ERR: Shorten traceback in groupby _cython_agg_general

doc/source/whatsnew/v2.1.0.rst

@@ -109,6 +109,7 @@ Other enhancements
 - Added ``engine_kwargs`` parameter to :meth:`DataFrame.to_excel` (:issue:`53220`)
 - Added a new parameter ``by_row`` to :meth:`Series.apply`. When set to ``False`` the supplied callables will always operate on the whole Series (:issue:`53400`).
 - Groupby aggregations (such as :meth:`DataFrameGroupby.sum`) now can preserve the dtype of the input instead of casting to ``float64`` (:issue:`44952`)
+- Improved error message when :meth:`DataFrameGroupBy.agg` failed (:issue:`52930`)
 - Many read/to_* functions, such as :meth:`DataFrame.to_pickle` and :func:`read_csv`, support forwarding compression arguments to lzma.LZMAFile (:issue:`52979`)
 - Performance improvement in :func:`concat` with homogeneous ``np.float64`` or ``np.float32`` dtypes (:issue:`52685`)
 - Performance improvement in :meth:`DataFrame.filter` when ``items`` is given (:issue:`52941`)

pandas/core/groupby/groupby.py

@@ -1711,7 +1711,7 @@ def _agg_general(
         return result.__finalize__(self.obj, method="groupby")
 
     def _agg_py_fallback(
-        self, values: ArrayLike, ndim: int, alt: Callable
+        self, how: str, values: ArrayLike, ndim: int, alt: Callable
     ) -> ArrayLike:
         """
         Fallback to pure-python aggregation if _cython_operation raises
@@ -1737,7 +1737,12 @@ def _agg_py_fallback(
         # We do not get here with UDFs, so we know that our dtype
         #  should always be preserved by the implemented aggregations
         # TODO: Is this exactly right; see WrappedCythonOp get_result_dtype?
-        res_values = self.grouper.agg_series(ser, alt, preserve_dtype=True)
+        try:
+            res_values = self.grouper.agg_series(ser, alt, preserve_dtype=True)
+        except Exception as err:
+            msg = f"agg function failed [how->{how},dtype->{ser.dtype}]"
+            # preserve the kind of exception that raised
+            raise type(err)(msg) from err
 
         if ser.dtype == object:
             res_values = res_values.astype(object, copy=False)
@@ -1779,8 +1784,10 @@ def array_func(values: ArrayLike) -> ArrayLike:
                 # TODO: shouldn't min_count matter?
                 if how in ["any", "all", "std", "sem"]:
                     raise  # TODO: re-raise as TypeError?  should not be reached
-                result = self._agg_py_fallback(values, ndim=data.ndim, alt=alt)
+            else:
+                return result
 
+            result = self._agg_py_fallback(how, values, ndim=data.ndim, alt=alt)
             return result
 
         new_mgr = data.grouped_reduce(array_func)

pandas/tests/extension/base/groupby.py

@@ -1,3 +1,5 @@
+import re
+
 import pytest
 
 from pandas.core.dtypes.common import (
@@ -141,7 +143,16 @@ def test_in_numeric_groupby(self, data_for_grouping):
             result = df.groupby("A").sum().columns
         else:
             expected = pd.Index(["C"])
-            with pytest.raises(TypeError, match="does not support"):
-                df.groupby("A").sum().columns
+
+            msg = "|".join(
+                [
+                    # period/datetime
+                    "does not support sum operations",
+                    # all others
+                    re.escape(f"agg function failed [how->sum,dtype->{dtype}"),
+                ]
+            )
+            with pytest.raises(TypeError, match=msg):
+                df.groupby("A").sum()
             result = df.groupby("A").sum(numeric_only=True).columns
         tm.assert_index_equal(result, expected)

pandas/tests/extension/test_arrow.py

@@ -585,7 +585,8 @@ def test_groupby_extension_agg(self, as_index, data_for_grouping, request):
         super().test_groupby_extension_agg(as_index, data_for_grouping)
 
