishes to exit early may use the ``return`` statement without a value. Functions with this decorator return a `.Future`. .. warning:: When exceptions occur inside a coroutine, the exception information will be stored in the `.Future` object. You must examine the result of the `.Future` object, or the exception may go unnoticed by your code. This means yielding the function if called from another coroutine, using something like `.IOLoop.run_sync` for top-level calls, or passing the `.Future` to `.IOLoop.add_future`. .. versionchanged:: 6.0 The ``callback`` argument was removed. Use the returned awaitable object instead. """ @functools.wraps(func) def wrapper(*args, **kwargs): future = _create_future() try: result = func(*args, **kwargs) except (Return, StopIteration) as e: result = _value_from_stopiteration(e) except Exception: future_set_exc_info(future, sys.exc_info()) try: return future finally: # Avoid circular references future = None else: if isinstance(result, Generator): # Inline the first iteration of Runner.run. This lets us # avoid the cost of creating a Runner when the coroutine # never actually yields, which in turn allows us to # use "optional" coroutines in critical path code without # performance penalty for the synchronous case. try: yielded = next(result) except (StopIteration, Return) as e: future_set_result_unless_cancelled(future, _value_from_stopiteration(e)) except Exception: future_set_exc_info(future, sys.exc_info()) else: # Provide strong references to Runner objects as long # as their result future objects also have strong # references (typically from the parent coroutine's # Runner). This keeps the coroutine's Runner alive. # We do this by exploiting the public API # add_done_callback() instead of putting a private # attribute on the Future. # (Github issues #1769, #2229). runner = Runner(result, future, yielded) future.add_done_callback(lambda _: runner) yielded = None try: return future finally: # Subtle memory optimization: if next() raised an exception, # the future's exc_info contains a traceback which # includes this stack frame. This creates a cycle, # which will be collected at the next full GC but has # been shown to greatly increase memory usage of # benchmarks (relative to the refcount-based scheme # used in the absence of cycles). We can avoid the # cycle by clearing the local variable after we return it. future = None future_set_result_unless_cancelled(future, result) return future wrapper.__wrapped__ = func wrapper.__tornado_coroutine__ = True return wrapper
[docs]def is_coroutine_function(func): """Return whether *func* is a coroutine function, i.e. a function wrapped with `~.gen.coroutine`. .. versionadded:: 4.5 """ return getattr(func, '__tornado_coroutine__', False)
[docs]class Return(Exception): """Special exception to return a value from a `coroutine`. If this exception is raised, its value argument is used as the result of the coroutine:: @gen.coroutine def fetch_json(url): response = yield AsyncHTTPClient().fetch(url) raise gen.Return(json_decode(response.body)) In Python 3.3, this exception is no longer necessary: the ``return`` statement can be used directly to return a value (previously ``yield`` and ``return`` with a value could not be combined in the same function). By analogy with the return statement, the value argument is optional, but it is never necessary to ``raise gen.Return()``. The ``return`` statement can be used with no arguments instead. """ def __init__(self, value=None): super(Return, self).__init__() self.value = value # Cython recognizes subclasses of StopIteration with a .args tuple. self.args = (value,)
[docs]class WaitIterator(object): """Provides an iterator to yield the results of futures as they finish. Yielding a set of futures like this: ``results = yield [future1, future2]`` pauses the coroutine until both ``future1`` and ``future2`` return, and then restarts the coroutine with the results of both futures. If either future is an exception, the expression will raise that exception and all the results will be lost. If you need to get the result of each future as soon as possible, or if you need the result of some futures even if others produce errors, you can use ``WaitIterator``:: wait_iterator = gen.WaitIterator(future1, future2) while not wait_iterator.done(): try: result = yield wait_iterator.next() except Exception as e: print("Error {} from {}".format(e, wait_iterator.current_future)) else: print("Result {} received from {} at {}".format( result, wait_iterator.current_future, wait_iterator.current_index)) Because results are returned as soon as they are available the output from the iterator *will not be in the same order as the input arguments*. If you need to know which future produced the current result, you can use the attributes ``WaitIterator.current_future``, or ``WaitIterator.current_index`` to get the index of the future from the input list. (if keyword arguments were used