gwpy.io: silence some NDS2 host warnings

by default GWpy will _always_ emit a warning whenever you try to dynamically work out the NDS2 host for a non-LIGO interferometer. This commit changes the default behaviour such that if `NDSSERVER` is set (or similar), then a warning will not be emitted.

gwpy/io/nds2.py

@@ -266,7 +266,11 @@ def host_resolution_order(ifo, env='NDSSERVER', epoch='now',
         try:
             host, port = DEFAULT_HOSTS[difo]
         except KeyError:
-            warnings.warn('No default host found for ifo %r' % ifo)
+            # unknown default NDS2 host for detector, if we don't have
+            # hosts already defined (either by NDSSERVER or similar)
+            # we should warn the user
+            if not hosts:
+                warnings.warn('No default host found for ifo %r' % ifo)
         else:
             if (host, port) not in hosts:
                 hosts.append((host, port))

gwpy/io/tests/test_nds2.py

@@ -125,9 +125,13 @@ def test_nds2_host_order():
                    ('nds.ligo.caltech.edu', 31200)]
 
     # test warnings for unknown IFO
-    with pytest.warns(UserWarning):
+    with pytest.warns(UserWarning) as record:
+        # should produce warning
         hro = io_nds2.host_resolution_order('X1')
         assert hro == [('nds.ligo.caltech.edu', 31200)]
+        # should _not_ produce warning
+        hro = io_nds2.host_resolution_order('X1', env='TESTENV')
+    assert len(record) == 1  # make sure only one warning was emitted
 
 
 @skip_missing_dependency('nds2')