metrics diff error message in --no-scm projects

dvc/repo/brancher.py

@@ -1,6 +1,7 @@
 from funcy import group_by
 
 from dvc.scm.tree import WorkingTree
+from dvc.scm.base import NOSCMError
 
 
 def brancher(  # noqa: E302
@@ -46,5 +47,7 @@ def brancher(  # noqa: E302
             for sha, names in group_by(scm.resolve_rev, revs).items():
                 self.tree = scm.get_tree(sha)
                 yield ", ".join(names)
+    except AttributeError:
+        raise NOSCMError()
     finally:
         self.tree = saved_tree

dvc/repo/diff.py

@@ -1,9 +1,9 @@
 import os
 
-from dvc.exceptions import DvcException
 from dvc.repo import locked
 from dvc.scm.git import Git
 from dvc.scm.tree import is_working_tree
+from dvc.scm.base import NOSCMError
 
 
 @locked
@@ -16,7 +16,7 @@ def diff(self, a_rev="HEAD", b_rev=None):
     `dvc diff` would be the same as `dvc diff HEAD`.
     """
     if type(self.scm) is not Git:
-        raise DvcException("only supported for Git repositories")
+        raise NOSCMError
 
     def _paths_checksums():
         """

dvc/scm/base.py

@@ -24,6 +24,16 @@ class RevError(SCMError):
     pass
 
 
+class NOSCMError(SCMError):
+    def __init__(self):
+        msg = (
+            "Only supported for Git repositories. If you're\n"
+            "seeing this error in a Git repo, try updating the DVC "
+            "configuration with\n`dvc config core.no_scm False`."
+        )
+        super().__init__(msg)
+
+
 class Base(object):
     """Base class for source control management driver implementations."""
 

tests/func/test_diff.py

@@ -16,7 +16,7 @@ def digest(text):
 def test_no_scm(tmp_dir, dvc):
     tmp_dir.dvc_gen("file", "text")
 
-    with pytest.raises(DvcException, match=r"only supported for Git repos"):
+    with pytest.raises(DvcException, match=r"Only supported for Git repos"):
         dvc.diff()
 
 

tests/func/test_metrics.py

@@ -1005,3 +1005,13 @@ def test_metrics_diff_deleted_metric(tmp_dir, scm, dvc):
             "a.b.e": {"old": "3", "new": None},
         }
     }
+
+
+def test_metrics_without_scm(tmp_dir, dvc):
+    metrics = {"acc": 0.97, "recall": 0.95}
+    metrics_name = "metrics.json"
+    tmp_dir.gen({metrics_name: json.dumps(metrics)})
+    dvc.add(metrics_name)
+    dvc.metrics.add(metrics_name)
+    with pytest.raises(DvcException, match=r"Only supported for Git repos"):
+        dvc.metrics.diff()