cmd2 Application Lifecycle and Hooks¶

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.

Application Lifecycle Hooks¶

You can 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):
        self.poutput("before the loop begins")

To retain backwards compatibility with cmd.Cmd, after all registered preloop hooks have been called, the 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):
        self.poutput("before the loop begins")

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

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

Application Lifecycle Attributes¶

There are numerous attributes (member variables of the cmd2.Cmd) which have a significant effect on the application behavior upon entering or during the main loop. A partial list of some of the more important ones is presented here:

intro: str - if provided this serves as the intro banner printed once at start of application, after preloop runs
allow_cli_args: bool - if True (default), then searches for -t or –test at command line to invoke transcript testing mode instead of a normal main loop and also processes any commands provided as arguments on the command line just prior to entering the main loop
echo: bool - if True, then the command line entered is echoed to the screen (most useful when running scripts)
prompt: str - sets the prompt which is displayed, can be dynamically changed based on application state and/or command results

Command Processing Loop¶

When you call .cmdloop(), the following sequence of events are repeated until the application exits:

Output the prompt
Accept user input
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()

By registering hook methods, steps 4, 8, 12, and 16 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 part of your application. 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 a cmd2.EmptyStatement 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 - no error message will be displayed to the user

If a hook raises any other 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¶

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

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 PostparsingData object which we will refer to as params. params contains two attributes. params.statement is a Statement object which describes the parsed user input. There are many useful attributes in the 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 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 influece 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 Statement object and return it in params.statement. Don’t try and directly modify the contents of a Statement object, there be dragons. Instead, use the various attributes in a Statement object to construct a new string, and then parse that string to create a new Statement object.

cmd2.Cmd() uses an instance of cmd2.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 self.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 PostparsingData object with the 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¶

Precommand hooks can modify the user input, but can not 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 started, all the hooks registered with 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

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 Statement with different properties (see above). If you do so, assign your new Statement object to data.statement.

The precommand hook must return a 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, self.precmd(statement) will be called. To retain full backward compatibility with cmd.Cmd, this method is passed a Statement, not a PrecommandData object.

Postcommand Hooks¶

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 stop

Your hook will be passed a PostcommandData object, which has a 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(statement) will be called to retain full backward compatibility with cmd.Cmd.

If any postcommand hook (registered or self.postcmd()) returns 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.

Any postcommand hook can change the value of the stop parameter 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 requst 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.

Command Finalization Hooks¶

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, stop, statement):
        return stop

Command Finalization hooks must check whether the statement object is None. There are certain circumstances where these hooks may be called before the statement has been parsed, so you can’t always rely on having a statement.

If any prior postparsing or precommand hook has requested the application to terminate, the value of the stop parameter passed to the first command finalization hook will be True. Any command finalization hook can change the value of the stop parameter 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 requst 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.