The typical way of starting a cmd2 application is as follows:

import cmd2
class App(cmd2.Cmd):
    # customized attributes and methods here

if __name__ == '__main__':
    app = App()
    app.cmdloop()

There are several pre-existing methods and attributes which you can tweak to control the overall behavior of your application before, during, and after the command processing loop.

You can run a script on initialization by passing the script filename in the startup_script parameter of cmd2.Cmd.__init__.

You can also register methods to be called at the beginning of the command loop:

    class App(cmd2.Cmd):
        def __init__(self, *args, **kwargs):
            super().__init__(*args, **kwargs)
            self.register_preloop_hook(self.myhookmethod)

        def myhookmethod(self) -> None:
            self.poutput("before the loop begins")

To retain backwards compatibility with cmd.Cmd, after all registered preloop hooks have been called, the cmd2.Cmd.preloop method is called.

A similar approach allows you to register functions to be called after the command loop has finished:

    class App(cmd2.Cmd):
        def __init__(self, *args, **kwargs):
            super().__init__(*args, **kwargs)
            self.register_postloop_hook(self.myhookmethod)

        def myhookmethod(self) -> None:
            self.poutput("after the loop ends")

To retain backwards compatibility with cmd.Cmd, after all registered postloop hooks have been called, the cmd2.Cmd.postloop method is called.

Preloop and postloop hook methods are not passed any parameters and any return value is ignored.

The approach of registering hooks instead of overriding methods allows multiple hooks to be called before the command loop begins or ends. Plugin authors should review this page carefully in full for best practices writing hooks.

There are numerous attributes on [cmd2.Cmd][] which affect application behavior upon entering or during the command loop:

• [cmd2.Cmd.intro][] - if provided this serves as the intro banner printed once at start of application, after cmd2.Cmd.preloop is called
• [cmd2.Cmd.prompt][] - see Prompt for more information
• [cmd2.Cmd.continuation_prompt][] - The prompt issued to solicit input for the 2nd and subsequent lines of a Multiline Command
• [cmd2.Cmd.echo][] - if True write the prompt and the command into the output stream

In addition, several arguments to [cmd2.Cmd.__init__][cmd2.Cmd.init] also affect the command loop behavior:

• allow_cli_args - allows commands to be specified on the operating system command line which are executed before the command processing loop begins
• startup_script - run a script on initialization. See Scripting for more information

When you call [cmd2.Cmd.cmdloop][], the following sequence of events are repeated until the application exits:

1. Start the prompt-toolkit event loop
2. Call [cmd2.Cmd.pre_prompt][] for any behavior that should happen after event loop starts but before prompt is displayed
3. Output the prompt
4. Accept user input
5. Parse user input into a [cmd2.Statement][] object
6. Call methods registered with [cmd2.Cmd.register_postparsing_hook][]
7. Redirect output, if user asked for it and it's allowed
8. Start timer
9. Call methods registered with [cmd2.Cmd.register_precmd_hook][]
10. Call [cmd2.Cmd.precmd][] - for backwards compatibility with cmd.Cmd
11. Add statement to History
12. Call do_command method
13. Call methods registered with [cmd2.Cmd.register_postcmd_hook][]
14. Call [cmd2.Cmd.postcmd][] - for backwards compatibility with cmd.Cmd
15. Stop timer and display the elapsed time
16. Stop redirecting output if it was redirected
17. Call methods registered with [cmd2.Cmd.register_cmdfinalization_hook][]

By registering hook methods, multiple steps allow you to run code during, and control the flow of the command processing loop. Be aware that plugins also utilize these hooks, so there may be code running that is not directly part of your application code. Methods registered for a hook are called in the order they were registered. You can register a function more than once, and it will be called each time it was registered.

Postparsing, precommand, and postcommand hook methods share some common ways to influence the command processing loop.

If a hook raises an exception:

• no more hooks (except command finalization hooks) of any kind will be called
• if the command has not yet been executed, it will not be executed
• the exception message will be displayed for the user.

Specific types of hook methods have additional options as described below.

Postparsing hooks are called after the user input has been parsed but before execution of the command. These hooks can be used to:

• modify the user input
• run code before every command executes
• cancel execution of the current command
• exit the application

When postparsing hooks are called, output has not been redirected, nor has the timer for command execution been started.

To define and register a postparsing hook, do the following:

class App(cmd2.Cmd):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.register_postparsing_hook(self.myhookmethod)

    def myhookmethod(self, params: cmd2.plugin.PostparsingData) -> cmd2.plugin.PostparsingData:
        # the statement object created from the user input
        # is available as params.statement
        return params

cmd2.Cmd.register_postparsing_hook checks the method signature of the passed callable, and raises a TypeError if it has the wrong number of parameters. It will also raise a TypeError if the passed parameter and return value are not annotated as PostparsingData.

The hook method will be passed one parameter, a [cmd2.plugin.PostparsingData][] object which we will refer to as params. params contains two attributes. params.statement is a cmd2.Statement object which describes the parsed user input. There are many useful attributes in the cmd2.Statement object, including .raw which contains exactly what the user typed. params.stop is set to False by default.

The hook method must return a cmd2.plugin.PostparsingData object, and it is very convenient to just return the object passed into the hook method. The hook method may modify the attributes of the object to influence the behavior of the application. If params.stop is set to True, a fatal failure is triggered prior to execution of the command, and the application exits.

