Updated define_glacier_region to handle a list of DEM sources (#1506)

* Updated define_glacier_region to handle a list of DEM sources

* Update PR number in whats-new.rst

* Update gis.py

small simplifications

Co-authored-by: Fabien Maussion <fabien.maussion@uibk.ac.at>

docs/whats-new.rst

@@ -69,6 +69,9 @@ Bug fixes
 - corrected a but in ``apparent_mb_from_any_mb``, where only two years of MB
   would be used instead of a range of years (:pull:`1426`).
   By `Bowen <https://github.com/bowenbelongstonature>`_
+- Corrected ``source`` argument in ``tasks.define_glacier_region`` to handle a
+  list of DEM sources. (:pull:`1506`).
+  By `Daniel Otto <https://github.com/d-otto>`_
 
 Migrating guide
 ~~~~~~~~~~~~~~~

oggm/core/gis.py

@@ -333,7 +333,16 @@ def define_glacier_region(gdir, entity=None, source=None):
 
     # Open DEM
     # We test DEM availability for glacier only (maps can grow big)
-    if not is_dem_source_available(source, *gdir.extent_ll):
+    if isinstance(source, list):  # when multiple sources are provided, try them sequentially
+        for src in source:
+            source_exists = is_dem_source_available(src, *gdir.extent_ll)
+            if source_exists:
+                source = src  # pick the first source which exists
+                break
+    else:
+        source_exists = is_dem_source_available(source, *gdir.extent_ll)
+
+    if not source_exists:
         raise InvalidWorkflowError(f'Source: {source} is not available for '
                                    f'glacier {gdir.rgi_id} with border '
                                    f"{cfg.PARAMS['border']}")