From 15c5163508ef655766349666854c95f10ba13049 Mon Sep 17 00:00:00 2001
From: David-Wu1119 <133224895+David-Wu1119@users.noreply.github.com>
Date: Wed, 23 Sep 2026 13:05:03 -0400
Subject: [PATCH] BUG: Fix Backtest() with a non-datetime index on pandas 3

`Backtest.__init__` checks `data.index.is_numeric()` for every index that
is neither a DatetimeIndex nor a RangeIndex. pandas 3.0 removed
`Index.is_numeric()`, so `Backtest(data, ...)` raised AttributeError for
data indexed by epoch timestamps or by date strings. The conversion call
also passed `infer_datetime_format=True`, which pandas 3.0 removed as
well (TypeError, not caught by the surrounding `except ValueError`).

Use `pd.api.types.is_numeric_dtype()` (available in every supported
pandas) and drop `infer_datetime_format`, which has no effect on numeric
input.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 backtesting/backtesting.py |  4 ++--
 backtesting/test/_test.py  | 14 ++++++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/backtesting/backtesting.py b/backtesting/backtesting.py
index d356b211..38e94f73 100644
--- a/backtesting/backtesting.py
+++ b/backtesting/backtesting.py
@@ -1224,10 +1224,10 @@ def __init__(self,
         if (not isinstance(data.index, pd.DatetimeIndex) and
             not isinstance(data.index, pd.RangeIndex) and
             # Numeric index with most large numbers
-            (data.index.is_numeric() and
+            (pd.api.types.is_numeric_dtype(data.index) and
              (data.index > pd.Timestamp('1975').timestamp()).mean() > .8)):
             try:
-                data.index = pd.to_datetime(data.index, infer_datetime_format=True)
+                data.index = pd.to_datetime(data.index)
             except ValueError:
                 pass
 
diff --git a/backtesting/test/_test.py b/backtesting/test/_test.py
index d74fde9f..f507ac45 100644
--- a/backtesting/test/_test.py
+++ b/backtesting/test/_test.py
@@ -1283,6 +1283,20 @@ def test_optimize_range_index(self):
         res = bt.optimize(fast=range(2, 3), slow=range(4, 5))
         self.assertGreater(res['# Trades'], 0)
 
+    def test_numeric_and_string_index(self):
+        # `Index.is_numeric()` and `to_datetime(infer_datetime_format=)` were
+        # removed in pandas 3.0, so any index that was neither a DatetimeIndex
+        # nor a RangeIndex made `Backtest()` raise.
+        data = GOOG.iloc[:100].copy()
+        data.index = pd.Index(data.index.values.astype('datetime64[ns]').astype('int64'))
+        bt = Backtest(data, SmaCross)
+        self.assertIsInstance(bt._data.index, pd.DatetimeIndex)
+        self.assertTrue(bt._data.index.equals(GOOG.index[:100]))
+
+        data.index = GOOG.index[:100].strftime('%Y-%m-%d')
+        with self.assertWarnsRegex(UserWarning, 'index is not datetime'):
+            Backtest(data, SmaCross)
+
     def test_sl_tp_values_in_trades_df(self):
         class S(_S):
             def next(self):