pyrsistent-as-rpds-py

what is this?

This is a thin adapter library which implements a subset of rpds-py using pyrsistent. The provided functionality is sufficient to run the jsonschema test cases successfully.

why is this?

In 2023, the jsonschema devs replaced the pure-Python pyrsistent library dependency with a binary dependency called rpds-py. This replacement was done in order to improve performance (as CPython is quite slow). However, PyPy users may still prefer the pure-Python version both for speed reasons and also to avoid the hassle of building binary packages.

(The rpds-py API also just feels better than the pyrsistent one, in my humble opinion. If I wanted to build something with persistent data structures, I would probably either use this module or rpds-py.)

how to use

use case 1: I can't use rpds-py for whatever reason, some other package requires it, and I need a drop-in replacement

Pip doesn't seem to have a way to override or substitute dependencies (at least without disabling dependency resolution entirely). Therefore, I suggest running something like this:

# install package "pyrsistent-as-rpds-py"
pip install --no-index --no-build-isolation .

# install fake "rpds" package to make pip happy
cd extra/fake_rpds
pip install --no-index --no-build-isolation .

Now you can install jsonschema or other packages that depend on rpds-py normally.

use case 2: I am developing a new project and I need a persistent data structures library

Just add pyrsistent-as-rpds-py to your requirements.txt / pyproject.toml dependencies like you would any other dependency. Then you can write:

from pyrsistent_as_rpds.auto import List

lst = List(1, 2, 3)
print(lst.rest)

This will try to import the original rpds-py module first. If it fails (because rpds-py is not installed), then it use the pure Python version instead.

This library needs at least one of rpds-py or pyrsistent to be installed. If you are an end-user application packager / system integrator, you must choose to install either pyrsistent-as-rpds-py[pyrsistent] or pyrsistent-as-rpds-py[rpds-py]. This is unfortunately necessary because pip does not support alternative dependencies (e.g., require either X or Y to be installed).