     def test_in_numeric_groupby(self, data_for_grouping):
-        if is_string_dtype(data_for_grouping.dtype):
+        dtype = data_for_grouping.dtype
+        if is_string_dtype(dtype):
             df = pd.DataFrame(
                 {
                     "A": [1, 1, 2, 2, 3, 3, 1, 4],
@@ -595,8 +596,9 @@ def test_in_numeric_groupby(self, data_for_grouping):
             )
 
             expected = pd.Index(["C"])
-            with pytest.raises(TypeError, match="does not support"):
-                df.groupby("A").sum().columns
+            msg = re.escape(f"agg function failed [how->sum,dtype->{dtype}")
+            with pytest.raises(TypeError, match=msg):
+                df.groupby("A").sum()
             result = df.groupby("A").sum(numeric_only=True).columns
             tm.assert_index_equal(result, expected)
         else:

pandas/tests/frame/test_stack_unstack.py

@@ -1,6 +1,7 @@
 from datetime import datetime
 from io import StringIO
 import itertools
+import re
 
 import numpy as np
 import pytest
@@ -1897,7 +1898,8 @@ def test_stack_multiple_bug(self):
         multi = df.set_index(["DATE", "ID"])
         multi.columns.name = "Params"
         unst = multi.unstack("ID")
-        with pytest.raises(TypeError, match="Could not convert"):
+        msg = re.escape("agg function failed [how->mean,dtype->object]")
+        with pytest.raises(TypeError, match=msg):
             unst.resample("W-THU").mean()
         down = unst.resample("W-THU").mean(numeric_only=True)
         rs = down.stack("ID")

pandas/tests/groupby/test_function.py

@@ -1,5 +1,6 @@
 import builtins
 from io import StringIO
+import re
 
 import numpy as np
 import pytest
@@ -249,8 +250,10 @@ def _check(self, df, method, expected_columns, expected_columns_numeric):
             msg = "|".join(
                 [
                     "Categorical is not ordered",
-                    "function is not implemented for this dtype",
                     f"Cannot perform {method} with non-ordered Categorical",
+                    re.escape(f"agg function failed [how->{method},dtype->object]"),
+                    # cumsum/cummin/cummax/cumprod
+                    "function is not implemented for this dtype",
                 ]
             )
             with pytest.raises(exception, match=msg):
@@ -259,12 +262,9 @@ def _check(self, df, method, expected_columns, expected_columns_numeric):
             msg = "|".join(
                 [
                     "category type does not support sum operations",
-                    "[Cc]ould not convert",
-                    "can't multiply sequence by non-int of type 'str'",
+                    re.escape(f"agg function failed [how->{method},dtype->object]"),
                 ]
             )
-            if method == "median":
-                msg = r"Cannot convert \['a' 'b'\] to numeric"
             with pytest.raises(exception, match=msg):
                 getattr(gb, method)()
         else:
@@ -274,16 +274,13 @@ def _check(self, df, method, expected_columns, expected_columns_numeric):
         if method not in ("first", "last"):
             msg = "|".join(
                 [
-                    "[Cc]ould not convert",
                     "Categorical is not ordered",
                     "category type does not support",
-                    "can't multiply sequence",
                     "function is not implemented for this dtype",
                     f"Cannot perform {method} with non-ordered Categorical",
+                    re.escape(f"agg function failed [how->{method},dtype->object]"),
                 ]
             )
-            if method == "median":
-                msg = r"Cannot convert \['a' 'b'\] to numeric"
             with pytest.raises(exception, match=msg):
                 getattr(gb, method)(numeric_only=False)
         else:
@@ -1464,16 +1461,14 @@ def test_numeric_only(kernel, has_arg, numeric_only, keys):
         msg = "|".join(
             [
                 "not allowed for this dtype",
-                "must be a string or a number",
                 "cannot be performed against 'object' dtypes",
-                "must be a string or a real number",
+                # On PY39 message is "a number"; on PY310 and after is "a real number"
+                "must be a string or a.* number",
                 "unsupported operand type",
-                "not supported between instances of",
                 "function is not implemented for this dtype",
+                re.escape(f"agg function failed [how->{kernel},dtype->object]"),
             ]
         )
-        if kernel == "median":
-            msg = r"Cannot convert \[<class 'object'> <class 'object'>\] to numeric"
         with pytest.raises(exception, match=msg):
             method(*args, **kwargs)
     elif not has_arg and numeric_only is not lib.no_default:

