#!/usr/bin/env python
# coding: utf-8

# Adding cell timeouts to IPython.
# 
# Let's say you want to add a timeout to cells, so that cells that run for too long get interrupted.
# 
# A handy way to do this is with [signal.alarm](https://docs.python.org/3/library/signal.html#signal.alarm).

# In[1]:


import signal
import time


class TookTooLong(Exception):
    """A special error to raise when a cell takes too long"""
    def __str__(self):
        return "The cell took too long to execute"


def raise_on_alarm(*args):
    """signal handler to raise our special error"""
    raise TookTooLong()


# register our handler when the alarm signal fires:
signal.signal(signal.SIGALRM, raise_on_alarm)
# start the alarm timer:
signal.alarm(1)
# and run some code that will take too long:
time.sleep(2)


# We can register starting the timer before each cell
# and clearing it after it's done with IPython events:

# In[2]:


ip = get_ipython()
# start the alarm when the cell starts
ip.events.register('pre_run_cell', lambda : signal.alarm(1))
# cancel the alarm when it finishes
ip.events.register('post_run_cell', lambda : signal.alarm(0))


# This one is fine because it takes less than a second:

# In[3]:


time.sleep(0.5)


# But two seconds is too long!

# In[4]:


time.sleep(2)


# You can enable this by default by putting it in an [IPython extension](https://ipython.readthedocs.io/en/stable/config/extensions/).