exp branch: Make branch arg optional.

Default to `f"{ref_info.name}-branch"` where ref_info is inferred from the `exp_rev` argument.

Co-authored-by: Dave Berenbaum <[email protected]>

dvc/commands/experiments/branch.py

@@ -26,5 +26,13 @@ def add_parser(experiments_subparsers, parent_parser):
     experiments_branch_parser.add_argument(
         "experiment", help="Experiment to be promoted."
     )
-    experiments_branch_parser.add_argument("branch", help="Name for the new Git branch")
+    experiments_branch_parser.add_argument(
+        "branch",
+        nargs="?",
+        default=None,
+        help=(
+            "Optional name for the new Git branch. "
+            "Defaults to '{experiment-name}-branch'."
+        ),
+    )
     experiments_branch_parser.set_defaults(func=CmdExperimentsBranch)

dvc/repo/experiments/branch.py

@@ -13,7 +13,7 @@ logger = logging.getLogger(__name__)
 
 @locked
 @scm_context
-def branch(repo, exp_rev, branch_name, *args, **kwargs):
+def branch(repo, exp_rev, branch_name=None, **kwargs):
     from dvc.scm import resolve_rev
 
     try:
@@ -46,6 +46,8 @@ def branch(repo, exp_rev, branch_name, *args, **kwargs):
     if not ref_info:
         raise InvalidExpRevError(exp_rev)
 
+    branch_name = branch_name or f"{ref_info.name}-branch"
+
     branch_ref = f"refs/heads/{branch_name}"
     if repo.scm.get_ref(branch_ref):
         raise InvalidArgumentError(f"Git branch '{branch_name}' already exists.")

tests/func/experiments/test_experiments.py

@@ -246,11 +246,12 @@ def test_branch(tmp_dir, scm, dvc, exp_stage):
 
     with pytest.raises(InvalidArgumentError):
         dvc.experiments.branch("foo", "branch-exists")
+    dvc.experiments.branch("foo")
     dvc.experiments.branch("foo", "branch-name")
     dvc.experiments.branch(exp_a, "branch-rev")
     dvc.experiments.branch(ref_a, "branch-ref")
 
-    for name in ["branch-name", "branch-rev", "branch-ref"]:
+    for name in ["foo-branch", "branch-name", "branch-rev", "branch-ref"]:
         assert name in scm.list_branches()
         assert scm.resolve_rev(name) == exp_a
 

tests/unit/command/test_experiments.py

@@ -187,15 +187,21 @@ def test_experiments_gc(dvc, scm, mocker):
 
 
 def test_experiments_branch(dvc, scm, mocker):
-    cli_args = parse_args(["experiments", "branch", "expname", "branchname"])
+    m = mocker.patch("dvc.repo.experiments.branch.branch", return_value={})
+
+    cli_args = parse_args(["experiments", "branch", "expname"])
     assert cli_args.func == CmdExperimentsBranch
 
     cmd = cli_args.func(cli_args)
-    m = mocker.patch("dvc.repo.experiments.branch.branch", return_value={})
-
     assert cmd.run() == 0
 
-    m.assert_called_once_with(cmd.repo, "expname", "branchname")
+    m.assert_called_with(cmd.repo, "expname", None)
+
+    cli_args = parse_args(["experiments", "branch", "expname", "branchname"])
+    cmd = cli_args.func(cli_args)
+    assert cmd.run() == 0
+
+    m.assert_called_with(cmd.repo, "expname", "branchname")
 
 
 def test_experiments_list(dvc, scm, mocker):