Error handling using cmd module

[leifeng]

I'm using Python's cmd module to write my own command line interpreter. I'm not sure how to handle errors using the modules.

Let's say my command eat requires 1 argument.

The function usually goes as follows:

    def do_eat(self, arg):
        args = arg.split()
        if len(args) < 1:
            print("Missing arguments!")
            return

However, I was wondering if one could write it as:

    def do_eat(self, arg):
        args = arg.split()
        if len(args) < 1:
            raise "Missing arguments" # Or raise an enum

[Gribouillis]

I suggest

print("Error: missing arguments.")

It will print an error message, but will not exit the intrepreter as an exception would. Also the word "Error" must appear so that the user knows immediatly that something is going wrong.

[DeaD_EyE]

It's the question of responsibility. If you raise an Error, you must catch it on the caller side, but the caller is in this case the loop of cmd. Without changing code, you can't raise an Exception and catch it outside the function.

By the way, I was asking myself how to stop the command loop:

import cmd


class Cmd(cmd.Cmd):
    def do_quit(self, agrg):
        return True


Cmd().cmdloop()

If you enter quit, the method do_quit is called and True is returned to the postcmd method and stops the command loop.

[leifeng]

Here is my solution:

Every command has the following template:

    def do_eat(self, arg):
        try:
            # eat
        except Exception as exc:
            return exc

Then I print the errors:

    def postcmd(self, exc, line):
        if exc != True:
            print("Error: {}".format(exc))

Later on, when I unittest:

        self.assertEqual(self.cmd_class.do_eat("args"), None) # if success
        self.assertIsInstance(self.cmd_class.do_eat("args"), ExceptionClass) # if fail