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I stream data using context manager to close the connection when the program exits. I run my program as a daemon in the background.

How can I make the context manager handle the case when the daemon is interrupted by a SIGINT or SIGTERM or any interrupt signal sent by the kill command ?

I am running Python 3 on a Raspberry Pi and Ubuntu.

I have seen this: How do I capture SIGINT in Python? Which is helpful, but I am not sure how to use that with python's context manager ? ie. let's say I have an object that I have built as a context manager:

class Sensor:

    def __init__(self, name: str):
        self.name = name

    def __enter__(self):
        self._connect()
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.close()

I use that object inside a script that is run as daemon. Is there a pythonic way to specify to specify that the __exit__ function has to be called also on SIGINT and SIGTERM exceptions ?

Neuron
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gohu
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    Does this answer your question? [How do I capture SIGINT in Python?](https://stackoverflow.com/questions/1112343/how-do-i-capture-sigint-in-python) – mkrieger1 Jun 29 '20 at 16:42
  • Not exactly, I have updated the question in that regard – gohu Jun 29 '20 at 17:09
  • Doesn’t your context manager *already* work for `SIGINT`? – Davis Herring Jun 29 '20 at 20:01
  • You need to register whichever function you want to be called as a signal handler, for example `self.close`. – mkrieger1 Jun 29 '20 at 21:23
  • Yes, and it might already work as you expect without doing that because it’s a context manager. Can you show a [mre] where the script does not work as you expect when you use the kill command? – mkrieger1 Jun 29 '20 at 21:27
  • I have tried some stuff using signal on my side. What makes it more complex is that I use threading to stream and write the data. I will update my answer with what works so far and see if there is a better idea to try – gohu Jun 30 '20 at 10:22

1 Answers1

4

You could create a internal method (e.g. _handle_interrupt) for your context manager that calls sys.exit() and register it as a signal handler once __enter__ is called:

class Sensor:

    def __init__(self, name: str):
        self.name = name

    def _handle_interrupt(self):
       sys.exit()  # will trigger a exception, causing __exit__ to be called

    def __enter__(self):
        signal.signal(signal.SIGINT, self._handle_interrupt)
        signal.signal(signal.SIGTERM, self._handle_interrupt)
        self._connect()
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.close()

You should also have a look at PEP 419, which could be helpful for your setup. As for your requirements with regards to threading it is hard to tell without more information.

nehtor.t
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  • How could this be used with https://docs.python.org/3/library/contextlib.html#contextlib.contextmanager? – AlexM Nov 13 '20 at 18:29
  • I guess you could use the above code and extend [ContextDecorator](https://docs.python.org/3/library/contextlib.html#contextlib.ContextDecorator), which will allow you to use it as decorator with any function – nehtor.t Nov 14 '20 at 16:01