alternate_event_loops.md

Alternate Event Loops

Throughout this documentation we have focused on the 90% use case, that is the use case we believe around 90+% of our user base is looking for. This focuses on ease of use and the best out-of-the-box experience where developers get the most functionality for the least amount of effort. We are talking about running cmd2 applications with the cmdloop() method:

from cmd2 import Cmd
class App(Cmd):
    # customized attributes and methods here
app = App()
app.cmdloop()

However, there are some limitations to this way of using cmd2, mainly that cmd2 owns the inner loop of a program. This can be unnecessarily restrictive and can prevent using libraries which depend on controlling their own event loop.

Many Python concurrency libraries involve or require an event loop which they are in control of such as asyncio, gevent, Twisted, etc.

cmd2 applications can be executed in a fashion where cmd2 doesn't own the main loop for the program by using code like the following:

import cmd2

class Cmd2EventBased(cmd2.Cmd):
    def __init__(self):
        cmd2.Cmd.__init__(self)

    # ... your class code here ...

if __name__ == '__main__':
    app = Cmd2EventBased()
    app.preloop()

    # Do this within whatever event loop mechanism you wish to run a single command
    cmd_line_text = "help history"
    app.runcmds_plus_hooks([cmd_line_text])

    app.postloop()

The cmd2.Cmd.runcmds_plus_hooks() method runs multiple commands via cmd2.Cmd.onecmd_plus_hooks.

The cmd2.Cmd.onecmd_plus_hooks() method will do the following to execute a single command in a normal fashion:

1. Parse user input into a cmd2.Statement object
2. Call methods registered with cmd2.Cmd.register_postparsing_hook()
3. Redirect output, if user asked for it and it's allowed
4. Start timer
5. Call methods registered with cmd2.Cmd.register_precmd_hook
6. Call cmd2.Cmd.precmd - for backwards compatibility with cmd.Cmd
7. Add statement to History
8. Call do_command method
9. Call methods registered with cmd2.Cmd.register_postcmd_hook()
10. Call cmd2.Cmd.postcmd - for backwards compatibility with cmd.Cmd
11. Stop timer and display the elapsed time
12. Stop redirecting output if it was redirected
13. Call methods registered with cmd2.Cmd.register_cmdfinalization_hook()

Running in this fashion enables the ability to integrate with an external event loop. However, how to integrate with any specific event loop is beyond the scope of this documentation. Please note that running in this fashion comes with several disadvantages, including:

• Requires the developer to write more code
• Does not support transcript testing
• Does not allow commands at invocation via command-line arguments