How do you make a console application which accepts commands?

Question

I am relatively new to Python and programming in general. I am working on writing a console application.

How do you write a console application that accepts commands in a terminal? For instance, like how a terminal itself accepts commands and does a corresponding task. Are the "commands" actually just functions in the application that are called by the user? Is the console interface itself just a function? E.g. :

def console_interface(): user_input = input() if user_input == "some_function": some_function() if user_input == "some_other_function": some_other_function()

Although it is not efficient, I know the above works because I have tested it. Is this general idea correct or is it way off?

Have you had a look at the cmd module? I would recommend you using this: docs.python.org/2/library/cmd.html — user4695347
– user4695347, Commented Mar 21, 2015 at 15:30
Also argparse docs.python.org/2/library/argparse.html#module-argparse — Paulo Bu
– Paulo Bu, Commented Mar 21, 2015 at 15:30
You might want to look at eval() if all you need to do is dispatch the input. — AChampion
– AChampion, Commented Mar 21, 2015 at 15:34
In general, yes, parsing the input of a user is what you are doing. Other solutions and libraries are built upon this and may add additional functionalities or a framework so you can easier extend it by without having to reinvent everything all the time. — poke
– poke, Commented Mar 21, 2015 at 15:35

Alex Martelli · Accepted Answer · 2015-03-21 15:45:28Z

Python's standard library offers a module that encapsulates exactly the "console application that accepts commands" functionality: see https://docs.python.org/3/library/cmd.html .

In that module, the commands are actually methods of your class, which subclasses cmd.Cmd: do_this, do_that, etc, by naming convention. The example at https://docs.python.org/3/library/cmd.html#cmd-example is a rich "console accepting commands" for turtle graphics, so you can play with it.

Didactically, you may want to start with far simpler examples given at http://pymotw.com/2/cmd/ -- that's Python 2 but the functionality is just about the same. The excellent series of examples need a little adaptation to run in Python 3, but it shouldn't be too hard.

For example, consider the very first one:

import cmd class HelloWorld(cmd.Cmd): """Simple command processor example.""" def do_greet(self, line): print "hello" def do_EOF(self, line): return True if __name__ == '__main__': HelloWorld().cmdloop()

The do_EOF is what happens when the user terminates standard input (control-D on Unix); as https://docs.python.org/3/library/cmd.html#cmd.Cmd.cmdloop says,

An end-of-file on input is passed back as the string 'EOF'.

(In this case, the return True terminates the program).

The only thing you need to change to run this in Python 2 rather than 3 is the one line:

 print "hello"

which must become

 print("hello")

because print, which was a statement in Python 2, is now a function in Python 3.

I find the cmd.py sources at http://www.opensource.apple.com/source/python/python-3/python/Lib/cmd.py to also be quite instructive and I would recommend studying them as an introduction to the world of "dispatching"...!

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