Zarr (Python 3.9 Backport)

Building

1. Clone this repository:

   git clone https://github.com/chip44/zarr-python-39.git
   cd zarr-python-39

2. Set up virtual environment:

   python -m venv venv
   source venv/bin/activate
   pip install hatch

3. Build the package:

   hatch build

Code Changes

Syntax

• Converted all match-case statements to if-else blocks (only appeared in test files)
• Replaced star-unpacking in indices (arr[*selection]) with explicit tuple conversion (arr[tuple(selection)]) (only appeared in test files)
• Reordered class fields to follow Python 3.9's requirement (non-default fields must precede fields with defaults) (might cause breakage?)

Types

• Converted union type operators (|) to Union[] declarations (only in type declarations, since type hints in methods/classes are not checked at runtime)
• Replaced isinstance() union checks (|) with tuple-based checks
• Added from __future__ import annotations to all files using type hints
• Replaced typing imports with typing_extensions for modern type hints (e.g. Self, TypeAlias)

Polyfills

• zip(strict=True): Implemented a custom wrapper (_compat.py) and imported globally
• @dataclass(kw_only=True): Implemented a custom wrapper (_compat.py) and imported globally
• itertools.pairwise(): Implemented an equivalent replacement (single occurrence)
• importlib.metadata: Replaced with importlib_metadata (also added as a dependency)

Configuration Changes

• Set minimum Python version to 3.9 in pyproject.toml
• Renamed package to zarr-py39
• Bundled numcodecs dependency (for simplified maintenance):
  • Includes all compatibility modifications listed above
  • Note: Since numcodecs contains C extensions, the zarr-py39 wheel must be compiled per Python version (e.g., cp39 Python 3.9 wheel), unlike the original py3 generic wheel