Raise asynchronous exceptions in other threads, control the timeout of blocks or callables with two context managers and two decorators.
Attention!
API Changes
Users of 1.0.0 should upgrade their source code:
stopit.Timeoutis renamedstopit.ThreadingTimeoutstopit.timeoutableis renamedstopit.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.
Note
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.
Both work identically:
easy_install stopit
pip install stopit# You should prefer forking if you have a Github account
git clone https://github.com/glenfant/stopit.git
cd stopit
python setup.py develop
# Does it work for you ?
python setup.py testA 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.
Warning
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)
tidis the thread identifier as provided by theidentattribute of a thread object. See the documentation of thethreadingmodule for further information.exceptionis 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)
secondsis the number of seconds allowed to the execution of the context managed block.swallow_exc: ifFalse, the possiblestopit.TimeoutExceptionwill be re-raised when quitting the context managed block. Attention: aTruevalue 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_mgr.state == to_ctx_mgr.EXECUTED:
# All's fine, everything was executed within 10 seconds
elif to_ctx_mgr.state == to_ctx_mgr.EXECUTING:
# Hmm, that's not possible outside the block
elif to_ctx_mgr.state == to_ctx_mgr.TIMED_OUT:
# Eeek the 10 seconds timeout occurred while executing the block
elif to_ctx_mgr.state == to_ctx_mgr.INTERRUPTED:
# Oh you raised specifically the TimeoutException in the block
elif to_ctx_mgr.state == to_ctx_mgr.CANCELED:
# Oh you called to_ctx_mgr.cancel() method within the block but it
# executed till the end
else:
# That's not possibleNotice 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 consistentA 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]])
defaultis 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
Nonevalue 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 atimeoutnamed parameter for whatever reason. This empowers you to change the name of thetimeoutparameter in the decorated function signature to whatever suits, and prevent a potential naming conflict.@stopit.threading_timeoutable(timeout_param='my_timeout') def some_slow_function(a, b, timeout='whatever'): # As its name says... result = some_slow_function(1, 2, timeout="something", my_timeout=2)
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.
Warning
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')
stopit_logger.setLevel(logging.ERROR)| 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 |
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:
- https://docs.python.org/2.7/library/sys.html#sys.getcheckinterval
- https://docs.python.org/2.7/library/sys.html#sys.getcheckinterval
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.
Threading timeout control as mentioned in Threading based resources does not work as expected when used in the context of a gevent worker.
See the discussion in Issue 13 for more details.
>>> import threading
>>> from stopit import async_raise, TimeoutExceptionIn a real application, you should either use threading based timeout resources:
>>> from stopit import ThreadingTimeout as Timeout, threading_timeoutable as timeoutable #doctest: +SKIPOr the POSIX signal based resources:
>>> from stopit import SignalTimeout as Timeout, signal_timeoutable as timeoutable #doctest: +SKIPLet'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()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()
TrueWe 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():
... passAnd we can notice that we stopped the thread before it stopped by itself:
>>> time.time() - start_time < 0.5
True
>>> len(exc_traces)
1
>>> exc_traces[-1].__class__.__name__
'LookupError'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.
We check that the fast functions return as outside our context manager:
>>> with Timeout(5.0) as timeout_ctx:
... result = fast_func()
>>> result
0
>>> timeout_ctx.state == timeout_ctx.EXECUTED
TrueAnd the context manager is considered as True (the block executed its last
line):
>>> bool(timeout_ctx)
TrueWe 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
True
>>> timeout_ctx.state == timeout_ctx.TIMED_OUT
TrueAnd the context manager is considered as False since the block did timeout.
>>> bool(timeout_ctx)
FalseOther 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
True
>>> result
'exception_seen'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
True
>>> result
'exception_seen'
>>> timeout_ctx.state == timeout_ctx.TIMED_OUT
TrueOther 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
True
>>> result
'exception_seen'
>>> timeout_ctx.state == timeout_ctx.EXECUTING
TrueThis 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)
6You 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
TrueOr any other value provided to the timeoutable decorator parameter:
>>> @timeoutable('unexpected')
... def infinite():
... while True:
... pass
... return 'whatever'
>>> infinite(timeout=1)
'unexpected'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)
'unexpected'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)
'unexpected'- Source code (clone, fork, ...)
- https://github.com/glenfant/stopit
- Issues tracker
- https://github.com/glenfant/stopit/issues
- PyPI
- https://pypi.python.org/pypi/stopit
- 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.
This software is open source delivered under the terms of the MIT license. See the LICENSE file of this repository.
