interruptingcow

Interrupting Cow

Interruptingcow is a generic utility that can relatively gracefully interrupt your Python code when it doesn't execute within a specific number of seconds::

from interruptingcow import timeout

try:
    with timeout(5, exception=RuntimeError):
        # perform a potentially very slow operation
        pass
except RuntimeError:
    print "didn't finish within 5 seconds"

Timeouts are specified in seconds (as floats with theoretical microsecond precision).

Installation

::

$ pip install interruptingcow

Reentrant

Interruptingcow is fully reentrant, which means that you can have nested timeouts::

from interruptingcow import timeout

class Outer(RuntimeError): pass

class Inner(RuntimeError): pass

try:
    with timeout(20.0, Outer):
        try:
            with timeout(1.0, Inner):
                # some expensive operation
                try_the_expensive_thing()
        except Inner:
            do_the_cheap_thing_instead()

except Outer:
    print 'Program as a whole failed to return in 20 secs'

Nested timeouts allow a large outer timeout to contain smaller timeouts. If the inner timeout is larger than the outer timeout, it is treated as a no-op.

Function Decorators

Interruptingcow can be used both as inline with-statements, as shown in the above examples, as well as function decorator::

from interruptingcow import timeout

@timeout(.5)
def foo():
    with timeout(.3):
        # some expensive operation
        pass

Quotas

You can allocate a quota of time and then share it across multiple invocations to timeout(). This is especially useful if you need to use timeouts inside a loop::

from interruptingcow import timeout, Quota

quota = Quota(1.0)
for i in something:
    try:
        with timeout(quota, RuntimeError):
            # perform a slow operation
            pass
    except RuntimeError:
        # do a cheaper thing instead

Here the first iterations of the loop will be able to perform the expensive operation, until the shared quota of 1 second runs out and then the remaining iterations will perform the cheaper alternative.

A single quota instance can also be shared across all calls to timeout() your application makes (including nested calls), to give place an upper bound on the total runtime, regardless of how many calls to timeout() you have.

Caveats

Interruptingcow uses signal(SIGALRM) to let the operating system interrupt program execution. This has the following limitations:

1. Python signal handlers only apply to the main thread, so you cannot use this from other threads
2. You must not use this in a program that uses SIGALRM itself (this includes certain profilers)