Pypsi - Python Pluggable Shell Interface

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Develop extensible and powerful command line interface shells with minimal code. Want to jump straight into the API? Take a look at Pypsi Shell and Command Tutorial.

Python Pluggable Shell Interface, or pypsi, is a framework for developing command line based shell interfaces, akin to bash or csh. It is intended to be a replacement for the builtin Python cmd module.

Pypsi is targetted towards both rapid prototype interfaces and large stable shells. The bootstraping code is very small with very little boilerplate. Pypsi ships with a great deal of capabilities out of the box, all of which can be used or ignored. Pypsi is pluggable which allows commands, features, and plugins to be developed independently in their own source files and/or Python classes. This results in a very clean source repository. The actual code to setup and run the shell is exetremely small, on the order of ~20-50 lines of code.

Pypsi, at its core, is pluggable. There are many hooks that allow plugin authors to extend and modify the core behavior of pypsi. Commands are isolated classes that make distribution, sharing, and modification easy.

Releases

The pypsi source code is hosted at GitHub and releases are stored at PyPI. The latest version can also be install via pip:

$ pip install pypsi

Documentation can be found on GitHub Pages.

Features

The following capabilities ship with pypsi and are available out of the box.

  • I/O redirection
  • Flexible API
  • Tab completion
  • Multiplatform
  • Minimal dependencies
  • Colors
  • Session tips and message of the day (MOTD)
  • Automated help, usage messages, and argument parsing
  • Word wrapping
  • Term highlighting (grep)
  • Tables
  • Prompt wizards
  • cmd plugin to migrate existing cmd commands into pypsi

Demo

The demo.py source file can be run to show off some of the base commands and features that ship with pypsi (the demo.py file can be downloaded from the git repo at https://github.com/ameily/pypsi/blob/master/demo.py). The commands displayed below are all optional: pypsi does not require the use of any command or plugin. The demo.py file is meant to be a reference to the Pypsi API and design. Use it as a starting point for your first shell.

Variables

pypsi)> var name = "Paul"

pypsi)> var house = "Atredis"

pypsi)> echo My name is $name, and I belong to House $house

My name is Paul, and I belong to House Atredis

pypsi)> var -l

name     Paul
house    Atredis

pypsi)> var -d name

pypsi)> echo $name

pypsi)> var name = "Paul $house"

pypsi)> echo $name

Paul Atredis

I/O redirection

pypsi)> echo Hello

Hello

pypsi)> echo Hello > output.txt

pypsi)> echo Goodbye

pypsi)> xargs -I{} "echo line: {}" < output.txt

line: Hello
line: Goodbye

pypsi)> cat output.txt | grep ll

Hello

System commands

Allows execution of external applications. Command mimics Python’s os.system() function.

pypsi)> ls

pypsi: ls: command not found

pypsi)> system ls

include/
src/
README.md

pypsi)> system ls | system grep md

README.md

Fallback command

Allows the developer to set which command gets called if one does not exist in the current shell. This is very useful, for example, if you want to fallback on any OS installed executables. In this example, the fallback command is system.

pypsi)> ls

include/
src/
README.md

Command chaining

pypsi)> echo Hello && echo --bad-arg && echo goodbye

Hello
echo: unrecgonized arguments: --bad-arg

pypsi)> echo Hello ; echo --bad-arg ; echo goodbye

Hello
echo: unrecgonized arguments: --bad-arg
goodbye

pypsi)> echo --bad-arg || echo first failed

echo: unrecgonized arguments: --bad-arg
first failed

Multiline commands

pypsi)> echo Hello, \
> Dave

Hello, Dave

pypsi)> echo This \
> is \
> pypsi \
> and it rocks

This is pypsi and it rocks

Macros

Macros are analogous to functions in bash. They provide the ability to create new commands in the shell.

pypsi)> macro hello
> echo Hello, $1
> echo Goodbye from macro $0
> end

pypsi)> hello Adam

Hello, Adam
Goodbye from macro hello

Tab Complete

Tab completion is easier than ever with PyPsi. Using the included command_completer() function, arguments and sub-commands are completed automatically when the tab key is pressed. To get started, add the use of command_completer to your custom command’s complete function:

def complete(self, shell, args, prefix):
    from pypsi.completers import command_completer
    return completions = command_completer(self.parser, shell, args, prefix)

Just pass command_completer the parser you created for the command, along with the standard arguments to the complete function, and let PyPsi work it’s magic!

pypsi)> macro -<tab>
--delete --help   --list   --show   -d       -h       -l       -s

For each argument added to a PyPsi Argument parser, a callback function to get the possible completions can be specified via the completer argument. The callback function will be called from command_completer anytime tab is pressed while the user is currently entering that argument’s value. Ex:

# Snippet from macro.py
self.parser.add_argument(
     '-s', '--show', help='print macro body',
     metavar='NAME', completer=self.complete_macros
)
...
def complete_macros(self, shell, args, prefix):
    # returns a list of macro names in the current shell
    return list(shell.ctx.macros.keys())

pypsi)> macro --show <tab>
hello   goodbye

See tail.py, help.py, and macro.py for examples.

Prompt Wizards

Prompt wizards ask the user a series of questions and request input. Input is tab completed, validated, and returned. The wizard can be used for easy configuration of components that require a substantial amount of input.

pypsi)> wizard
+-----------------------------------------------------------------------------+
|                    Entering Example Configuration Wizard                    |
+-----------------------------------------------------------------------------+
Shows various examples of wizard steps

To exit, enter either Ctrl+C, Ctrl+D, or 'quit'. For help about the current
step, enter 'help' or '?'.

IP Address: <enter>

Error: Value is required
Local IP Address or Host name

IP Address: 192.168.0.10

TCP Port [1337]: <enter>

File path: /var/lo<tab>

local/  lock/   log/

File path: /var/log/<tab>

Xorg.1.log        btmp              faillog           upstart/
Xorg.1.log.old    dist-upgrade/     fontconfig.log    wtmp
alternatives.log  distccd.log       fsck/
apt/              dmesg             lastlog
bootstrap.log     dpkg.log          mongodb/

File path: /var/log/dpkg.log

Shell mode [local]: asdf

Error: Invalid choice

Mode of the shell

Shell mode [local]: remote

Config ID    Config Value
================================================================================
ip_addr      172.16.11.204
port         1337
path         /var/log/dpkg.log
mode         remote

Background

I developed Pypsi while working on a commerical product with a command line interface. Originally, we used the cmd module, which was fine when we only had a few commands that didn’t accept complex arguments. As we added more commands and more features, maintainability and extensibility became extremely complicated and time consuming.

I took what I had learned from the cmd module, ORM libraries such as MongoEngine, and features from proven great command line interfaces such as Git and then I developed Pypsi. In order for Pypsi to be viable for our project, I knew that Pypsi had to be compatible with cmd, the porting process had to take as little time as possible, and it had to be easy to understand and maintain.

The porting process from cmd to Pypsi for our commerical project took place in January 2014. Since then, we’ve had 4 stable releases, had real world feedback, and have successfully created many Pypsi commands and plugins with ease.

License

pypsi is released under the ISC premissive license.

Indices and tables