in the construction of the `WaitIterator`, ``current_index`` will use the corresponding keyword). On Python 3.5, `WaitIterator` implements the async iterator protocol, so it can be used with the ``async for`` statement (note that in this version the entire iteration is aborted if any value raises an exception, while the previous example can continue past individual errors):: async for result in gen.WaitIterator(future1, future2): print("Result {} received from {} at {}".format( result, wait_iterator.current_future, wait_iterator.current_index)) .. versionadded:: 4.1 .. versionchanged:: 4.3 Added ``async for`` support in Python 3.5. """ def __init__(self, *args, **kwargs): if args and kwargs: raise ValueError( "You must provide args or kwargs, not both") if kwargs: self._unfinished = dict((f, k) for (k, f) in kwargs.items()) futures = list(kwargs.values()) else: self._unfinished = dict((f, i) for (i, f) in enumerate(args)) futures = args self._finished = collections.deque() self.current_index = self.current_future = None self._running_future = None for future in futures: future_add_done_callback(future, self._done_callback)
[docs] def done(self): """Returns True if this iterator has no more results.""" if self._finished or self._unfinished: return False # Clear the 'current' values when iteration is done. self.current_index = self.current_future = None return True
[docs] def next(self): """Returns a `.Future` that will yield the next available result. Note that this `.Future` will not be the same object as any of the inputs. """ self._running_future = Future() if self._finished: self._return_result(self._finished.popleft()) return self._running_future
def _done_callback(self, done): if self._running_future and not self._running_future.done(): self._return_result(done) else: self._finished.append(done) def _return_result(self, done): """Called set the returned future's state that of the future we yielded, and set the current future for the iterator. """ chain_future(done, self._running_future) self.current_future = done self.current_index = self._unfinished.pop(done) def __aiter__(self): return self def __anext__(self): if self.done(): # Lookup by name to silence pyflakes on older versions. raise getattr(builtins, 'StopAsyncIteration')() return self.next()
[docs]def multi(children, quiet_exceptions=()): """Runs multiple asynchronous operations in parallel. ``children`` may either be a list or a dict whose values are yieldable objects. ``multi()`` returns a new yieldable object that resolves to a parallel structure containing their results. If ``children`` is a list, the result is a list of results in the same order; if it is a dict, the result is a dict with the same keys. That is, ``results = yield multi(list_of_futures)`` is equivalent to:: results = [] for future in list_of_futures: results.append(yield future) If any children raise exceptions, ``multi()`` will raise the first one. All others will be logged, unless they are of types contained in the ``quiet_exceptions`` argument. In a ``yield``-based coroutine, it is not normally necessary to call this function directly, since the coroutine runner will do it automatically when a list or dict is yielded. However, it is necessary in ``await``-based coroutines, or to pass the ``quiet_exceptions`` argument. This function is available under the names ``multi()`` and ``Multi()`` for historical reasons. Cancelling a `.Future` returned by ``multi()`` does not cancel its children. `asyncio.gather` is similar to ``multi()``, but it does cancel its children. .. versionchanged:: 4.2 If multiple yieldables fail, any exceptions after the first (which is raised) will be logged. Added the ``quiet_exceptions`` argument to suppress this logging for selected exception types. .. versionchanged:: 4.3 Replaced the class ``Multi`` and the function ``multi_future`` with a unified function ``multi``. Added support for yieldables other than ``YieldPoint`` and `.Future`. """ return multi_future(children, quiet_exceptions=quiet_exceptions)
Multi = multi
[docs]def multi_future(children, quiet_exceptions=()): """Wait for multiple asynchronous futures in parallel. Since Tornado 6.0, this function is exactly the same as `multi`. .. versionadded:: 4.0 .. versionchanged:: 4.2 If multiple ``Futures`` fail, any exceptions after the first (which is raised) will be logged. Added the ``quiet_exceptions`` argument to suppress this logging for selected exception types. .. deprecated:: 4.3 Use `multi` instead. """ if isinstance(children, dict): keys = list(children.keys()) children = children.values() else: keys = None children = list(map(convert_yielded, children)) assert all(is_future(i) or isinstance(i, _NullFuture) for i in children) unfinished_children = set(children) future = _create_future() if not children: future_set_result_unless_cancelled(future, {} if keys is not None else []) def callback(f): unfinished_children.remove(f) if not unfinished_children: result_list = [] for f in children: try: result_list.append(f.result()) except Exception as e: if future.done(): if not isinstance(e, quiet_exceptions): app_log.error("Multiple exceptions in yield list", exc_info=True) else: future_set_exc_info(future, sys.exc_info()) if not future.done(): if keys is not None: future_set_result_unless_cancelled(future, dict(zip(keys, result_list))) else: future_set_result_unless_cancelled(future, result_list) listening = set() for f in children: if f not in listening: listening.add(f) future_add_done_callback(f, callback) return future
