Fix IndexError in _read_rdb when NWIS returns no data rows

When an NWIS service responds with only comment lines (e.g. "No sites
found matching all criteria"), _read_rdb tried to index past the end of
the lines list and raised an unhelpful IndexError.  Now it detects the
empty-data case, extracts the Response-Message from the comment block,
and raises a ValueError with that message instead.

Fixes #171.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: thodson-usgs

dataretrieval/nwis.py

@@ -1039,16 +1039,26 @@ def _read_rdb(rdb):
         )
 
     count = 0
+    lines = rdb.splitlines()
 
-    for line in rdb.splitlines():
+    for line in lines:
         # ignore comment lines
         if line.startswith("#"):
             count = count + 1
 
         else:
             break
 
-    fields = rdb.splitlines()[count].split("\t")
+    if count >= len(lines):
+        # All lines are comments — no data rows. Extract the NWIS message.
+        msg = "No data returned from the NWIS service."
+        for line in lines:
+            if "Response-Message:" in line:
+                msg = line.split("Response-Message:")[-1].strip()
+                break
+        raise ValueError(msg)
+
+    fields = lines[count].split("\t")
     fields = [field.replace(",", "").strip() for field in fields if field.strip()]
     dtypes = {
         "site_no": str,

tests/nwis_test.py

@@ -9,6 +9,7 @@
 
 from dataretrieval.nwis import (
     NWIS_Metadata,
+    _read_rdb,
     get_discharge_measurements,
     get_gwlevels,
     get_info,
@@ -321,3 +322,44 @@ def test_variable_info_deprecated(self):
         ):
             result = md.variable_info
         assert result is None
+
+
+class TestReadRdb:
+    """Tests for the _read_rdb helper.
+
+    Notes
+    -----
+    Related to GitHub Issue #171.
+    """
+
+    # Minimal valid RDB response with one data row
+    _VALID_RDB = (
+        "# comment\n"
+        "site_no\tvalue\n"
+        "5s\t10n\n"
+        "01491000\t42\n"
+    )
+
+    # NWIS response when no sites match the query criteria
+    _NO_SITES_RDB = (
+        "# //Output-Format: RDB\n"
+        "# //Response-Status: OK\n"
+        "# //Response-Message: No sites found matching all criteria\n"
+    )
+
+    def test_valid_rdb_returns_dataframe(self):
+        """_read_rdb returns a DataFrame for a well-formed RDB response."""
+        df = _read_rdb(self._VALID_RDB)
+        assert isinstance(df, pd.DataFrame)
+        assert "site_no" in df.columns
+
+    def test_no_sites_raises_value_error(self):
+        """_read_rdb raises ValueError when NWIS returns no data rows (issue #171)."""
+        with pytest.raises(ValueError, match="No sites found"):
+            _read_rdb(self._NO_SITES_RDB)
+
+    def test_empty_comments_raises_value_error(self):
+        """_read_rdb raises a generic ValueError when response has only comments."""
+        rdb = "# just a comment\n# another comment\n"
+        with pytest.raises(ValueError):
+            _read_rdb(rdb)