To modify the user input, you create a new cmd2.Statement object and return it in params.statement. Don't try and directly modify the contents of a cmd2.Statement object, there be dragons. Instead, use the various attributes in a cmd2.Statement object to construct a new string, and then parse that string to create a new cmd2.Statement object.

cmd2.Cmd uses an instance of [cmd2.parsing.StatementParser][] to parse user input. This instance has been configured with the proper command terminators, multiline commands, and other parsing related settings. This instance is available as the cmd2.Cmd.statement_parser attribute. Here's a simple example which shows the proper technique:

def myhookmethod(self, params: cmd2.plugin.PostparsingData) -> cmd2.plugin.PostparsingData:
    if not '|' in params.statement.raw:
        newinput = params.statement.raw + ' | less'
        params.statement = self.statement_parser.parse(newinput)
    return params

If a postparsing hook returns a cmd2.plugin.PostparsingData object with the cmd2.plugin.PostparsingData.stop attribute set to True:

• no more hooks of any kind (except Command Finalization Hooks) will be called
• the command will not be executed
• no error message will be displayed to the user
• the application will exit

Precommand hooks can modify the user input, but cannot request the application terminate. If your hook needs to be able to exit the application, you should implement it as a postparsing hook.

Once output is redirected and the timer is started, all the hooks registered with cmd2.Cmd.register_precmd_hook are called. Here's how to do it:

class App(cmd2.Cmd):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.register_precmd_hook(self.myhookmethod)

    def myhookmethod(self, data: cmd2.plugin.PrecommandData) -> cmd2.plugin.PrecommandData:
        # the statement object created from the user input
        # is available as data.statement
        return data

cmd2.Cmd.register_precmd_hook checks the method signature of the passed callable, and raises a TypeError if it has the wrong number of parameters. It will also raise a TypeError if the parameters and return value are not annotated as PrecommandData.

You may choose to modify the user input by creating a new cmd2.Statement with different properties (see above). If you do so, assign your new cmd2.Statement object to data.statement.

The precommand hook must return a [cmd2.plugin.PrecommandData][] object. You don't have to create this object from scratch, you can just return the one passed into the hook.

After all registered precommand hooks have been called, cmd2.Cmd.precmd will be called. To retain full backward compatibility with cmd.Cmd, this method is passed a cmd2.Statement, not a cmd2.plugin.PrecommandData object.

Once the command method has returned (i.e. the do_command(self, statement) method has been called and returns, all postcommand hooks are called. If output was redirected by the user, it is still redirected, and the command timer is still running.

Here's how to define and register a postcommand hook:

class App(cmd2.Cmd):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.register_postcmd_hook(self.myhookmethod)

    def myhookmethod(self, data: cmd2.plugin.PostcommandData) -> cmd2.plugin.PostcommandData:
        return data

Your hook will be passed a [cmd2.plugin.PostcommandData][] object, which has a cmd2.plugin.PostcommandData.statement attribute that describes the command which was executed. If your postcommand hook method gets called, you are guaranteed that the command method was called, and that it didn't raise an exception.

If any postcommand hook raises an exception, the exception will be displayed to the user, and no further postcommand hook methods will be called. Command finalization hooks, if any, will be called.

After all registered postcommand hooks have been called, self.postcmd will be called to retain full backward compatibility with cmd.Cmd.

If any postcommand hook (registered or self.postcmd) returns a cmd2.plugin.PostcommandData object with the stop attribute set to True, subsequent postcommand hooks will still be called, as will the command finalization hooks, but once those hooks have all been called, the application will terminate. Likewise, if self.postcmd returns True, the command finalization hooks will be called before the application terminates.

Any postcommand hook can change the value of the stop attribute before returning it, and the modified value will be passed to the next postcommand hook. The value returned by the final postcommand hook will be passed to the command finalization hooks, which may further modify the value. If your hook blindly returns False, a prior hook's request to exit the application will not be honored. It's best to return the value you were passed unless you have a compelling reason to do otherwise.

To purposefully and silently skip postcommand hooks, commands can raise any of the following exceptions.

• [cmd2.exceptions.SkipPostcommandHooks][]
• [cmd2.exceptions.Cmd2ArgparseError][]

Command finalization hooks are called even if one of the other types of hooks or the command method raise an exception. Here's how to create and register a command finalization hook:

class App(cmd2.Cmd):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.register_cmdfinalization_hook(self.myhookmethod)

    def myhookmethod(self, data: cmd2.plugin.CommandFinalizationData) -> cmd2.plugin.CommandFinalizationData:
        return data

Command Finalization hooks must check whether the cmd2.plugin.CommandFinalizationData.statement attribute of the passed [cmd2.plugin.CommandFinalizationData][] object contains a value. There are certain circumstances where these hooks may be called before the user input has been parsed, so you can't always rely on having a cmd2.plugin.CommandFinalizationData.statement.

If any prior postparsing or precommand hook has requested the application to terminate, the value of the cmd2.plugin.CommandFinalizationData.stop attribute passed to the first command finalization hook will be True. Any command finalization hook can change the value of the cmd2.plugin.CommandFinalizationData.stop attribute before returning it, and the modified value will be passed to the next command finalization hook. The value returned by the final command finalization hook will determine whether the application terminates or not.

This approach to command finalization hooks can be powerful, but it can also cause problems. If your hook blindly returns False, a prior hook's request to exit the application will not be honored. It's best to return the value you were passed unless you have a compelling reason to do otherwise.

If any command finalization hook raises an exception, no more command finalization hooks will be called. If the last hook to return a value returned True, then the exception will be rendered, and the application will terminate.