nicer error message

Author: nicolaskruchten

Diff for: packages/python/plotly/plotly/express/_core.py

@@ -279,10 +279,21 @@ def make_trace_kwargs(args, trace_spec, trace_data, mapping_labels, sizeref):
                         x = x.astype(int) / 10 ** 9  # convert to unix epoch seconds
                         x_is_date = True
                     elif x.dtype.type == np.object_:
-                        x = x.astype(np.float64)
-
+                        try:
+                            x = x.astype(np.float64)
+                        except ValueError:
+                            raise ValueError(
+                                "Could not convert value of 'x' ('%s') into a numeric type. "
+                                "If 'x' contains stringified dates, please convert to a datetime column."
+                                % args["x"]
+                            )
                     if y.dtype.type == np.object_:
-                        y = y.astype(np.float64)
+                        try:
+                            y = y.astype(np.float64)
+                        except ValueError:
+                            raise ValueError(
+                                "Could not convert value of 'y' into a numeric type."
+                            )
 
                     if attr_value == "lowess":
                         # missing ='drop' is the default value for lowess but not for OLS (None)

Diff for: packages/python/plotly/plotly/tests/test_core/test_px/test_trendline.py

@@ -94,6 +94,13 @@ def test_no_slope_ols_trendline():
 @pytest.mark.parametrize("mode", ["ols", "lowess"])
 def test_trendline_on_timeseries(mode):
     df = px.data.stocks()
+
+    with pytest.raises(ValueError) as err_msg:
+        px.scatter(df, x="date", y="GOOG", trendline=mode)
+    assert "Could not convert value of 'x' ('date') into a numeric type." in str(
+        err_msg.value
+    )
+
     df["date"] = pd.to_datetime(df["date"])
     fig = px.scatter(df, x="date", y="GOOG", trendline=mode)
     assert len(fig.data) == 2