fix: clearing assets in collections

I'm generally uneasy with the way that the base catalog `extra_fields` holds
values that might be mutated on the collection. I think this is an argument
_for_ #296, namely that while the inheritance makes some things easier, it also
causes problems.

Author: gadomski

pystac/collection.py

@@ -569,6 +569,8 @@ def to_dict(
             d["summaries"] = self.summaries.to_dict()
         if any(self.assets):
             d["assets"] = {k: v.to_dict() for k, v in self.assets.items()}
+        elif "assets" in d:
+            del d["assets"]
 
         return d
 

tests/test_collection.py

@@ -226,6 +226,15 @@ def test_assets(self) -> None:
         collection = pystac.Collection.from_dict(data)
         collection.validate()
 
+    def test_clearing_assets_then_to_dict(self) -> None:
+        path = TestCases.get_path("data-files/collections/with-assets.json")
+        collection = pystac.Collection.from_file(path)
+        assert collection.assets
+        collection.assets.clear()
+        assert not collection.assets
+        collection_as_dict = collection.to_dict()
+        assert "assets" not in collection_as_dict
+
     def test_from_dict_preserves_dict(self) -> None:
         path = TestCases.get_path("data-files/collections/with-assets.json")
         with open(path) as f: