[FIX] flagGeneric: don't fail on empty data

saqc/funcs/generic.py

@@ -267,7 +267,7 @@ def flagGeneric(
             f"the generic function returned {len(result.columns)} field(s), but only {len(targets)} target(s) were given"
         )
 
-    if not (result.dtypes == bool).all():
+    if not result.empty and not (result.dtypes == bool).all():
         raise TypeError(f"generic expression does not return a boolean array")
 
     meta = {

tests/funcs/test_generic_api_functions.py

@@ -8,6 +8,7 @@ from dios.dios.dios import DictOfSeries
 from saqc.constants import BAD, UNFLAGGED, FILTER_ALL
 from saqc.core.flags import Flags
 from saqc import SaQC
+from saqc.core.register import _isflagged
 from saqc.lib.tools import toSequence
 
 from tests.common import initData
@@ -18,6 +19,19 @@ def data():
     return initData()
 
 
+def test_emptyData():
+    # test that things do not break with empty data sets
+    saqc = SaQC(data=pd.DataFrame({"x": [], "y": []}))
+
+    saqc.flagGeneric("x", func=lambda x: x < 0)
+    assert saqc.data.empty
+    assert saqc.flags.empty
+
+    saqc = saqc.processGeneric(field="x", target="y", func=lambda x: x + 2)
+    assert saqc.data.empty
+    assert saqc.flags.empty
+
+
 def test_writeTargetFlagGeneric(data):
     params = [
         (["tmp"], lambda x, y: pd.Series(True, index=x.index.union(y.index))),