pytest-drift 0.1.2

Pytest plugin for regression testing via branch comparison

pytest-drift

A pytest plugin for regression testing via branch comparison. When a test returns a value, the plugin runs the same test on a base git branch and compares the results — catching regressions before they merge.

How it works

1. You run pytest --drift BASE_BRANCH
2. For every test that returns a non-None value, the plugin:
   • Records the return value from the current branch (HEAD)
   • Simultaneously runs the same tests on BASE_BRANCH in a git worktree
   • Compares the two results at the end of the session
3. Tests returning None (the default for normal pytest tests) are ignored entirely

The base branch runs in parallel with your HEAD tests, so total wall time is approximately max(HEAD_time, BASE_time) rather than HEAD_time + BASE_time.

Installation

pip install pytest-drift

# With smart DataFrame diff reports (recommended):
pip install "pytest-drift[datacompy]"

Usage

CLI flag

pytest --drift main
pytest --drift origin/main

Environment variable

export PYTEST_DRIFT_BASE_BRANCH=main
pytest

Writing regression tests

Return a value from your test — that's it:

def test_revenue_calculation():
    df = compute_revenue(load_data())
    return df  # compared against the same function on BASE_BRANCH

def test_model_accuracy():
    return evaluate_model()  # compared as a float

def test_pipeline_output():
    return run_pipeline()  # compared as a dict, list, DataFrame, etc.

Normal tests (returning None) are unaffected and run as usual.

Comparison logic

The plugin dispatches comparison based on the return type:

Type	Comparison method
pd.DataFrame	Auto-detects join columns; uses datacompy if installed, else pd.testing.assert_frame_equal
pd.Series	Converted to DataFrame, same path as above
float / np.floating	math.isclose with rtol=1e-5, atol=1e-8
np.ndarray	np.testing.assert_array_almost_equal (5 decimal places)
dict	Recursive key-by-key comparison
list / tuple	Element-wise comparison
Everything else	==, with repr() diff on failure

Pandas index auto-detection

When comparing DataFrames, the plugin automatically finds the best join key:

1. Named index: if the DataFrame already has a named (non-RangeIndex) index, it's used directly
2. MultiIndex: all named index levels are used
3. Column heuristic: searches combinations of up to 3 non-float columns with full cardinality (every row is unique in that combination)
4. Positional fallback: if no unique key is found, rows are compared positionally

You can also pass join_columns explicitly by calling compare_dataframes directly from pandas_utils.

Terminal output

At the end of the session a regression summary is printed:

========================================================================
REGRESSION COMPARISON SUMMARY
========================================================================
PASSED tests/test_revenue.py::test_revenue_calculation
FAILED tests/test_model.py::test_model_accuracy
    Float mismatch:
      head: 0.923
      base: 0.941
------------------------------------------------------------------------
1 passed, 1 failed (2 total regression comparisons)

How branch switching works

The plugin uses git worktree add to check out BASE_BRANCH into a temporary directory — your working tree is never touched. The worktree is cleaned up automatically after the session.

HEAD tests run         ─────────────────────────▶  sessionfinish
                                                       │
git worktree add ──▶  BASE tests run in parallel  ────┘  compare

Requirements

Package	Required	Purpose
pytest >= 7.0	Yes	Core
cloudpickle >= 3.0	Yes	Serialization of return values
pandas >= 1.5	Yes	DataFrame/Series support
datacompy >= 0.9	Optional	Rich DataFrame diff reports
pyarrow >= 10.0	Optional	Parquet storage for large DataFrames

Comparison with similar tools

	pytest-drift	syrupy / pytest-snapshot	pytest-regressions
Baseline source	git branch (live re-run)	committed snapshot file	committed YAML/CSV file
Baseline stays fresh	yes — base branch always re-runs	only when you update snapshots	only when you update fixtures
Detects environment drift	yes — same code path, different branch	no	no
Test changes required	no — just return a value	yes — use a snapshot fixture	yes — use a regression fixture
DataFrame support	yes, with datacompy	via custom serializer	yes, via dataframe_regression

When to use pytest-drift — you want to catch regressions introduced by your current branch without manually maintaining baseline files. Ideal for data pipelines, model outputs, or any function whose output is hard to specify upfront but easy to compare.

When to use snapshot tools — you want a stable, reviewable artifact in version control. Snapshots are better when the baseline should be human-readable or when you're not working in a git-branch workflow.

Caveats

• The base branch subprocess uses the same Python environment as HEAD — if your project uses tox or nox, point to the correct environment
• Session-scoped fixtures with side effects (e.g. starting a server) will run twice — once per session
• Tests that fail on HEAD are not compared (no base result is fetched for them)
• Tests that fail on BASE produce a "base branch test failed, cannot compare" warning