fix: use positional indexing to detect dates in pandas dataframes

Close #36

Author: juba

src/pyobsplot/data.py

@@ -62,7 +62,7 @@ def pd_to_arrow(df: pd.DataFrame) -> bytes:
     # Convert dates to timestamps
     for colname in df.columns:
         col = df[colname].dropna()
-        if col is not None and isinstance(col[0], date):
+        if col is not None and len(col) > 0 and isinstance(col.iloc[0], date):
             try:
                 df[colname] = pd.to_datetime(df[colname])
             except ValueError:

tests/test_data.py

@@ -4,6 +4,7 @@
 
 import io
 
+import numpy as np
 import pandas as pd
 import polars as pl
 from polars.testing import assert_frame_equal
@@ -42,3 +43,12 @@ def test_data_frame_polars(self):
             pl.Float64,
             pl.Utf8,
         ]
+
+    def test_pandas_indexing(self):
+        df = pd.DataFrame({"x": [1, 2, 3, 4, 5], "y": [2, 1, 4, 3, 5]})
+        filtered = df[df.x > 1]
+        pd_to_arrow(filtered)
+
+    def test_pandas_dropna(self):
+        df = pd.DataFrame({"x": [np.nan, np.nan, np.nan], "y": [2, 1, 4]})
+        pd_to_arrow(df)