fix(nwis): handle empty peaks response instead of raising KeyError

dataretrieval/nwis.py

@@ -196,6 +196,13 @@ def preformat_peaks_response(df: pd.DataFrame) -> pd.DataFrame:
         The formatted data frame
 
     """
+    if "peak_dt" not in df.columns:
+        # An empty peaks response (e.g. "No sites found") parses to a
+        # column-less frame, so there is no peak_dt to reformat. Return it
+        # unchanged and let format_response's empty-frame path handle it,
+        # matching how the other services treat empty results (issue #171).
+        return df
+
     df["datetime"] = pd.to_datetime(df.pop("peak_dt"), errors="coerce")
     df.dropna(subset=["datetime"], inplace=True)
     return df

tests/nwis_test.py

@@ -11,6 +11,7 @@
 from dataretrieval.nwis import (
     NWIS_Metadata,
     _read_rdb,
+    format_response,
     get_discharge_measurements,
     get_gwlevels,
     get_iv,
@@ -19,6 +20,7 @@
     get_record,
     get_water_use,
     preformat_peaks_response,
+    read_rdb,
 )
 
 START_DATE = "2018-01-24"
@@ -303,3 +305,22 @@ def test_no_sites_flows_through_format_response(self):
         df = _read_rdb(no_sites_rdb)
         assert isinstance(df, pd.DataFrame)
         assert df.empty
+
+    def test_no_peaks_flows_through_format_response(self):
+        """The 'peaks' service takes an extra formatting step
+        (preformat_peaks_response) before the empty-frame check, so an empty
+        peaks response has to survive that too. Previously this raised
+        KeyError('peak_dt'); now it returns an empty frame like the other
+        services (same empty-result contract as issue #171).
+        """
+        no_peaks_rdb = (
+            "# //Output-Format: RDB\n"
+            "# //Response-Status: OK\n"
+            "# //Response-Message: No sites found matching all criteria\n"
+        )
+        # Mirror get_discharge_peaks: raw read_rdb, then the peaks-specific
+        # format_response (not _read_rdb, which formats with service=None).
+        df = read_rdb(no_peaks_rdb)
+        df = format_response(df, service="peaks")
+        assert isinstance(df, pd.DataFrame)
+        assert df.empty