Fix bug that prevents prefixes from being omitted in udfs

The types for a udf schema allow the prefix to be None, but our code
doesn't work in that scenario. In addition, the annotation interface for
udfs don't even let you have a None prefix. This change fixes both of
those issues.

python/tests/experimental/core/test_udf_schema.py

@@ -71,6 +71,14 @@ def f2(x: Union[Dict[str, List], pd.DataFrame]) -> Union[Dict[str, List], pd.Dat
         return pd.DataFrame({"foo": x["xx1"], "bar": x["xx2"]})
 
 
+@register_multioutput_udf(["xx1", "xx2"], no_prefix=True)
+def no_prefix_udf(x: Union[Dict[str, List], pd.DataFrame]) -> Union[Dict[str, List], pd.DataFrame]:
+    if isinstance(x, dict):
+        return {"foo": [x["xx1"][0]], "bar": [x["xx2"][0]]}
+    else:
+        return pd.DataFrame({"foo": x["xx1"], "bar": x["xx2"]})
+
+
 def test_multioutput_udf_row() -> None:
     schema = udf_schema()
     row = {"xx1": 42, "xx2": 3.14}
@@ -79,6 +87,8 @@ def test_multioutput_udf_row() -> None:
     assert results.get_column("f1.bar") is not None
     assert results.get_column("blah.foo") is not None
     assert results.get_column("blah.bar") is not None
+    assert results.get_column("foo") is not None
+    assert results.get_column("bar") is not None
 
 
 def test_multioutput_udf_dataframe() -> None:
@@ -89,6 +99,8 @@ def test_multioutput_udf_dataframe() -> None:
     assert results.get_column("f1.bar") is not None
     assert results.get_column("blah.foo") is not None
     assert results.get_column("blah.bar") is not None
+    assert results.get_column("foo") is not None
+    assert results.get_column("bar") is not None
 
 
 @register_dataset_udf(["col1"], schema_name="unit-tests")

python/whylogs/experimental/core/udf_schema.py

@@ -135,10 +135,13 @@ def _apply_udf_on_dataframe(
     udf(Union[Dict[str, List], pd.DataFrame]) -> Union[Dict[str, List], pd.DataFrame]
     """
 
+    def add_prefix(col):
+        return prefix + "." + col if prefix else col
+
     try:
         # TODO: I think it's OKAY if udf returns a dictionary
         udf_output = pd.DataFrame(udf(pandas))
-        udf_output = udf_output.rename(columns={old: prefix + "." + old for old in udf_output.keys()})  # type: ignore
+        udf_output = udf_output.rename(columns={old: add_prefix(old) for old in udf_output.keys()})
         for new_col in udf_output.keys():
             new_df[new_col] = udf_output[new_col]
     except Exception as e:  # noqa
@@ -271,6 +274,7 @@ def register_multioutput_udf(
     prefix: Optional[str] = None,
     namespace: Optional[str] = None,
     schema_name: str = "",
+    no_prefix: bool = False,
 ) -> Callable[[Any], Any]:
     """
     Decorator to easily configure UDFs for your data set. Decorate your UDF
@@ -294,7 +298,10 @@ def decorator_register(func):
         global _multicolumn_udfs
         name = udf_name or func.__name__
         name = f"{namespace}.{name}" if namespace else name
-        output_prefix = prefix if prefix else name
+        if no_prefix:
+            output_prefix = None
+        else:
+            output_prefix = prefix if prefix else name
         _multicolumn_udfs[schema_name].append(UdfSpec(col_names, prefix=output_prefix, udf=func, name=name))
         return func