lazydataclasses

Dataclass constructor, but lazier.

This module exposes a decorator @dataclass that should return a class that looks like, smells like, behaves like a dataclasses.dataclass. The difference (for our purposes) is that its __dataclass_fields__ are loaded lazily rather than at import time. This speeds up imports by around 5x at the expense of slowing down init time (around 10x slower).

pip install lazydataclasses

Compatability/Completeness

Included are a copy of the stdlib tests as of 3.14.

Most stuff passes, however the following features are unsupported:

• No __slots__ support.
• Some errors have moved from class-creation time to field-lookup time. Everywhere dataclasses.fields(...) appears it is because there is an error that no longer appears at class-creation time, most methods (eg. __init__) do a field-lookup, so these should eventually get raised.
• No InitVar support. This is pure laziness, I've never seen one in the wild.
• No generated docstring support, again pure laziness.
• inspect.signature(MyDataclass.__init__) no longer works the same as the methods aren't codegenned at import time, there are *args, **kwargs.
• A few other very small things - see @pytest.mark.xfail.

Development

uv run pytest
uv run mypy src tests --strict

Updating test suite

rm -r tests/test_dataclasses
curl 'https://github.com/python/cpython/archive/refs/tags/v3.14.0b3.zip'
unzip ...
cp -r ../cpython/Lib/test/test_dataclasses tests/
mv tests/test_dataclasses/__init__.py tests/test_dataclasses/test_stdlib.py
# add `from lazydataclasses import dataclass` import below `from dataclasses import *`