metrics diff error message in --no-scm projects (#3650)

* fix #3605

* changed exception raise position

* Updated error msg

* message changed

* metrics diff: output edits per
per https://github.com/iterative/dvc/pull/3650
and https://github.com/iterative/dvc/pull/3650#pullrequestreview-396837915

* Update tests/func/test_metrics.py

Co-Authored-By: Ruslan Kuprieiev <[email protected]>

* NoSCM now had a default attribute getting method

`NoSCM` Object now raise `NoSCMError` when calling attributes which only in `Git` Object

* Joined lines

Mail change test

* tests: diff: use NoSCMError

Co-authored-by: myname <[email protected]>
Co-authored-by: karajan1001 <[email protected]>
Co-authored-by: Jorge Orpinel <[email protected]>
Co-authored-by: Ruslan Kuprieiev <[email protected]>
Co-authored-by: Ruslan Kuprieiev <[email protected]>

dvc/repo/diff.py

@@ -1,8 +1,6 @@
 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
 
 
@@ -15,8 +13,6 @@ def diff(self, a_rev="HEAD", b_rev=None):
     the concept of `index`, but it keeps the same interface, thus,
     `dvc diff` would be the same as `dvc diff HEAD`.
     """
-    if type(self.scm) is not Git:
-        raise DvcException("only supported for Git repositories")
 
     def _paths_checksums():
         """

dvc/scm/__init__.py

@@ -1,13 +1,14 @@
 """Manages source control systems (e.g. Git)."""
 
-from dvc.scm.base import Base
+from dvc.scm.base import Base, NoSCMError
 from dvc.scm.git import Git
 
 
 # Syntactic sugar to signal that this is an actual implementation for a DVC
 # project under no SCM control.
 class NoSCM(Base):
-    pass
+    def __getattr__(self, name):
+        raise NoSCMError
 
 
 def SCM(

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 "
+            "seeing this error in a Git repo, try updating the DVC "
+            "configuration with `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

@@ -14,9 +14,11 @@ def digest(text):
 
 
 def test_no_scm(tmp_dir, dvc):
+    from dvc.scm.base import NoSCMError
+
     tmp_dir.dvc_gen("file", "text")
 
-    with pytest.raises(DvcException, match=r"only supported for Git repos"):
+    with pytest.raises(NoSCMError):
         dvc.diff()
 
 

tests/func/test_metrics.py

@@ -12,6 +12,7 @@ from dvc.main import main
 from dvc.repo import Repo as DvcRepo
 from dvc.repo.metrics.show import NO_METRICS_FILE_AT_REFERENCE_WARNING
 from dvc.utils import relpath
+from dvc.scm.base import NoSCMError
 from tests.basic_env import TestDvcGit
 
 
@@ -1005,3 +1006,13 @@ 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(NoSCMError):
+        dvc.metrics.diff()