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Handling Exceptions
As you’ve seen already, discord.py is an event-driven system. This focus on events extends even to exceptions. When one event handler raises an Exception, Discord calls on_error(). The default behavior of on_error() is to write the error message and stack trace to stderr. To test this, you’ll add a special message handler to on_message().
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This is part 10. Now, let’s take a look at exception handling. As you would have already seen, discord.py is an event-driven system. This focus on events extends all the way to exceptions.
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When one event handler raises an Exception, Discord calls on_error(). The default behavior of on_error() is to write the error message and stack trace to stderr (standard error). To test this, add a special message handler to on_message().
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You can see it’s been added here and it will raise the DiscordException. This new raise-exception message handler allows you to raise a DiscordException on command. Run the program, wait for it to connect,
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and type raise-exception into the Discord channel. You should now see the Exception that was raised by your on_message() handler in the console, just at the bottom here, Discord.errors.DiscordException, so you know that the exception handler is working.
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The Exception was caught by the default error handler, so the output contains a message Ignoring exception in on_message.
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If we go back—there it is. Ignoring exception in on_message, just there. Let’s fix that by handling
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that particular error. In order to do so, you’ll catch the DiscordException and write to a file, instead. The on_error() event handler takes the event as the first argument.
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In this case, we expect the event to be 'on_message'.
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It also accepts *args and **kwargs as flexible, positional and keyword arguments passed to the original event handler. So, since on_message() takes a single argument, message, we expect args[0] to be the message that the user sent in the Discord channel.
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If the Exception originated in the on_message() event handler, which is what this line does, you write a formatted string to the file err.log, which is what this line does. If another event raises an Exception, then we simply want our handler to re-raise the Exception to invoke the default behavior. Rerun bot.py and send the raise-exception message again, to view the output in err.log.
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This is err.log, I’ve just done a little bit of formatting, so it’s a little bit more readable because it comes, by default, in one big line, makes it a little bit annoying, but you can still see it. So, as you can see, instead of only a stack trace, you have a more informative error showing the message that caused on_message() to raise the DiscordException.
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Save to a file for longer persistence. As a quick technical note, if you wish to take the actual Exception into account when you are writing your error messages to the error log, or err.log, then you can use functions from sys, such as exc_info(). Well done!
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You now have some experience in handling different events and interacting with Discord APIs. Next up, you will learn about a subclass of Client called Bot, which implements some useful, bot-specific functionality.
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