Integrating cmd2 with external tools

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()

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.

Integrating cmd2 with event loops

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):

    # ... your class code here ...

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

    # Do this within whatever event loop mechanism you wish to run a single command
    cmd_line_text = "help history"


The runcmds_plus_hooks() method is a convenience method to run multiple commands via onecmd_plus_hooks(). It properly deals with load commands which under the hood put commands in a FIFO queue as it reads them in from a script file.

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

  1. Parse the command line text
  2. Execute postparsing_precmd()
  3. Add the command to the history
  4. Apply output redirection, if present
  5. Execute precmd()
  6. Execute onecmd() - this is what actually runs the command
  7. Execute postcmd()
  8. Undo output rediriection (if present) and perform piping, if present
  9. Execute postparsing_postcmd()

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

Here is a little more info on runcmds_plus_hooks: