Add support in vinca-pinning-render to completly delete pinning key from conda-forge-pinnings

README.md

@@ -69,6 +69,8 @@ pinning_overrides:
     - false
   python_impl:
     - cpython
+  # Remove a pin inherited from conda-forge-pinning.
+  docker_image: null
 ```
 
 Render the build-tool input with:
@@ -80,6 +82,9 @@ vinca-pinning-render
 The renderer downloads that exact `conda-forge-pinning` package and starts with its
 `conda_build_config.yaml`. It sorts the named migrations by `migrator_ts`, just as
 `conda-smithy` does, and combines each one using the [CFEP-9 variant algebra](https://github.com/conda-forge/cfep/blob/main/cfep-09.md).
+An override set to `null` removes that key from the generated config, including
+pins added by migrations. To remove a key in a `zip_keys` group, set every member
+of that group to `null`; the group is removed as well.
 
 For a full rebuild, update the base and eligible migrations with:
 

vinca/pinning.py

@@ -220,10 +220,22 @@ def _migration_name(name: str) -> str:
 
 
 def _overlay(target: Any, source: Any) -> None:
+    removed_keys = {key for key, value in source.items() if value is None}
     for key, value in source.items():
         if key == "migrator_ts" or str(key).startswith("__"):
             continue
-        target[key] = value
+        if value is None:
+            target.pop(key, None)
+        else:
+            target[key] = value
+    if removed_keys and "zip_keys" in target:
+        groups = [
+            group for group in target["zip_keys"] if not set(group) <= removed_keys
+        ]
+        if groups:
+            target["zip_keys"] = groups
+        else:
+            target.pop("zip_keys")
 
 
 def _migration_timestamp(payload: bytes) -> float:
@@ -244,6 +256,13 @@ def _validate_zipped_overrides(rendered: Any, overrides: Any) -> None:
                 f"{', '.join(sorted(touched))} also requires "
                 f"{', '.join(sorted(missing))}"
             )
+        removed = {key for key in group if key in overrides and overrides[key] is None}
+        if removed and removed != group:
+            raise PinningError(
+                "Pinning overrides must remove every member of a zip_keys group; "
+                f"{', '.join(sorted(removed))} also requires "
+                f"{', '.join(sorted(group - removed))}: null"
+            )
 
 
 def _format_rendered_yaml(payload: str) -> str:

vinca/test_pinning.py

@@ -100,6 +100,77 @@ def test_render_applies_base_migration_then_override(tmp_path):
     assert not any(line.endswith(" ") for line in output_path.read_text().splitlines())
 
 
+def test_render_null_override_removes_base_and_migrated_pins(tmp_path):
+    config_path = tmp_path / "vinca_pinning.yaml"
+    config_path.write_text(
+        """\
+conda_forge_pinning_version: '1'
+migrations:
+  - libboost190
+pinning_overrides:
+  python: null
+  libboost_devel: null
+  absent: null
+"""
+    )
+    output_path = tmp_path / "conda_build_config.yaml"
+
+    rendered = render_pinning(
+        config_path,
+        output_path,
+        package=(BASE_CONFIG, {"libboost190": BOOST_MIGRATION}),
+    )
+
+    assert dict(rendered) == {"keep": ["base"]}
+    assert dict(ruamel.yaml.YAML(typ="safe").load(output_path.read_text())) == {
+        "keep": ["base"]
+    }
+
+
+def test_render_null_override_removes_entire_zip_group(tmp_path):
+    config_path = tmp_path / "vinca_pinning.yaml"
+    config_path.write_text(
+        """\
+conda_forge_pinning_version: '1'
+pinning_overrides:
+  python: null
+  is_python_min: null
+"""
+    )
+    base = b"""\
+python: ['3.13']
+is_python_min: [true]
+zip_keys:
+  - [python, is_python_min]
+keep: [base]
+"""
+
+    rendered = render_pinning(config_path, tmp_path / "output.yaml", package=(base, {}))
+
+    assert dict(rendered) == {"keep": ["base"]}
+
+
+def test_render_rejects_mixed_removal_and_override_in_zip_group(tmp_path):
+    config_path = tmp_path / "vinca_pinning.yaml"
+    config_path.write_text(
+        """\
+conda_forge_pinning_version: '1'
+pinning_overrides:
+  python: null
+  is_python_min: [false]
+"""
+    )
+    base = b"""\
+python: ['3.13']
+is_python_min: [true]
+zip_keys:
+  - [python, is_python_min]
+"""
+
+    with pytest.raises(PinningError, match="remove every member"):
+        render_pinning(config_path, tmp_path / "output.yaml", package=(base, {}))
+
+
 def test_render_keeps_selector_on_empty_sequence_item(tmp_path):
     config_path = tmp_path / "vinca_pinning.yaml"
     config_path.write_text(