# `tornado.concurrent` — Work with threads and futures

Utilities for working with threads and `Futures`.

`Futures` are a pattern for concurrent programming introduced in Python 3.2 in the `concurrent.futures` package. This package defines a mostly-compatible `Future` class designed for use from coroutines, as well as some utility functions for interacting with the `concurrent.futures` package.

class `tornado.concurrent.``Future`

  Placeholder for an asynchronous result.

  A `Future` encapsulates the result of an asynchronous operation. In synchronous applications `Futures` are used to wait for the result from a thread or process pool; in Tornado they are normally used with `IOLoop.add_future` or by yielding them in a `gen.coroutine`.

  `tornado.concurrent.Future` is similar to `concurrent.futures.Future`, but not thread-safe (and therefore faster for use with single-threaded event loops).

  In addition to `exception` and `set_exception`, methods `exc_info` and `set_exc_info` are supported to capture tracebacks in Python 2. The traceback is automatically available in Python 3, but in the Python 2 futures backport this information is discarded. This functionality was previously available in a separate class `TracebackFuture`, which is now a deprecated alias for this class.

  Changed in version 4.0: `tornado.concurrent.Future` is always a thread-unsafe `Future` with support for the `exc_info` methods. Previously it would be an alias for the thread-safe `concurrent.futures.Future` if that package was available and fall back to the thread-unsafe implementation if it was not.

  Changed in version 4.1: If a `Future` contains an error but that error is never observed (by calling `result()`, `exception()`, or `exc_info()`), a stack trace will be logged when the `Future` is garbage collected. This normally indicates an error in the application, but in cases where it results in undesired logging it may be necessary to suppress the logging by ensuring that the exception is observed: `f.add_done_callback(lambda f: f.exception())`.

## Consumer methods

`Future.``result`(timeout=None)

  If the operation succeeded, return its result. If it failed, re-raise its exception.

  This method takes a `timeout` argument for compatibility with `concurrent.futures.Future` but it is an error to call it before the `Future` is done, so the `timeout` is never used.

`Future.``exception`(timeout=None)

  If the operation raised an exception, return the `Exception` object. Otherwise returns None.

  This method takes a `timeout` argument for compatibility with `concurrent.futures.Future` but it is an error to call it before the `Future` is done, so the `timeout` is never used.

`Future.``exc_info`()

  Returns a tuple in the same format as `sys.exc_info` or None.

  New in version 4.0.

`Future.``add_done_callback`(fn)

  Attaches the given callback to the `Future`.

  It will be invoked with the `Future` as its argument when the Future has finished running and its result is available. In Tornado consider using `IOLoop.add_future` instead of calling `add_done_callback` directly.

`Future.``done`()

  Returns True if the future has finished running.

`Future.``running`()

  Returns True if this operation is currently running.

`Future.``cancel`()

  Cancel the operation, if possible.

  Tornado `Futures` do not support cancellation, so this method always returns False.

`Future.``cancelled`()

  Returns True if the operation has been cancelled.

  Tornado `Futures` do not support cancellation, so this method always returns False.

## Producer methods

`Future.``set_result`(result)

  Sets the result of a `Future`.

  It is undefined to call any of the `set` methods more than once on the same object.

`Future.``set_exception`(exception)

  Sets the exception of a `Future.`

`Future.``set_exc_info`(exc_info)

  Sets the exception information of a `Future.`

  Preserves tracebacks on Python 2.

  New in version 4.0.

`tornado.concurrent.``run_on_executor`(*args, **kwargs)

  Decorator to run a synchronous method asynchronously on an executor.

  The decorated method may be called with a `callback` keyword argument and returns a future.

  The `IOLoop` and executor to be used are determined by the `io_loop` and `executor` attributes of `self`. To use different attributes, pass keyword arguments to the decorator:

      @run_on_executor(executor='_thread_pool')
      def foo(self):
          pass

  Changed in version 4.2: Added keyword arguments to use alternative attributes.

`tornado.concurrent.``return_future`(f)

  Decorator to make a function that returns via callback return a `Future`.

  The wrapped function should take a `callback` keyword argument and invoke it with one argument when it has finished. To signal failure, the function can simply raise an exception (which will be captured by the `StackContext` and passed along to the `Future`).

  From the caller’s perspective, the callback argument is optional. If one is given, it will be invoked when the function is complete with `Future.result()` as an argument. If the function fails, the callback will not be run and an exception will be raised into the surrounding `StackContext`.

  If no callback is given, the caller should use the `Future` to wait for the function to complete (perhaps by yielding it in a `gen.engine` function, or passing it to `IOLoop.add_future`).

  Usage:

      @return_future
      def future_func(arg1, arg2, callback):
          # Do stuff (possibly asynchronous)
          callback(result)

      @gen.engine
      def caller(callback):
          yield future_func(arg1, arg2)
          callback()

  Note that `@return_future` and `@gen.engine` can be applied to the same function, provided `@return_future` appears first. However, consider using `@gen.coroutine` instead of this combination.

`tornado.concurrent.``chain_future`(a, b)

  Chain two futures together so that when one completes, so does the other.

  The result (success or failure) of `a` will be copied to `b`, unless `b` has already been completed or cancelled by the time `a` finishes.