pandas/tests/groupby/test_groupby.py

@@ -1,5 +1,6 @@
 from datetime import datetime
 from decimal import Decimal
+import re
 
 import numpy as np
 import pytest
@@ -641,7 +642,7 @@ def test_frame_multi_key_function_list_partial_failure():
 
     grouped = data.groupby(["A", "B"])
     funcs = [np.mean, np.std]
-    msg = "Could not convert string 'dullshinyshiny' to numeric"
+    msg = re.escape("agg function failed [how->mean,dtype->object]")
     with pytest.raises(TypeError, match=msg):
         grouped.agg(funcs)
 
@@ -915,9 +916,10 @@ def test_groupby_multi_corner(df):
 
 def test_raises_on_nuisance(df):
     grouped = df.groupby("A")
-    with pytest.raises(TypeError, match="Could not convert"):
+    msg = re.escape("agg function failed [how->mean,dtype->object]")
+    with pytest.raises(TypeError, match=msg):
         grouped.agg(np.mean)
-    with pytest.raises(TypeError, match="Could not convert"):
+    with pytest.raises(TypeError, match=msg):
         grouped.mean()
 
     df = df.loc[:, ["A", "C", "D"]]
@@ -965,10 +967,12 @@ def test_omit_nuisance_agg(df, agg_function, numeric_only):
     if agg_function in no_drop_nuisance and not numeric_only:
         # Added numeric_only as part of GH#46560; these do not drop nuisance
         # columns when numeric_only is False
-        klass = ValueError if agg_function in ("std", "sem") else TypeError
-        msg = "|".join(["[C|c]ould not convert", "can't multiply sequence"])
-        if agg_function == "median":
-            msg = r"Cannot convert \['one' 'three' 'two'\] to numeric"
+        if agg_function in ("std", "sem"):
+            klass = ValueError
+            msg = "could not convert string to float: 'one'"
+        else:
+            klass = TypeError
+            msg = re.escape(f"agg function failed [how->{agg_function},dtype->object]")
         with pytest.raises(klass, match=msg):
             getattr(grouped, agg_function)(numeric_only=numeric_only)
     else:
@@ -993,9 +997,10 @@ def test_raise_on_nuisance_python_single(df):
 
 def test_raise_on_nuisance_python_multiple(three_group):
     grouped = three_group.groupby(["A", "B"])
-    with pytest.raises(TypeError, match="Could not convert"):
+    msg = re.escape("agg function failed [how->mean,dtype->object]")
+    with pytest.raises(TypeError, match=msg):
         grouped.agg(np.mean)
-    with pytest.raises(TypeError, match="Could not convert"):
+    with pytest.raises(TypeError, match=msg):
         grouped.mean()
 
 
@@ -1035,7 +1040,8 @@ def test_wrap_aggregated_output_multindex(mframe):
     df["baz", "two"] = "peekaboo"
 
     keys = [np.array([0, 0, 1]), np.array([0, 0, 1])]
-    with pytest.raises(TypeError, match="Could not convert"):
+    msg = re.escape("agg function failed [how->mean,dtype->object]")
+    with pytest.raises(TypeError, match=msg):
         df.groupby(keys).agg(np.mean)
     agged = df.drop(columns=("baz", "two")).groupby(keys).agg(np.mean)
     assert isinstance(agged.columns, MultiIndex)

pandas/tests/groupby/test_raises.py

@@ -3,6 +3,7 @@
 # test file.
 
