api: params_show: Raise exception if no params found.

Fixes #7926

dvc/api/params.py

@@ -3,6 +3,7 @@
 
 from funcy import first
 
+from dvc.exceptions import DvcException
 from dvc.repo import Repo
 
 
@@ -50,6 +51,9 @@ def params_show(
     Returns:
         Dict: See Examples below.
 
+    Raises:
+        DvcException: If no params are found in `repo`.
+
     Examples:
 
         - No arguments.
@@ -253,6 +257,9 @@ def _postprocess(params):
         if "workspace" in processed:
             del processed["workspace"]
 
+        if not processed:
+            raise DvcException("No params found")
+
         return processed[first(processed)]
 
     with Repo.open(repo) as _repo:

tests/func/api/test_params.py

@@ -3,6 +3,7 @@
 import pytest
 
 from dvc import api
+from dvc.exceptions import DvcException
 
 
 @pytest.fixture
@@ -128,3 +129,19 @@ def test_params_show_repo(tmp_dir, erepo_dir):
             params=["foo"],
         )
     assert api.params_show(repo=erepo_dir) == {"foo": 1}
+
+
+def test_params_show_no_params_found(tmp_dir, dvc):
+    # Empty repo
+    with pytest.raises(DvcException, match="No params found"):
+        api.params_show()
+
+    # params.yaml but no dvc.yaml
+    (tmp_dir / "params.yaml").dump({"foo": 1})
+    assert api.params_show() == {"foo": 1}
+
+    # dvc.yaml but no params.yaml
+    (tmp_dir / "params.yaml").unlink()
+    dvc.stage.add(name="echo", cmd="echo foo")
+    with pytest.raises(DvcException, match="No params found"):
+        api.params_show()