Skip to content

# Completion

`cmd2.Cmd` adds tab completion of file system paths for all built-in commands where it makes sense, including:

- edit
- run_pyscript
- run_script
- shell

`cmd2.Cmd` also adds tab completion of shell commands to the shell command.

It is easy to add identical file system path completion to your own custom commands. Suppose you have defined a custom command `foo` by implementing the `do_foo` method. To enable path completion for the `foo` command, then add a line of code similar to the following to your class which inherits from `cmd2.Cmd`:

    complete_foo = cmd2.Cmd.path_complete

This will effectively define the `complete_foo` prompt-toolkit completer method in your class and make it utilize the same path completion logic as the built-in commands.

The built-in logic allows for a few more advanced path completion capabilities, such as cases where you only want to match directories. Suppose you have a custom command `bar` implemented by the `do_bar` method. You can enable path completion of directories only for this command by adding a line of code similar to the following to your class which inherits from `cmd2.Cmd`:

    # Make sure you have an "import functools" somewhere at the top
    complete_bar = functools.partialmethod(cmd2.Cmd.path_complete, path_filter=os.path.isdir)

## Included Tab Completion Functions

cmd2.Cmd provides the following tab completion functions

- basic_complete - helper method for tab completion against a list

- path_complete - helper method provides flexible tab completion of file system paths

  > - See the paged_output example for a simple use case
  > - See the python_scripting example for a more full-featured use case

- delimiter_complete - helper method for tab completion against a list but each match is split on a delimiter

  > - See the basic_completion example for a demonstration of how to use this feature

## Raising Exceptions During Completion

There are times when an error occurs while tab completing and a message needs to be reported to the user. These include the following example cases:

- Reading a database to retrieve a tab completion data set failed
- A previous command line argument that determines the data set being completed is invalid
- Tab completion hints

`cmd2` provides the CompletionError exception class for this capability. If an error occurs in which it is more desirable to display a message than a stack trace, then raise a `CompletionError`. By default, the message displays in red like an error. However, `CompletionError` has a member called `apply_style`. Set this False if the error style should not be applied. For instance, `ArgparseCompleter` sets it to False when displaying completion hints.

## Tab Completion Using the argparse Decorator

When using `cmd2`'s @with_argparser decorator, `cmd2` provides automatic tab completion of flag names.

Tab completion of argument values can be configured by using one of three parameters to `Cmd2ArgumentParser.add_argument()`.

- `choices`
- `choices_provider`
- `completer`

See the argparse_example example for a demonstration of how to use the `choices` parameter. See the argparse_completion example for a demonstration of how to use the `choices_provider` parameter. See the argparse_example or argparse_completion example for a demonstration of how to use the `completer` parameter.

When tab completing flags or argument values for a `cmd2` command using the `@with_argparser` decorator, `cmd2` keeps track of state so that once a flag has already previously been provided, it won't attempt to tab complete it again. When no completion results exist, a hint for the current argument will be displayed to help the user.

## CompletionItem For Providing Extra Context

When tab completing things like a unique ID from a database, it can often be beneficial to provide the user with some extra context about the item being completed, such as a description. To facilitate this, `cmd2` defines the CompletionItem class which can be returned from any of the 3 completion parameters: `choices`, `choices_provider`, and `completer`.

See the argparse_completion example or the implementation of the built-in set command for demonstration of how this is used.

## Custom Completion with `read_input()`

`cmd2` provides cmd2.Cmd.read_input as an alternative to Python's `input()` function. `read_input` supports configurable tab completion and up-arrow history at the prompt. See read_input example for a demonstration.

## Custom Completion Menu Colors

`cmd2` provides the ability to customize the foreground and background colors of the completion menu items and their associated help text. See Customizing Completion Menu Colors in the Theme documentation for more details.

## For More Information

See cmd2's argparse_utils API for a more detailed discussion of argparse completion.