[docs]def maybe_future(x): """Converts ``x`` into a `.Future`. If ``x`` is already a `.Future`, it is simply returned; otherwise it is wrapped in a new `.Future`. This is suitable for use as ``result = yield gen.maybe_future(f())`` when you don't know whether ``f()`` returns a `.Future` or not. .. deprecated:: 4.3 This function only handles ``Futures``, not other yieldable objects. Instead of `maybe_future`, check for the non-future result types you expect (often just ``None``), and ``yield`` anything unknown. """ if is_future(x): return x else: fut = _create_future() fut.set_result(x) return fut
[docs]def with_timeout(timeout, future, quiet_exceptions=()): """Wraps a `.Future` (or other yieldable object) in a timeout. Raises `tornado.util.TimeoutError` if the input future does not complete before ``timeout``, which may be specified in any form allowed by `.IOLoop.add_timeout` (i.e. a `datetime.timedelta` or an absolute time relative to `.IOLoop.time`) If the wrapped `.Future` fails after it has timed out, the exception will be logged unless it is of a type contained in ``quiet_exceptions`` (which may be an exception type or a sequence of types). The wrapped `.Future` is not canceled when the timeout expires, permitting it to be reused. `asyncio.wait_for` is similar to this function but it does cancel the wrapped `.Future` on timeout. .. versionadded:: 4.0 .. versionchanged:: 4.1 Added the ``quiet_exceptions`` argument and the logging of unhandled exceptions. .. versionchanged:: 4.4 Added support for yieldable objects other than `.Future`. """ # It's tempting to optimize this by cancelling the input future on timeout # instead of creating a new one, but A) we can't know if we are the only # one waiting on the input future, so cancelling it might disrupt other # callers and B) concurrent futures can only be cancelled while they are # in the queue, so cancellation cannot reliably bound our waiting time. future = convert_yielded(future) result = _create_future() chain_future(future, result) io_loop = IOLoop.current() def error_callback(future): try: future.result() except Exception as e: if not isinstance(e, quiet_exceptions): app_log.error("Exception in Future %r after timeout", future, exc_info=True) def timeout_callback(): if not result.done(): result.set_exception(TimeoutError("Timeout")) # In case the wrapped future goes on to fail, log it. future_add_done_callback(future, error_callback) timeout_handle = io_loop.add_timeout( timeout, timeout_callback) if isinstance(future, Future): # We know this future will resolve on the IOLoop, so we don't # need the extra thread-safety of IOLoop.add_future (and we also # don't care about StackContext here. future_add_done_callback( future, lambda future: io_loop.remove_timeout(timeout_handle)) else: # concurrent.futures.Futures may resolve on any thread, so we # need to route them back to the IOLoop. io_loop.add_future( future, lambda future: io_loop.remove_timeout(timeout_handle)) return result
[docs]def sleep(duration): """Return a `.Future` that resolves after the given number of seconds. When used with ``yield`` in a coroutine, this is a non-blocking analogue to `time.sleep` (which should not be used in coroutines because it is blocking):: yield gen.sleep(0.5) Note that calling this function on its own does nothing; you must wait on the `.Future` it returns (usually by yielding it). .. versionadded:: 4.1 """ f = _create_future() IOLoop.current().call_later(duration, lambda: future_set_result_unless_cancelled(f, None)) return f
class _NullFuture(object): """_NullFuture resembles a Future that finished with a result of None. It's not actually a `Future` to avoid depending on a particular event loop. Handled as a special case in the coroutine runner. """ def result(self): return None def done(self): return True # _null_future is used as a dummy value in the coroutine runner. It differs # from moment in that moment always adds a delay of one IOLoop iteration # while _null_future is processed as soon as possible. _null_future = _NullFuture() moment = _NullFuture() moment.