Opaque Python CLI Wrapper
A scrim is a piece of cloth that’s opaque until lit from behind. Like a scrim, Scrim provides opaque scripts to wrap python command line tools and a transparent python api to send shell commands up to the user’s shell.
Why would I need Scrim?
When writing command line tools in python, changes to–let’s say–environment variables, don’t persist when the python process exits. This means we can’t write certain types of tools in pure python, we need to revert to our shell’s scripting language instead. That’s why tools like virtualenvwrapper are written in shell scripting languages instead of pure python. Scrim provides opaque shell scripts that wrap your python cli, so you can continue writing the interfaces for your python programs in python.
How does it work?
In this diagram the Scrim Script is the opaque shell script provided by the scrim package. Following along with the diagram, the User calls the cli command. This invokes the Scrim Script which sets these environment variables:
- SCRIM_SHELL - name of the shell process (powershell.exe, cmd.exe)
- SCRIM_PATH - path to temp script which python will write
- SCRIM_SCRIPT - full path to the Scrim Script
- SCRIM_AUTO_WRITE - whether or not to automatically write the temp script when python exits.
Then the Scrim Script invokes the actual Python CLI passing all arguments from the User. The Python CLI can now use the scrim api to append commands to a list. When the python program exits, the list of commands is written to a temporary script file. The Scrim Script now continues and executes the temporary script file if it exists. Finally the Scrim Script removes any temporary files and unsets the above environment variables.
First add Scrim scripts to your project using Scrim’s cli.
> cd mytool > scrim add --entry_point pymytool Creating scripts for: pymytool Created bin/mytool.bat Created bin/mytool.ps1 Created bin/mytool.sh Add the following section to your package setup: scripts=[ 'bin/mytool.bat', 'bin/mytool.ps1', 'bin/mytool.sh' ],
Assume that mytool is a python project containing a setup.py file. Here we’ve provided scrim add with the name of the entry_point to our python cli. If you’ve got multiple entry_points already defined in your package you can use:
> scrim add --all_entry_points ...
This will add Scrim Scripts to each console_script you’ve defined in entry_points.
Now that you’re project has Scrim added to it let’s take a look at the python side.
import click from scrim import get_scrim scrim = get_scrim() @click.command() def mytool(): scrim.set_env('MYTOOL', "Hello World!") if __name__ == '__main__': mytool()
We use get_scrim to get an instance of Scrim. Then we append commands to the scrim and those will be written to a shell script when python exits. After python exits the scrim script will check to see if a shell script exists and execute it. In this case the environment variable MYTOOL will be set to Hello World!.
Installing a library that uses Scrim
A simple pip install will do you. The scrim scripts will be picked up normally from your command line, you won’t even know they are there!
In addition to your standard pip install you also have to source the scrim script after installation. It usually ends up in one of these locations:
- More tests…
- Support more shells: fish, csh, zsh…
- Add more commands to the Scrim
- Extend scrim cli to better support a variety of entry_points scenarios
- Currently we only capture setup entry_points when defined as a dict.
- We also only support entry_points beginning with py.
- Parse setup.cfg in addition to setup.py
Use nose to manually run the scrim test suite.
> nosetests -v -s --with-doctest
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