fix Link 'label:assets' field type as list

CHANGELOG.md

@@ -1,6 +1,11 @@
 
 ## Unreleased
 
+- Fix invalid `label:assets` type in `stac_pydantic.links.Link`.
+  A single `str` was specified instead of `List[str]` as expected by the extension field
+  (see [Label Extension - Links](https://github.com/stac-extensions/label?tab=readme-ov-file#links-source-imagery)) (#183, @fmigneault).
+- Add validation of `Link` ensuring that `rel=source` is employed if `label:assets` is specified (#183, @fmigneault).
+
 ## 3.3.2 (2025-06-18)
 
 - Remove restriction on valid media types for links (#182, @mishaschwartz)

stac_pydantic/links.py

@@ -1,8 +1,8 @@
 from enum import auto
-from typing import Iterator, List, Optional, Union
+from typing import Iterator, List, Optional, Union, Any
 from urllib.parse import urljoin
 
-from pydantic import ConfigDict, Field, RootModel
+from pydantic import AliasChoices, ConfigDict, Field, RootModel, ValidationInfo, field_validator
 
 from stac_pydantic.shared import MimeTypes, StacBaseModel
 from stac_pydantic.utils import AutoValueEnum
@@ -19,13 +19,29 @@ class Link(StacBaseModel):
     title: Optional[str] = None
 
     # Label extension
-    label: Optional[str] = Field(default=None, alias="label:assets")
+    label: Optional[List[str]] = Field(
+        default=None,
+        min_length=1,
+        alias="label:assets",
+        validation_alias=AliasChoices("label:assets", "label_assets", "label"),
+    )
     model_config = ConfigDict(use_enum_values=True, extra="allow")
 
     def resolve(self, base_url: str) -> None:
         """resolve a link to the given base URL"""
         self.href = urljoin(base_url, self.href)
 
+    @field_validator("label", mode="after")
+    @classmethod
+    def validate_label_rel_source(cls, label: Optional[List[str]], info: ValidationInfo) -> Optional[List[str]]:
+        # check requirement: https://github.com/stac-extensions/label#links-source-imagery
+        if label and info.data["rel"] != "source":
+            raise ValueError(
+                "Label extension link with 'label:assets' is only allowed for link with 'rel=source'. "
+                f"Values ('rel': {info.data['rel']}, 'label:assets': {label})"
+            )
+        return label
+
 
 class Links(RootModel[List[Link]]):
     root: List[Link]

tests/example_stac/roads_item.json

@@ -156,7 +156,9 @@
         "href": "roads_source.json",
         "rel": "source",
         "title": "The source imagery these road labels were derived from",
-        "label:assets": "road_labels"
+        "label:assets": [
+          "road_labels"
+        ]
       }
     ],
     "assets": {

tests/test_models.py

@@ -105,6 +105,15 @@ def test_label_extension() -> None:
     dict_match(test_item, valid_item)
 
 
+def test_label_extension_rel_source() -> None:
+    with pytest.raises(ValidationError):
+        Link(href=LABEL_EXTENSION, rel="random", label_assets=["road_labels"])
+
+    link = Link(href=LABEL_EXTENSION, rel="source", label_assets=["road_labels"])
+    assert link.rel == "source"
+    assert link.label == ["road_labels"]
+
+
 def test_explicit_extension_validation() -> None:
     test_item = request(EO_EXTENSION)