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` readline 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

- flag_based_complete - helper method for tab completion based on a particular flag preceding the token being completed

- index_based_complete - helper method for tab completion based on a fixed position in the input string

  > - See the basic_completion example for a demonstration of how to use these features
  > - `flag_based_complete()` and `index_based_complete()` are basic methods and should only be used if you are not familiar with argparse. The recommended approach for tab completing positional tokens and flags is to use argparse-based completion.

## 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 argparse.ArgumentParser.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.

## For More Information

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