Fix non-daily rebalancing (never fired), flat-bench beta div-by-zero, nil-token columns

- portfolio._rebalance_dates: Grouper labels groups with period-END
  timestamps (Sundays, month/quarter/year ends) that never exist in a
  trading-day index, so 1W/1ME/QE/YE silently never rebalanced (only
  1D and buy-and-hold worked). Now takes the first trading day of each
  group.
- metrics.beta_alpha: guard var(bench) == 0 (flat benchmark) instead of
  dividing by zero.
- data._read_csv_clean: the downloader has written Go nil tokens
  ('%!f(<nil>)') into price cells (89 symbols, one bad day each), which
  made pandas infer the whole column as string and crash every
  pct_change on any portfolio containing them. Coerce all columns to
  numeric (nil tokens -> NaN); parquet cache rebuilt (0 string cols).
- new tests/test_portfolio.py (18 checks: rebalance dates land on
  trading days and actually fire, flat-benchmark beta, nil-token
  columns), wired into run_tests.sh (now 5 sections).
- full suite: 30 + 22 + 18 + 38 AppTest + 12 browser e2e, all passing

data.py

@@ -233,6 +233,11 @@ def _read_csv_clean(f: Path) -> pd.DataFrame:
     df = pd.read_csv(f, index_col=0)
     df.index = pd.to_datetime(df.index, format="mixed", errors="coerce")
     df = df[~df.index.isna()]
+    # The downloader has written Go nil tokens into numeric cells (e.g.
+    # '%!f(<nil>)' in Close/Adj Close). A single such token would make
+    # pandas infer the whole column as string and crash every downstream
+    # division, so coerce all columns to numeric and turn them into NaN.
+    df = df.apply(pd.to_numeric, errors="coerce")
     df = df[~df.index.duplicated(keep="last")].sort_index()
     return df
 

metrics.py

@@ -70,10 +70,11 @@ def beta_alpha(returns: pd.Series, bench: pd.Series,
     r = excess(returns, rf).dropna()
     b = excess(bench, rf).dropna()
     r, b = r.align(b, join="inner")
-    beta = np.cov(r, b)[0, 1] / np.var(b)
+    var_b = float(np.var(b))
+    beta = float(np.cov(r, b)[0, 1] / var_b) if var_b > 0 else 0.0
     # r and b are already rf-adjusted
     alpha_daily = r.mean() - beta * b.mean()
-    return float(beta), float(alpha_daily * ANN)
+    return beta, float(alpha_daily * ANN)
 
 
 def summary(price: pd.Series, bench: pd.Series | None = None,

portfolio.py

@@ -36,8 +36,13 @@ class PortfolioResult:
 
 def _rebalance_dates(index: pd.DatetimeIndex, freq: str) -> set:
     """First trading day of each period, including the first day."""
-    periods = index.to_series().groupby(pd.Grouper(freq=freq)).first()
+    if freq == "1D":
-    return set(periods.index.tolist())
+        return set(index)
+    # Grouper labels each group with its period-end (a Sunday / month-end /...
+    # none of which is in a trading-day index), so take the first index
+    # VALUE of each group instead of the group label.
+    groups = index.to_series().groupby(pd.Grouper(freq=freq))
+    return {g.index[0] for _, g in groups if len(g)}
 
 
 def portfolio_returns(adj: pd.DataFrame, weights: dict[str, float],

run_tests.sh

@@ -9,19 +9,23 @@ cd "$(dirname "$0")"
 
 fail=0
 
-echo "=== 1/4 data cache tests (incremental refresh) ==="
+echo "=== 1/5 data cache tests (incremental refresh) ==="
 .venv/bin/python tests/test_data.py || fail=1
 echo
 
-echo "=== 2/4 after-tax model tests (synthetic) ==="
+echo "=== 2/5 after-tax model tests (synthetic) ==="
 .venv/bin/python tests/test_tax.py || fail=1
 echo
 
-echo "=== 3/4 app tests (AppTest, no browser) ==="
+echo "=== 3/5 portfolio/metrics/data-loading tests ==="
+.venv/bin/python tests/test_portfolio.py || fail=1
+echo
+
+echo "=== 4/5 app tests (AppTest, no browser) ==="
 .venv/bin/python tests/test_app.py || fail=1
 echo
 
