Skip to content

# 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 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.

## Application Lifecycle Attributes

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__ 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

## Command Processing Loop

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

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

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.

## 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 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

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.