argcomp

Purpose

The argcomp package provides automatic argument completion support for all Python programs that use argparse's ArgumentParser class for their parameter handling. It does so by providing a drop-in replacement that transparently provides a (hidden) --_complete argument that can be invoked to automatically complete arguments. All that has to be done to hook it up is to register a completion by invocation of the program with this argument with the shell.

Rationale

There already exist a couple of packages that provide similar functionality. However, each was identified as inadequate.

• optcomplete: The optcomplete package, as the name suggests, only works with the optparse module which is deprecated and should not be used in new programs. It is also incompatible with Python 3, disqualifying it immediately.

• python-selfcompletion: This package works with argparse but is also not Python 3 ready. The code is not very well structured and inflexible because support for custom completer functions is missing.

• argcomplete: The argcomplete package appears to be Python 3 compatible. However, in order to provide its functionality it copies, pastes, and modifies the argparse code as well as the shlex module. This approach comes with the downside of requiring backports of fixes to those packages. Not speaking of it plainly being ugly. On the bright side, this package supports custom completers and is the only viable option but because of the code duplication it was discarded as well.

argcomp combines the best of the aforementioned packages. It is fully Python 3 compliant. It interfaces with argparse's ArgumentParser without duplicating all its code and without peeking into any internals. Lastly, it provides support for custom completers to allow for context sensitive completions.

Usage

To make use of the argcomp package, the user only needs to replace the usage of argparse's ArgumentParser with the provided CompletingArgumentParser class. Because the latter is fully compatible to the former no additional work is required.

--- example.py
+++ example.py
@@ -1,7 +1,7 @@
 #!/usr/bin/python

-from argparse import (
-  ArgumentParser,
+from deso.argcomp import (
+  CompletingArgumentParser as ArgumentParser,
 )

 parser = ArgumentParser(description="Process some integers.")

In order to make the completion functionality accessible from the shell, it needs to be registered first. Typically, this registration happens by sourcing a prepared completion file upon start of the shell. A sample completion file (valid for bash) looks like so:

_complete_example()
{
  local completions=$("${1}" --_complete "${COMP_CWORD}" "${COMP_WORDS[@]}")
  if [ $? -eq 0 ]; then
    readarray -t COMPREPLY < <(echo -n "${completions}")
  fi
}

complete -F _complete_example example.py

Here, the script example.py is registered to be completed by the newly created shell function _complete_example. Once this file is sourced in a shell, completion is available.

Completers

argcomp supports custom completers. A completer is a simple function that is registered along with the argument for which to provide completion. Such a function needs to have a given interface and yield its completions. Other than that there are practically no limitations on what a completer can use to provide completions.

A sample completer providing completions for local files and directories can look like this:

def localFileCompleter(parser, values, word):
  """A completer for files in the current working directory."""
  for value in listdir():
    if value.startswith(word):
      yield value

Registration is trivial and happens along with argument specification:

--- cat.py
+++ cat.py
@@ -13,7 +13,7 @@ def localFileCompleter(parser, values, word):

 parser = CompletingArgumentParser(prog="cat")
 parser.add_argument(
-  "files", nargs="+",
+  "files", nargs="+", completer=localFileCompleter,
   help="Files to cat."
 )

Note that completers are not part of argparse's ArgumentParser. As such, switching back to it requires removal of the completer keyword parameter.

Installation

The argcomp package has no dependencies other than a standard Python 3 installation. In order to install it it suffices to make the src/ directory known to Python by embedding it into the PYTHONPATH.

If you are using Gentoo Linux, there is an ebuild available that can be used directly.

Support

The module is tested with Python 3. There is no work going on to ensure compatibility with Python 2.