__doc__ = \ """A special object which may be yielded to allow the IOLoop to run for one iteration. This is not needed in normal use but it can be helpful in long-running coroutines that are likely to yield Futures that are ready instantly. Usage: ``yield gen.moment`` .. versionadded:: 4.0 .. deprecated:: 4.5 ``yield None`` (or ``yield`` with no argument) is now equivalent to ``yield gen.moment``. """ class Runner(object): """Internal implementation of `tornado.gen.coroutine`. Maintains information about pending callbacks and their results. The results of the generator are stored in ``result_future`` (a `.Future`) """ def __init__(self, gen, result_future, first_yielded): self.gen = gen self.result_future = result_future self.future = _null_future self.yield_point = None self.pending_callbacks = None self.results = None self.running = False self.finished = False self.had_exception = False self.io_loop = IOLoop.current() if self.handle_yield(first_yielded): gen = result_future = first_yielded = None self.run() def register_callback(self, key): """Adds ``key`` to the list of callbacks.""" if self.pending_callbacks is None: # Lazily initialize the old-style YieldPoint data structures. self.pending_callbacks = set() self.results = {} if key in self.pending_callbacks: raise KeyReuseError("key %r is already pending" % (key,)) self.pending_callbacks.add(key) def is_ready(self, key): """Returns true if a result is available for ``key``.""" if self.pending_callbacks is None or key not in self.pending_callbacks: raise UnknownKeyError("key %r is not pending" % (key,)) return key in self.results def set_result(self, key, result): """Sets the result for ``key`` and attempts to resume the generator.""" self.results[key] = result if self.yield_point is not None and self.yield_point.is_ready(): try: future_set_result_unless_cancelled(self.future, self.yield_point.get_result()) except: future_set_exc_info(self.future, sys.exc_info()) self.yield_point = None self.run() def pop_result(self, key): """Returns the result for ``key`` and unregisters it.""" self.pending_callbacks.remove(key) return self.results.pop(key) def run(self): """Starts or resumes the generator, running until it reaches a yield point that is not ready. """ if self.running or self.finished: return try: self.running = True while True: future = self.future if not future.done(): return self.future = None try: exc_info = None try: value = future.result() except Exception: self.had_exception = True exc_info = sys.exc_info() future = None if exc_info is not None: try: yielded = self.gen.throw(*exc_info) finally: # Break up a reference to itself # for faster GC on CPython. exc_info = None else: yielded = self.gen.send(value) except (StopIteration, Return) as e: self.finished = True self.future = _null_future if self.pending_callbacks and not self.had_exception: # If we ran cleanly without waiting on all callbacks # raise an error (really more of a warning). If we # had an exception then some callbacks may have been # orphaned, so skip the check in that case. raise LeakedCallbackError( "finished without waiting for callbacks %r" % self.pending_callbacks) future_set_result_unless_cancelled(self.result_future, _value_from_stopiteration(e)) self.result_future = None return except Exception: self.finished = True self.future = _null_future future_set_exc_info(self.result_future, sys.exc_info()) self.result_future = None return if not self.handle_yield(yielded): return yielded = None finally: self.running = False def handle_yield(self, yielded): try: self.future = convert_yielded(yielded) except BadYieldError: self.future = Future() future_set_exc_info(self.future, sys.exc_info()) if self.future is moment: self.io_loop.add_callback(self.run) return False elif not self.future.done(): def inner(f): # Break a reference cycle to speed GC. f = None # noqa self.run() self.io_loop.add_future( self.future, inner) return False return True def handle_exception(self, typ, value, tb): if not self.running and not self.finished: self.future = Future() future_set_exc_info(self.future, (typ, value, tb)) self.run() return True else: return False # Convert Awaitables into Futures. try: _wrap_awaitable = asyncio.ensure_future except AttributeError: # asyncio.ensure_future was introduced in Python 3.4.4, but # Debian jessie still ships with 3.4.2 so try the old name. _wrap_awaitable = getattr(asyncio, 'async')
[docs]def convert_yielded(yielded): """Convert a yielded object into a `.Future`. The default implementation accepts lists, dictionaries, and Futures. If the `~functools.singledispatch` library is available, this function may be extended to support additional types. For example:: @convert_yielded.register(asyncio.Future) def _(asyncio_future): return tornado.platform.asyncio.to_tornado_future(asyncio_future) .. versionadded:: 4.1 """ if yielded is None or yielded is moment: return moment elif yielded is _null_future: return _null_future elif isinstance(yielded, (list, dict)): return multi(yielded) elif is_future(yielded): return yielded elif isawaitable(yielded): return _wrap_awaitable(yielded) else: raise BadYieldError("yielded unknown object %r" % (yielded,))
convert_yielded = singledispatch(convert_yielded)