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Timeout control decorator and context managers, raise any exception in another thread

Project Description

Raise asynchronous exceptions in other threads, control the timeout of blocks or callables with two context managers and two decorators.


API Changes

Users of 1.0.0 should upgrade their source code:

  • stopit.Timeout is renamed stopit.ThreadingTimeout
  • stopit.timeoutable is renamed stopit.threading_timeoutable

Explications follow below…


This module provides:

  • a function that raises an exception in another thread, including the main thread.
  • two context managers that may stop its inner block activity on timeout.
  • two decorators that may stop its decorated callables on timeout.

Developed and tested with CPython 2.6, 2.7, 3.3 and 3.4 on MacOSX. Should work on any OS (xBSD, Linux, Windows) except when explicitly mentioned.


Signal based timeout controls, namely SignalTimeout context manager and signal_timeoutable decorator won’t work in Windows that has no support for signal.SIGALRM. Any help to work around this is welcome.


Using stopit in your application

Both work identically:

easy_install stopit
pip install stopit

Developing stopit

# You should prefer forking if you have a Github account
git clone
cd stopit
python develop

# Does it work for you ?
python test

Public API



A stopit.TimeoutException may be raised in a timeout context manager controlled block.

This exception may be propagated in your application at the end of execution of the context manager controlled block, see the swallow_ex parameter of the context managers.

Note that the stopit.TimeoutException is always swallowed after the execution of functions decorated with xxx_timeoutable(...). Anyway, you may catch this exception within the decorated function.

Threading based resources


Threading based resources will only work with CPython implementations since we use CPython specific low level API. This excludes Iron Python, Jython, Pypy, …

Will not stop the execution of blocking Python atomic instructions that acquire the GIL. In example, if the destination thread is actually executing a time.sleep(20), the asynchronous exception is effective after its execution.


A function that raises an arbitrary exception in another thread

async_raise(tid, exception)

  • tid is the thread identifier as provided by the ident attribute of a thread object. See the documentation of the threading module for further information.
  • exception is the exception class or object to raise in the thread.


A context manager that “kills” its inner block execution that exceeds the provided time.

ThreadingTimeout(seconds, swallow_exc=True)

  • seconds is the number of seconds allowed to the execution of the context managed block.
  • swallow_exc : if False, the possible stopit.TimeoutException will be re-raised when quitting the context managed block. Attention: a True value does not swallow other potential exceptions.

Methods and attributes

of a stopit.ThreadingTimeout context manager.

Method / Attribute Description
.cancel() Cancels the timeout control. This method is intended for use within the block that’s under timeout control, specifically to cancel the timeout control. Means that all code executed after this call may be executed till the end.
.state This attribute indicated the actual status of the timeout control. It may take the value of the EXECUTED, EXECUTING, TIMED_OUT, INTERRUPTED or CANCELED attributes. See below.
.EXECUTING The timeout control is under execution. We are typically executing within the code under control of the context manager.
.EXECUTED Good news: the code under timeout control completed normally within the assigned time frame.
.TIMED_OUT Bad news: the code under timeout control has been sleeping too long. The objects supposed to be created or changed within the timeout controlled block should be considered as non existing or corrupted. Don’t play with them otherwise informed.
.INTERRUPTED The code under timeout control may itself raise explicit stopit.TimeoutException for any application logic reason that may occur. This intentional exit can be spotted from outside the timeout controlled block with this state value.
.CANCELED The timeout control has been intentionally canceled and the code running under timeout control did complete normally. But perhaps after the assigned time frame.

A typical usage:

import stopit
# ...
with stopit.ThreadingTimeout(10) as to_ctx_mgr:
    assert to_ctx_mgr.state == to_ctx_mgr.EXECUTING
    # Something potentially very long but which
    # ...

# OK, let's check what happened
if to_ctx_mrg.state == to_ctx_mrg.EXECUTED:
    # All's fine, everything was executed within 10 seconds
elif to_ctx_mrg.state == to_ctx_mrg.EXECUTING:
    # Hmm, that's not possible outside the block
elif to_ctx_mrg.state == to_ctx_mrg.TIMED_OUT:
    # Eeek the 10 seconds timeout occurred while executing the block
elif to_ctx_mrg.state == to_ctx_mrg.INTERRUPTED:
    # Oh you raised specifically the TimeoutException in the block
elif to_ctx_mrg.state == to_ctx_mrg.CANCELED:
    # Oh you called to_ctx_mgr.cancel() method within the block but it
    # executed till the end
    # That's not possible

Notice that the context manager object may be considered as a boolean indicating (if True) that the block executed normally:

if to_ctx_mgr:
    # Yes, the code under timeout control completed
    # Objects it created or changed may be considered consistent


A decorator that kills the function or method it decorates, if it does not return within a given time frame.

stopit.threading_timeoutable([default [, timeout_param]])

  • default is the value to be returned by the decorated function or method of when its execution timed out, to notify the caller code that the function did not complete within the assigned time frame.

