Get link title from target, when possible (#607)

* Get link title from target, when possible

* Update CHANGELOG for #607

* Add tests for serialization and title in init

CHANGELOG.md

@@ -13,6 +13,7 @@
 - Enable [strict
   mode](https://mypy.readthedocs.io/en/latest/command_line.html?highlight=strict%20mode#cmdoption-mypy-strict)
   for `mypy` ([#591](https://github.com/stac-utils/pystac/pull/591))
+- Links will get their `title` from their target if no `title` is provided ([#607](https://github.com/stac-utils/pystac/pull/607))
 
 ### Fixed
 

pystac/link.py

@@ -56,9 +56,6 @@ class Link:
     """Optional description of the media type. Registered Media Types are preferred.
     See :class:`~pystac.MediaType` for common media types."""
 
-    title: Optional[str]
-    """Optional title for this link."""
-
     extra_fields: Dict[str, Any]
     """Optional, additional fields for this link. This is used by extensions as a
     way to serialize and deserialize properties on link object JSON."""
@@ -70,6 +67,7 @@ class Link:
 
     _target_href: Optional[str]
     _target_object: Optional["STACObject_Type"]
+    _title: Optional[str]
 
     def __init__(
         self,
@@ -103,6 +101,22 @@ def set_owner(self, owner: Optional["STACObject_Type"]) -> "Link":
         self.owner = owner
         return self
 
+    @property
+    def title(self) -> Optional[str]:
+        """Optional title for this link. If not provided during instantiation, this will
+        attempt to get the title from the STAC object that the link references."""
+        if self._title is not None:
+            return self._title
+        if self._target_object is not None and isinstance(
+            self._target_object, pystac.Catalog
+        ):
+            return self._target_object.title
+        return None
+
+    @title.setter
+    def title(self, v: Optional[str]) -> None:
+        self._title = v
+
     @property
     def href(self) -> str:
         """Returns the HREF for this link.

tests/test_link.py

@@ -130,6 +130,62 @@ def test_relative_self_href(self) -> None:
             finally:
                 os.chdir(previous)
 
+    def test_auto_title_when_resolved(self) -> None:
+        extent = pystac.Extent.from_items([self.item])
+        collection = pystac.Collection(
+            id="my_collection",
+            description="Test Collection",
+            extent=extent,
+            title="Collection Title",
+        )
+        link = pystac.Link("my rel", target=collection)
+
+        self.assertEqual(collection.title, link.title)
+
+    def test_auto_title_not_found(self) -> None:
+        extent = pystac.Extent.from_items([self.item])
+        collection = pystac.Collection(
+            id="my_collection",
+            description="Test Collection",
+            extent=extent,
+        )
+        link = pystac.Link("my rel", target=collection)
+
+        self.assertEqual(None, link.title)
+
+    def test_auto_title_is_serialized(self) -> None:
+        extent = pystac.Extent.from_items([self.item])
+        collection = pystac.Collection(
+            id="my_collection",
+            description="Test Collection",
+            extent=extent,
+            title="Collection Title",
+        )
+        link = pystac.Link("my rel", target=collection)
+
+        assert link.to_dict().get("title") == collection.title
+
+    def test_no_auto_title_if_not_resolved(self) -> None:
+        link = pystac.Link(
+            "my rel", target="https://www.some-domain.com/path/to/thing.txt"
+        )
+
+        assert link.title is None
+
+    def test_title_as_init_argument(self) -> None:
+        link_title = "Link title"
+        extent = pystac.Extent.from_items([self.item])
+        collection = pystac.Collection(
+            id="my_collection",
+            description="Test Collection",
+            extent=extent,
+            title="Collection Title",
+        )
+        link = pystac.Link("my rel", title=link_title, target=collection)
+
+        assert link.title == link_title
+        assert link.to_dict().get("title") == link_title
+
 
 class StaticLinkTest(unittest.TestCase):
     def setUp(self) -> None: