feat(stock-lab): ForeignBuy 노드 — 외국인 N일 누적 순매수 강도

stock-lab/app/test_screener_nodes_foreign_buy.py

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+import datetime as dt
+import pandas as pd
+
+from app.screener.engine import ScreenContext
+from app.screener.nodes.foreign_buy import ForeignBuy
+from app.screener._test_fixtures import make_master, make_prices, make_flow
+
+
+def _ctx(master, prices, flow):
+    return ScreenContext(master=master, prices=prices, flow=flow,
+                         kospi=pd.Series(dtype=float, name="kospi"),
+                         asof=dt.date(2026, 5, 12))
+
+
+def test_higher_foreign_buy_gets_higher_score():
+    asof = dt.date(2026, 5, 12)
+    master = make_master(["A", "B"])
+    prices = make_prices(["A", "B"], days=30, asof=asof)
+    flow = make_flow(["A", "B"], days=30, asof=asof,
+                     foreign_per_day={"A": 100_000_000, "B": 0})
+    out = ForeignBuy().compute(_ctx(master, prices, flow), {"window_days": 5})
+    assert out["A"] > out["B"]
+    assert 0 <= out.min() <= out.max() <= 100
+
+
+def test_all_zero_returns_50():
+    asof = dt.date(2026, 5, 12)
+    master = make_master(["A", "B"])
+    prices = make_prices(["A", "B"], days=30, asof=asof)
+    flow = make_flow(["A", "B"], days=30, asof=asof, foreign_per_day={"A": 0, "B": 0})
+    out = ForeignBuy().compute(_ctx(master, prices, flow), {"window_days": 5})
+    assert (out == 50.0).all()