    If this parameter is not provided, the decorated function or method will return a None value when its execution times out.

    @stopit.threading_timeoutable(default='not finished')
    def infinite_loop():
        # As its name says...
    result = infinite_loop(timeout=5)
    assert result == 'not finished'
  • timeout_param: The function or method you have decorated may require a timeout named parameter for whatever reason. This empowers you to change the name of the timeout parameter in the decorated function signature to whatever suits, and prevent a potential naming conflict.

    def some_slow_function(a, b, timeout='whatever'):
        # As its name says...
    result = some_slow_function(1, 2, timeout="something", my_timeout=2)

About the decorated function

or method…

As you noticed above, you just need to add the timeout parameter when calling the function or method. Or whatever other name for this you chose with the timeout_param of the decorator. When calling the real inner function or method, this parameter is removed.

Signaling based resources


Using signaling based resources will not work under Windows or any OS that’s not based on Unix.

stopit.SignalTimeout and stopit.signal_timeoutable have exactly the same API as their respective threading based resources, namely stopit.ThreadingTimeout and stopit.threading_timeoutable.

See the comparison chart that warns on the more or less subtle differences between the Threading based resources and the Signaling based resources.


The stopit named logger emits a warning each time a block of code execution exceeds the associated timeout. To turn logging off, just:

import logging
stopit_logger = logging.getLogger('stopit')

Comparing thread based and signal based timeout control

Feature Threading based resources Signaling based resources
GIL Can’t interrupt a long Python atomic instruction. e.g. if time.sleep(20.0) is actually executing, the timeout will take effect at the end of the execution of this line. Don’t care of it
Thread safety Yes : Thread safe as long as each thread uses its own ThreadingTimeout context manager or threading_timeoutable decorator. Not thread safe. Could yield unpredictable results in a multithreads application.
Nestable context managers Yes : you can nest threading based context managers No : never nest a signaling based context manager in another one. The innermost context manager will automatically cancel the timeout control of outer ones.
Accuracy Any positive floating value is accepted as timeout value. The accuracy depends on the GIL interval checking of your platform. See the doc on sys.getcheckinterval and sys.setcheckinterval for your Python version. Due to the use of signal.SIGALRM, we need provide an integer number of seconds. So a timeout of 0.6 seconds will ve automatically converted into a timeout of zero second!
Supported platforms Any CPython 2.6, 2.7 or 3.3 on any OS with threading support. Any Python 2.6, 2.7 or 3.3 with signal.SIGALRM support. This excludes Windows boxes

Issue about timeout accuracy

Important: the way CPython supports threading and asynchronous features has impacts on the accuracy of the timeout. In other words, if you assign a 2.0 seconds timeout to a context managed block or a decorated callable, the effective code block / callable execution interruption may occur some fractions of seconds after this assigned timeout.

For more background about this issue - that cannot be fixed - please read Python gurus thoughts about Python threading, the GIL and context switching like these ones:

This is the reason why I am more “tolerant” on timeout accuracy in the tests you can read thereafter than I should be for a critical real-time application (that’s not in the scope of Python).

It is anyway possible to improve this accuracy at the expense of the global performances decreasing the check interval which defaults to 100. See:

If this is a real issue for users (want a precise timeout and not an approximative one), a future release will add the optional check_interval parameter to the context managers and decorators. This parameter will enable to lower temporarily the threads switching check interval, having a more accurate timeout at the expense of the overall performances while the context managed block or decorated functions are executing.

Tests and demos

>>> import threading
>>> from stopit import async_raise, TimeoutException

In a real application, you should either use threading based timeout resources:

>>> from stopit import ThreadingTimeout as Timeout, threading_timeoutable as timeoutable  #doctest: +SKIP

Or the POSIX signal based resources:

>>> from stopit import SignalingTimeout as Timeout, signaling_timeoutable as timeoutable  #doctest: +SKIP

Let’s define some utilities:

>>> import time
>>> def fast_func():
...     return 0
>>> def variable_duration_func(duration):
...     t0 = time.time()
...     while True:
...         dummy = 0
...         if time.time() - t0 > duration:
...             break
>>> exc_traces = []
>>> def variable_duration_func_handling_exc(duration, exc_traces):
...     try:
...         t0 = time.time()
...         while True:
...             dummy = 0
...             if time.time() - t0 > duration:
...                 break
...     except Exception as exc:
...         exc_traces.append(exc)
>>> def func_with_exception():
...     raise LookupError()

async_raise function raises an exception in another thread

Testing async_raise() with a thread of 5 seconds:

