Today I learned how to customise the Python REPL on start-up.

How to customise the Python REPL on start-up

Customising the Python REPL on start-up allows you to run custom code whenever you open the Python REPL. Typically, people use this to import modules they use commonly or define functions that they use a lot.

To customise the Python REPL, you need two things:

1. You need to create the file .pythonrc in which you write the code you want to run when the REPL starts.
2. You need to set the environment variable PYTHONSTARTUP to point to the file.

Then, when the REPL starts, it will look at the environment variable PYTHONSTARTUP and if it is set to something, it will run the Python code in that file.

Example .pythonrc file

My .pythonrc file is very simple and looks like this:

from functools import partial

try:
    from rich import inspect, pretty, print, traceback

    help = partial(inspect, help=True)
    pretty.install()
    traceback.install()
    print("rich setup concluded.")
except ImportError:
    pass

It uses the module rich to override the built-ins help and print with two versions that are even more helpful and colourful: rich.inspect and rich.print. That way, whenever I use help or print in the REPL (which happens A LOT), I automatically use the version from the library rich, which I am really fond of.

Additionally, it uses rich.pretty to install automatic pretty-printing in the REPL and rich.traceback to pretty print exception tracebacks.

If you want to have the same .pythonrc file, just be sure to install rich with python -m pip install rich.

Setting the environment variable PYTHONSTARTUP

To set your environment variable PYTHONSTARTUP to point to your .pythonrc file, you can run the command

set PYTHONSTARTUP=C:\path\to\your\file\.pythonrc  # Windows

export PYTHONSTARTUP=/path/to/your/file/.pythonrc  # MacOS / Linux

To make sure everything is working, add a print("hello world") to your file .pythonrc and open the shell. You should see your greeting.

If you want this configuration to be set permanently, what you can do is figure out the file that your shell uses for start-up customisation. For example, when my shell starts, it runs the commands found in the file ~/.zshrc. So, if I add the line export PYTHONSTARTUP=/path/to/your/file/.pythonrc to my file ~/.zshrc, then python will always know where to find the file .pythonrc.

Backup your .pythonrc file

I first heard of the .pythonrc file when reading Adam Johnson's “Boost Your Git DX”¹, where Adam suggests that configuration files like these (as long as they don't have sensitive data, like credentials) could be kept in a GitHub repository. Then, you just create a symlink from the typical location ~/.pythonrc to the versioned file in your repository.

This way, when you change computers, you have all your configurations ready to be cloned from the repo and they're easier to setup on your new machine.

Customise Python on start-up

If you want to customise Python on start-up, and not just the REPL, you can read my short article on running custom code on Python start-up.

That's it for now! Stay tuned and I'll see you around!

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References

• “Boost Your Git DX”, by Adam Johnson;
• Python 3 Docs, The Interactive Startup File, https://docs.python.org/3/tutorial/appendix.html#the-interactive-startup-file [last accessed 27-07-2023];