-echo "=== 4/4 browser e2e (Playwright, needs the server) ==="
+echo "=== 5/5 browser e2e (Playwright, needs the server) ==="
 if curl -s -m 3 -o /dev/null http://localhost:8599/healthz; then
     .venv/bin/python tests/test_e2e_browser.py || fail=1
 else

tests/test_portfolio.py

@@ -0,0 +1,105 @@
+"""Unit tests for portfolio construction, metrics and data loading robustness.
+
+Regressions covered:
+  * _rebalance_dates must return the first TRADING day of each period.
+    (The old code returned the Grouper period-END labels — Sundays /
+    month-ends — which never exist in a trading-day index, so every
+    non-daily rebalance scheme silently never fired.)
+  * metrics.beta_alpha must not divide by zero on a flat benchmark.
+  * _read_csv_clean must coerce downloader nil tokens ('%!f(<nil>)')
+    to NaN, keeping price columns numeric (a single such token used to
+    turn the whole column into string and crash every pct_change).
+
+Run:  .venv/bin/python tests/test_portfolio.py
+"""
+from __future__ import annotations
+
+import sys
+import tempfile
+from pathlib import Path
+
+import numpy as np
+import pandas as pd
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+import data  # noqa: E402
+import metrics  # noqa: E402
+from portfolio import _rebalance_dates, portfolio_returns  # noqa: E402
+
+PASS, FAIL = 0, 0
+
+
+def check(name: str, ok: bool) -> None:
+    global PASS, FAIL
+    PASS += bool(ok)
+    FAIL += not ok
+    print(("  ok    " if ok else "  FAIL  ") + name)
+
+
+def main() -> int:
+    idx = pd.bdate_range("2023-01-02", "2023-12-29")   # full business year
+    i0, i1 = idx[0], idx[-1]
+
+    # --- rebalance dates land on real trading days ----------------------
+    for freq in ("1W", "1ME", "QE", "YE"):
+        dates = _rebalance_dates(idx, freq)
+        check(f"{freq}: all rebalance dates are trading days",
+              bool(dates) and dates <= set(idx))
+        check(f"{freq}: first day included", i0 in dates)
+
+    # bdate_range has every weekday, so the first trading day of each
+    # week/month is exactly each Monday / the min of each month's slice
+    weekly = sorted(_rebalance_dates(idx, "1W"))
+    check("1W: rebalances exactly on the Mondays (not the Sunday labels)",
+          weekly == [d for d in idx if d.weekday() == 0])
+    monthly = sorted(_rebalance_dates(idx, "1ME"))
+    check("1ME: first trading day of each month",
+          monthly == [d for m in range(1, 13)
+                      for d in [idx[idx.month == m].min()]])
+    check("YE: one rebalance, on the first day",
+          _rebalance_dates(idx, "YE") == {i0})
+    check("1D: every day", _rebalance_dates(idx, "1D") == set(idx))
+
+    # --- the fix actually makes non-daily rebalancing happen -----------
+    p = pd.DataFrame({"a": np.linspace(10, 12, len(idx)),
+                      "b": np.linspace(10, 8, len(idx))}, index=idx)
+    r_none = portfolio_returns(p, {"a": 0.5, "b": 0.5}, rebalance=None)
+    r_wk = portfolio_returns(p, {"a": 0.5, "b": 0.5}, rebalance="1W")
+    check("1W: rebalances fire (turnover > 0 on rebalance days)",
+          r_wk.turnover.loc[weekly[1:]].gt(0).all()
+          and float(r_wk.turnover.sum()) > float(r_none.turnover.sum()) + 1e-9)
+    check("1W: rebalance_dates reported on trading days",
+          set(r_wk.rebalance_dates) == set(weekly) and
+          set(r_wk.rebalance_dates) <= set(idx))
+
+    # --- beta_alpha: flat benchmark must not divide by zero ------------
+    r = pd.Series(np.random.default_rng(0).normal(0, 0.01, 100), index=idx[:100])
+    flat = pd.Series(0.0, index=idx[:100])
+    beta, alpha = metrics.beta_alpha(r, flat, 0.0)
+    check("beta_alpha: flat benchmark -> beta 0, no crash",
+          beta == 0.0 and np.isfinite(alpha))
+
+    # --- nil tokens in CSVs stay numeric --------------------------------
+    with tempfile.TemporaryDirectory() as td:
+        f = Path(td) / "x-history.csv"
+        f.write_text("Date,Open,High,Low,Close,Adj Close,Volume\n"
+                     "2024-01-02,1.0,1.1,0.9,1.0,1.0,100\n"
+                     "2024-01-03,1.0,1.1,0.9,%!f(<nil>),%!f(<nil>),100\n"
+                     "2024-01-04,1.0,1.1,0.9,1.2,1.2,100\n")
+        df = data._read_csv_clean(f)
+        close = df["Close"]
+        check("nil token: Close column stays numeric",
+              pd.api.types.is_numeric_dtype(close))
+        check("nil token: the bad cell becomes NaN, others intact",
+              close.isna().sum() == 1 and close.iloc[0] == 1.0 and close.iloc[2] == 1.2)
+        # the real pipeline drops NaN rows before pct_change
+        rets = df["Close"].dropna().pct_change().fillna(0.0)
+        check("nil token: dropna + pct_change works (the old crash)",
+              abs(float(rets.iloc[-1]) - (1.2 / 1.0 - 1)) < 1e-9)
+
+    print(f"\n{PASS} passed, {FAIL} failed")
+    return 1 if FAIL else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())