>>> five_seconds_threads = threading.Thread(
...     target=variable_duration_func_handling_exc, args=(5.0, exc_traces))
>>> start_time = time.time()
>>> five_seconds_threads.start()
>>> thread_ident = five_seconds_threads.ident
>>> five_seconds_threads.is_alive()

We raise a LookupError in that thread:

>>> async_raise(thread_ident, LookupError)

Okay but we must wait few milliseconds the thread death since the exception is asynchronous:

>>> while five_seconds_threads.is_alive():
...     pass

And we can notice that we stopped the thread before it stopped by itself:

>>> time.time() - start_time < 0.5
>>> len(exc_traces)
>>> exc_traces[-1].__class__.__name__

Timeout context manager

The context manager stops the execution of its inner block after a given time. You may manage the way the timeout occurs using a try: ... except: ... construct or by inspecting the context manager state attribute after the block.

Swallowing Timeout exceptions

We check that the fast functions return as outside our context manager:

>>> with Timeout(5.0) as timeout_ctx:
...     result = fast_func()
>>> result
>>> timeout_ctx.state == timeout_ctx.EXECUTED

And the context manager is considered as True (the block executed its last line):

>>> bool(timeout_ctx)

We check that slow functions are interrupted:

>>> start_time = time.time()
>>> with Timeout(2.0) as timeout_ctx:
...     variable_duration_func(5.0)
>>> time.time() - start_time < 2.2
>>> timeout_ctx.state == timeout_ctx.TIMED_OUT

And the context manager is considered as False since the block did timeout.

>>> bool(timeout_ctx)

Other exceptions are propagated and must be treated as usual:

>>> try:
...     with Timeout(5.0) as timeout_ctx:
...         result = func_with_exception()
... except LookupError:
...     result = 'exception_seen'
>>> timeout_ctx.state == timeout_ctx.EXECUTING
>>> result

Propagating TimeoutException

We can choose to propagate the TimeoutException too. Potential exceptions have to be handled:

>>> result = None
>>> start_time = time.time()
>>> try:
...     with Timeout(2.0, swallow_exc=False) as timeout_ctx:
...         variable_duration_func(5.0)
... except TimeoutException:
...     result = 'exception_seen'
>>> time.time() - start_time < 2.2
>>> result
>>> timeout_ctx.state == timeout_ctx.TIMED_OUT

Other exceptions must be handled too:

>>> result = None
>>> start_time = time.time()
>>> try:
...     with Timeout(2.0, swallow_exc=False) as timeout_ctx:
...         func_with_exception()
... except Exception:
...     result = 'exception_seen'
>>> time.time() - start_time < 0.1
>>> result
>>> timeout_ctx.state == timeout_ctx.EXECUTING

timeoutable callable decorator

This decorator stops the execution of any callable that should not last a certain amount of time.

You may use a decorated callable without timeout control if you don’t provide the timeout optional argument:

>>> @timeoutable()
... def fast_double(value):
...     return value * 2
>>> fast_double(3)

You may specify that timeout with the timeout optional argument. Interrupted callables return None:

>>> @timeoutable()
... def infinite():
...     while True:
...         pass
...     return 'whatever'
>>> infinite(timeout=1) is None

Or any other value provided to the timeoutable decorator parameter:

>>> @timeoutable('unexpected')
... def infinite():
...     while True:
...         pass
...     return 'whatever'
>>> infinite(timeout=1)

If the timeout parameter name may clash with your callable signature, you may change it using timeout_param:

>>> @timeoutable('unexpected', timeout_param='my_timeout')
... def infinite():
...     while True:
...         pass
...     return 'whatever'
>>> infinite(my_timeout=1)

It works on instance methods too:

>>> class Anything(object):
...     @timeoutable('unexpected')
...     def infinite(self, value):
...         assert type(value) is int
...         while True:
...             pass
>>> obj = Anything()
>>> obj.infinite(2, timeout=1)


  • This is a NIH package which is mainly a theft of Gabriel Ahtune’s recipe with tests, minor improvements and refactorings, documentation and setuptools awareness I made since I’m somehow tired to copy/paste this recipe among projects that need timeout control.
  • Gilles Lenfant: package creator and maintainer.

Changes log

1.1.1 - 2015-03-22

  • Fixed bug of timeout context manager as bool under Python 2.x
  • Tested with Python 3.4

1.1.0 - 2014-05-02

  • Added support for TIMER signal based timeout control (Posix OS only)
  • API changes due to new timeout controls
  • An exhaustive documentation.

1.0.0 - 2014-02-09

Initial version

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