fix: resolve string aggfun to callable in table_pivot step

frictionless/steps/table/__spec__/test_table_pivot.py

@@ -24,3 +24,33 @@ def test_step_table_pivot_issue_1220():
         {"region": "east", "boy": 33, "girl": 29},
         {"region": "west", "boy": 35, "girl": 23},
     ]
+
+
+def test_step_table_pivot_string_aggfun_issue_1764():
+    source = TableResource(path="data/transform-pivot.csv")
+    pipeline = Pipeline.from_descriptor(
+        {
+            "steps": [
+                {"type": "table-normalize"},
+                {
+                    "type": "table-pivot",
+                    "f1": "region",
+                    "f2": "gender",
+                    "f3": "units",
+                    "aggfun": "sum",
+                },
+            ]
+        }
+    )
+    target = source.transform(pipeline)
+    assert target.schema.to_descriptor() == {
+        "fields": [
+            {"name": "region", "type": "string"},
+            {"name": "boy", "type": "integer"},
+            {"name": "girl", "type": "integer"},
+        ]
+    }
+    assert target.read_rows() == [
+        {"region": "east", "boy": 33, "girl": 29},
+        {"region": "west", "boy": 35, "girl": 23},
+    ]

frictionless/steps/table/table_pivot.py

@@ -1,16 +1,34 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any
+import statistics
+from typing import TYPE_CHECKING, Any, Callable
 
 import attrs
 
+from ... import errors
+from ...exception import FrictionlessException
 from ...pipeline import Step
 from ...schema import Schema
 
 if TYPE_CHECKING:
     from ...resource import Resource
 
 
+# Mapping of petl aggregation function names to callables. This allows a
+# string `aggfun` (e.g. coming from a JSON/YAML descriptor) to mirror the
+# Python API, where a callable is passed directly.
+AGGREGATION_FUNCTIONS: dict[str, Callable[[Any], Any]] = {
+    "sum": sum,
+    "max": max,
+    "min": min,
+    "len": len,
+    "count": len,
+    "mean": statistics.mean,
+    "first": lambda values: list(values)[0],
+    "last": lambda values: list(values)[-1],
+}
+
+
 @attrs.define(kw_only=True, repr=False)
 class table_pivot(Step):
     """Pivot table.
@@ -40,14 +58,22 @@ class table_pivot(Step):
     aggfun: Any
     """
     Function to process and create data in the output pivot table.
-    The function can be "sum", "max", "min", "len" etc.
+    It can be a callable or, when defined via a descriptor, one of the
+    supported aggregation function names: "sum", "max", "min", "len",
+    "count", "mean", "first", "last".
     """
 
     # Transform
 
     def transform_resource(self, resource: Resource):
         table = resource.to_petl()  # type: ignore
-        resource.data = table.pivot(self.f1, self.f2, self.f3, self.aggfun)  # type: ignore
+        aggfun = self.aggfun
+        if isinstance(aggfun, str):
+            if aggfun not in AGGREGATION_FUNCTIONS:
+                note = f'aggregation function "{aggfun}" is not supported'
+                raise FrictionlessException(errors.StepError(note=note))
+            aggfun = AGGREGATION_FUNCTIONS[aggfun]
+        resource.data = table.pivot(self.f1, self.f2, self.f3, aggfun)  # type: ignore
         resource.schema = Schema()
         resource.infer()