Use GET with comma-separated values for multi-value waterdata queries

The OGC API now supports comma-separated values for fields like
monitoring_location_id, parameter_code, and statistic_id, making POST+CQL2
unnecessary for most services. Update _construct_api_requests to join list
params with commas and use GET for daily, continuous, latest-daily,
latest-continuous, field-measurements, time-series-metadata, and
channel-measurements.

The monitoring-locations endpoint does not yet support comma-separated GET
parameters (returns 400); it retains the POST+CQL2 path. Closes #210.

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

Author: thodson-usgs

dataretrieval/waterdata/utils.py

@@ -417,9 +417,11 @@ def _construct_api_requests(
     """
     Constructs an HTTP request object for the specified water data API service.
 
-    Depending on the input parameters (whether there's lists of multiple
-    argument values), the function determines whether to use a GET or POST
-    request, formats parameters appropriately, and sets required headers.
+    For most services, list parameters are comma-joined and sent as a single
+    GET request (e.g. ``parameter_code=["00060","00010"]`` becomes
+    ``parameter_code=00060,00010`` in the URL). For services that do not
+    support comma-separated values (currently only ``monitoring-locations``),
+    a POST request with CQL2 JSON is used instead.
 
     Parameters
     ----------
@@ -445,36 +447,41 @@ def _construct_api_requests(
     Notes
     -----
     - Date/time parameters are automatically formatted to ISO8601.
-    - If multiple values are provided for non-single parameters, a POST request
-    is constructed.
-    - The function sets appropriate headers for GET and POST requests.
     """
     service_url = f"{OGC_API_URL}/collections/{service}/items"
 
-    # Identify which parameters should be included in the POST content body
-    post_params = {
-        k: v
-        for k, v in kwargs.items()
-        if k not in _DATE_RANGE_PARAMS and isinstance(v, (list, tuple)) and len(v) > 1
-    }
-
-    # Everything else goes into the params dictionary for the URL
-    params = {k: v for k, v in kwargs.items() if k not in post_params}
-    # Set skipGeometry parameter (API expects camelCase)
-    params["skipGeometry"] = skip_geometry
+    # The monitoring-locations endpoint does not support comma-separated values
+    # for multi-value GET parameters; CQL2 POST is required for that service.
+    _cql2_required_services = {"monitoring-locations"}
 
-    # If limit is none or greater than 50000, then set limit to max results. Otherwise,
-    # use the limit
-    params["limit"] = 50000 if limit is None or limit > 50000 else limit
+    # Format date/time parameters to ISO8601 first — both routing paths need it.
+    for key in _DATE_RANGE_PARAMS:
+        if key in kwargs:
+            kwargs[key] = _format_api_dates(
+                kwargs[key],
+                date=(service == "daily" and key != "last_modified"),
+            )
 
-    # Indicate if function needs to perform POST conversion
-    POST = bool(post_params)
+    if service in _cql2_required_services:
+        # Legacy path: POST with CQL2 for multi-value params
+        post_params = {
+            k: v
+            for k, v in kwargs.items()
+            if k not in _DATE_RANGE_PARAMS
+            and isinstance(v, (list, tuple))
+            and len(v) > 1
+        }
+        params = {k: v for k, v in kwargs.items() if k not in post_params}
+    else:
+        # Join list/tuple values with commas for multi-value GET parameters.
+        post_params = {}
+        params = {
+            k: ",".join(str(x) for x in v) if isinstance(v, (list, tuple)) else v
+            for k, v in kwargs.items()
+        }
 
-    # Convert dates to ISO08601 format
-    for i in _DATE_RANGE_PARAMS:
-        if i in params:
-            dates = service == "daily" and i != "last_modified"
-            params[i] = _format_api_dates(params[i], date=dates)
+    params["skipGeometry"] = skip_geometry
+    params["limit"] = 50000 if limit is None or limit > 50000 else limit
 
     # `len()` instead of truthiness: a numpy ndarray would raise on `if bbox:`.
     if bbox is not None and len(bbox) > 0:
@@ -490,7 +497,7 @@ def _construct_api_requests(
 
     headers = _default_headers()
 
-    if POST:
+    if post_params:
         headers["Content-Type"] = "application/query-cql-json"
         request = requests.Request(
             method="POST",

tests/waterdata_test.py

@@ -30,6 +30,7 @@
 from dataretrieval.waterdata.utils import (
     _check_monitoring_location_id,
     _check_profiles,
+    _construct_api_requests,
     _normalize_str_iterable,
 )
 
@@ -111,6 +112,28 @@ def test_check_profiles():
         _check_profiles(service="results", profile="foo")
 
 
+def test_construct_api_requests_multivalue_get():
+    """Multi-value params use GET with comma-separated values for daily service."""
+    req = _construct_api_requests(
+        "daily",
+        monitoring_location_id=["USGS-05427718", "USGS-05427719"],
+        parameter_code=["00060", "00065"],
+    )
+    assert req.method == "GET"
+    assert "monitoring_location_id=USGS-05427718%2CUSGS-05427719" in req.url
+    assert "parameter_code=00060%2C00065" in req.url
+
+
+def test_construct_api_requests_monitoring_locations_post():
+    """monitoring-locations uses POST+CQL2 for multi-value params (API limitation)."""
+    req = _construct_api_requests(
+        "monitoring-locations",
+        hydrologic_unit_code=["010802050102", "010802050103"],
+    )
+    assert req.method == "POST"
+    assert req.body is not None
+
+
 def test_samples_results():
     """Test results call for proper columns"""
     df, _ = get_samples(