Integrating cmd2 with external tools¶

Integrating cmd2 with the shell¶

Typically you would invoke a cmd2 program by typing:

$ python mycmd2program.py

or:

$ mycmd2program.py

Either of these methods will launch your program and enter the cmd2 command loop, which allows the user to enter commands, which are then executed by your program.

You may want to execute commands in your program without prompting the user for any input. There are several ways you might accomplish this task. The easiest one is to pipe commands and their arguments into your program via standard input. You don’t need to do anything to your program in order to use this technique. Here’s a demonstration using the examples/example.py included in the source code of cmd2:

$ echo "speak -p some words" | python examples/example.py
omesay ordsway

Using this same approach you could create a text file containing the commands you would like to run, one command per line in the file. Say your file was called somecmds.txt. To run the commands in the text file using your cmd2 program (from a Windows command prompt):

c:\cmd2> type somecmds.txt | python.exe examples/example.py
omesay ordsway

By default, cmd2 programs also look for commands pass as arguments from the operating system shell, and execute those commands before entering the command loop:

$ python examples/example.py help

Documented commands (type help <topic>):
========================================
alias  help     load    orate  pyscript  say  shell      speak
edit   history  mumble  py     quit      set  shortcuts  unalias

(Cmd)

You may need more control over command line arguments passed from the operating system shell. For example, you might have a command inside your cmd2 program which itself accepts arguments, and maybe even option strings. Say you wanted to run the speak command from the operating system shell, but have it say it in pig latin:

$ python example/example.py speak -p hello there
python example.py speak -p hello there
usage: speak [-h] [-p] [-s] [-r REPEAT] words [words ...]
speak: error: the following arguments are required: words
*** Unknown syntax: -p
*** Unknown syntax: hello
*** Unknown syntax: there
(Cmd)

Uh-oh, that’s not what we wanted. cmd2 treated -p, hello, and there as commands, which don’t exist in that program, thus the syntax errors.

There is an easy way around this, which is demonstrated in examples/cmd_as_argument.py. By setting allow_cli_args=False you can so your own argument parsing of the command line:

$ python examples/cmd_as_argument.py speak -p hello there
ellohay heretay

Check the source code of this example, especially the main() function, to see the technique.

Integrating cmd2 with 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 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:

Parse user input into Statement object
Call methods registered with register_postparsing_hook()
Redirect output, if user asked for it and it’s allowed
Start timer
Call methods registered with register_precmd_hook()
Call precmd() - for backwards compatibility with cmd.Cmd
Add statement to history
Call do_command method
Call methods registered with register_postcmd_hook()
Call postcmd(stop, statement) - for backwards compatibility with cmd.Cmd
Stop timer and display the elapsed time
Stop redirecting output if it was redirected
Call methods registered with 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

Here is a little more info on runcmds_plus_hooks:

Cmd.runcmds_plus_hooks(cmds: List[str]) → bool¶

Convenience method to run multiple commands by onecmd_plus_hooks.

This method adds the given cmds to the command queue and processes the queue until completion or an error causes it to abort. Scripts that are loaded will have their commands added to the queue. Scripts may even load other scripts recursively. This means, however, that you should not use this method if there is a running cmdloop or some other event-loop. This method is only intended to be used in “one-off” scenarios.

NOTE: You may need this method even if you only have one command. If that command is a load, then you will need this command to fully process all the subsequent commands that are loaded from the script file. This is an improvement over onecmd_plus_hooks, which expects to be used inside of a command loop which does the processing of loaded commands.

Example: cmd_obj.runcmds_plus_hooks([‘load myscript.txt’])

Parameters:	cmds – command strings suitable for onecmd_plus_hooks.
Returns:	True implies the entire application should exit.