 import datetime
+import re
 
 import numpy as np
 import pytest
@@ -162,24 +163,20 @@ def test_groupby_raises_string(
         "max": (None, ""),
         "mean": (
             TypeError,
-            "Could not convert string '(xy|xyzwt|xyz|xztuo)' to numeric",
+            re.escape("agg function failed [how->mean,dtype->object]"),
         ),
         "median": (
             TypeError,
-            "|".join(
-                [
-                    r"Cannot convert \['x' 'y' 'z'\] to numeric",
-                    r"Cannot convert \['x' 'y'\] to numeric",
-                    r"Cannot convert \['x' 'y' 'z' 'w' 't'\] to numeric",
-                    r"Cannot convert \['x' 'z' 't' 'u' 'o'\] to numeric",
-                ]
-            ),
+            re.escape("agg function failed [how->median,dtype->object]"),
         ),
         "min": (None, ""),
         "ngroup": (None, ""),
         "nunique": (None, ""),
         "pct_change": (TypeError, "unsupported operand type"),
-        "prod": (TypeError, "can't multiply sequence by non-int of type 'str'"),
+        "prod": (
+            TypeError,
+            re.escape("agg function failed [how->prod,dtype->object]"),
+        ),
         "quantile": (TypeError, "cannot be performed against 'object' dtypes!"),
         "rank": (None, ""),
         "sem": (ValueError, "could not convert string to float"),
@@ -188,7 +185,10 @@ def test_groupby_raises_string(
         "skew": (ValueError, "could not convert string to float"),
         "std": (ValueError, "could not convert string to float"),
         "sum": (None, ""),
-        "var": (TypeError, "could not convert string to float"),
+        "var": (
+            TypeError,
+            re.escape("agg function failed [how->var,dtype->object]"),
+        ),
     }[groupby_func]
 
     _call_and_check(klass, msg, how, gb, groupby_func, args)
@@ -225,7 +225,7 @@ def test_groupby_raises_string_np(
         np.sum: (None, ""),
         np.mean: (
             TypeError,
-            "Could not convert string '(xyzwt|xy|xyz|xztuo)' to numeric",
+            re.escape("agg function failed [how->mean,dtype->object]"),
         ),
     }[groupby_func_np]
 

pandas/tests/resample/test_resample_api.py

@@ -1,4 +1,5 @@
 from datetime import datetime
+import re
 
 import numpy as np
 import pytest
@@ -186,7 +187,8 @@ def tests_raises_on_nuisance(test_frame):
     tm.assert_frame_equal(result, expected)
 
     expected = r[["A", "B", "C"]].mean()
-    with pytest.raises(TypeError, match="Could not convert"):
+    msg = re.escape("agg function failed [how->mean,dtype->object]")
+    with pytest.raises(TypeError, match=msg):
         r.mean()
     result = r.mean(numeric_only=True)
     tm.assert_frame_equal(result, expected)
@@ -886,8 +888,13 @@ def test_frame_downsample_method(method, numeric_only, expected_data):
 
     func = getattr(resampled, method)
     if isinstance(expected_data, str):
-        klass = TypeError if method in ("var", "mean", "median", "prod") else ValueError
-        with pytest.raises(klass, match=expected_data):
+        if method in ("var", "mean", "median", "prod"):
+            klass = TypeError
+            msg = re.escape(f"agg function failed [how->{method},dtype->object]")
+        else:
+            klass = ValueError
+            msg = expected_data
+        with pytest.raises(klass, match=msg):
             _ = func(**kwargs)
     else:
         result = func(**kwargs)
@@ -933,7 +940,8 @@ def test_series_downsample_method(method, numeric_only, expected_data):
         with pytest.raises(TypeError, match=msg):
             func(**kwargs)
     elif method == "prod":
-        with pytest.raises(TypeError, match="can't multiply sequence by non-int"):
+        msg = re.escape("agg function failed [how->prod,dtype->object]")
+        with pytest.raises(TypeError, match=msg):
             func(**kwargs)
     else:
         result = func(**kwargs)

pandas/tests/reshape/merge/test_join.py

@@ -1,3 +1,5 @@
+import re
+
 import numpy as np
 import pytest
 
@@ -567,7 +569,8 @@ def test_mixed_type_join_with_suffix(self):
         df.insert(5, "dt", "foo")
 
         grouped = df.groupby("id")
-        with pytest.raises(TypeError, match="Could not convert"):
+        msg = re.escape("agg function failed [how->mean,dtype->object]")
+        with pytest.raises(TypeError, match=msg):
             grouped.mean()
         mn = grouped.mean(numeric_only=True)
         cn = grouped.count()