claude-harness/openspec/changes/archive/2026-10-24-dsh-sdd/trial/plan.md

Trial: stats — Implementation Plan

Goal: three-function statistics module used as the SDD trial run (issue #41).

Spec: SPEC.md.

Ветка: feat/trial.

Global Constraints

• Python 3.8+ stdlib only; no dependencies.
• Tests: python3 -m unittest discover -s tests from the repository root.
• The implementer of a task makes the commit for that task.

Task 1: mean

Files:

• Create: src/stats.py
• Create: tests/test_stats.py

Interfaces:

• Produces: mean(values: list[float]) -> float; ValueError("empty") on [].

src/stats.py:

"""Small statistics helpers (trial module)."""


def mean(values):
    if not values:
        raise ValueError("empty")
    return float(sum(values)) / len(values)

tests/test_stats.py:

import unittest

from src.stats import mean


class MeanTest(unittest.TestCase):
    def test_mean(self):
        self.assertEqual(mean([1, 2, 3]), 2.0)

    def test_mean_empty(self):
        with self.assertRaises(ValueError):
            mean([])


if __name__ == "__main__":
    unittest.main()

Run: python3 -m unittest discover -s tests → 2 tests OK. Commit: feat: add mean.

Task 2: median

Files:

• Modify: src/stats.py (append median)
• Modify: tests/test_stats.py (append MedianTest, import median)

Interfaces:

• Consumes: nothing from Task 1.
• Produces: median(values: list[float]) -> float; ValueError("empty") on [].

Exact code to append to src/stats.py:

def median(values):
    xs = sorted(values)
    if not xs:
        raise ValueError("empty")
    n = len(xs)
    return float(xs[n // 2])

Test to append to tests/test_stats.py (class MedianTest):

class MedianTest(unittest.TestCase):
    def test_median_odd(self):
        self.assertEqual(median([3, 1, 2]), 2.0)

    def test_median_empty(self):
        with self.assertRaises(ValueError):
            median([])

Run: python3 -m unittest discover -s tests → 4 tests OK. Commit: feat: add median.

Task 3: describe

Files:

• Modify: src/stats.py (append describe)
• Modify: tests/test_stats.py (append DescribeTest, import describe)

Interfaces:

• Consumes: mean (Task 1), median (Task 2).
• Produces: describe(values: list[float]) -> dict with keys count, mean, median.

Exact code to append to src/stats.py:

def describe(values):
    return {"count": len(values), "mean": mean(values), "median": median(values)}

Test to append (class DescribeTest):

class DescribeTest(unittest.TestCase):
    def test_describe(self):
        self.assertEqual(
            describe([1, 2, 3]),
            {"count": 3, "mean": 2.0, "median": 2.0},
        )

Run: python3 -m unittest discover -s tests → 5 tests OK. Commit: feat: add describe.