Timeout using signals in Python

Question

I'm trying to prevent my UDP client program from waiting forever in cases of a lost packet from the server. I want to use signals to implement this. My code is:

def handler(signum):
    raise TimeoutException("Timeout occured!")

signal.signal(signal.SIGALRM, sig_alarm)
sock.sendto(data,serv)
signal.alarm(5)

try:
    received_time, servaddr = sock.recvfrom(size)
except TimeoutException:
    print "timeout"

signal.alarm(0)

When running this code in the instance in which the remote server is down, I get the following error:

global name 'TimeoutException' is not defined

I assume the program does not "see" my handler. What may cause it?

Answer 1

There is no TimeoutException exception type in either the Python 2 or Python 3 standard libraries. In Python 3.3 and newer, there is a TimeoutError type which is raised when an operating system call times out in the system (corresponding to errno being set to the ETIMEDOUT value). However, I do not recommend you raise this value yourself, due to its specific intent of being raised in response to timeouts returned by the operating system.

Correct use of signal.signal

In your comment, after defining a custom timeout exception type, you indicate a further error arises: handler() takes exactly 1 argument (2 given).

This occurs because your signal handler does not adhere to the definition of a signal handler function as expected by the signal package (those docs for Python 3, but identical to Python 2):

signal.signal(signum, handler)

Set the handler for signal signalnum to the function handler... The handler is called with two arguments: the signal number and the current stack frame

To remedy this, adjust the definition of your custom signal handler to receive the stack frame as the second argument. You need to accept this in the function call, as the standard library expects, but you don't need to do anything with the value passed in.

def handler(signum, frame):
    raise TimeoutException("Timeout occurred!")

Better way? Use socket.settimeout

There is built-in support in the socket library for what you are trying to accomplish: the settimeout method of socket objects. I would recommend using this in your code, rather than attempting to reinvent the wheel; it declares intent while abstracting away the details of the implementation and has a built-in exception type (socket.timeout) you can catch to determine when timeouts occur.

Using this method, you would simply call socket.settimeout(5) on your socket prior to making blocking calls which may timeout. You can also set a default timeout on all sockets using socket.setdefaulttimeout before opening the first socket in the application.

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Where did TimeoutException come from originally?

Obviously, your use of this may have originated from anywhere. However, I note there is a TimeoutException in Selenium, which is a widely-used Python library used for automating browsers. It is widely used and examples/Q&A associated with Selenium is common, so I would not be surprised if you had found documentation or examples which led you to believe this was an exception type present in the standard library whereas it is actually a type specific to that library. It does not appear that anything related to Selenium is applicable to your present usecase.

Answer 2

For this error : global name 'TimeoutException' is not defined that means u must declare your custom exception .. the simple way to do so is by adding this to your code :

class TimeoutException(Exception): pass

one thing to mention is that selenium has an exception named TimeoutException which is not an exception type present in the standard library but specific to the selenium library :

from selenium.common.exceptions import TimeoutException

and finally you will run to an another error ,because your signal handler must recieve the stack frame as the second argument (The handler is called with two arguments: the signal number and the current stack frame) :

def handler(signum, frame): raise TimeoutException("Timeout occurred!")

please read the following example from the docs minimal example program