Add module with Ellipsis typing workaround from daf_butler.

python/lsst/utils/ellipsis.py

@@ -0,0 +1,38 @@
+# This file is part of utils.
+#
+# Developed for the LSST Data Management System.
+# This product includes software developed by the LSST Project
+# (https://www.lsst.org).
+# See the COPYRIGHT file at the top-level directory of this distribution
+# for details of code ownership.
+#
+# Use of this source code is governed by a 3-clause BSD-style
+# license that can be found in the LICENSE file.
+
+from __future__ import annotations
+
+__all__ = ("Ellipsis", "EllipsisType")
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    # Workaround for `...` not having an exposed type in Python, borrowed from
+    # https://github.com/python/typing/issues/684#issuecomment-548203158
+    # Along with that, we need to either use `Ellipsis` instead of `...` for
+    # the actual sentinal value internally, and tell MyPy to ignore conversions
+    # from `...` to `Ellipsis` at the public-interface boundary.
+    #
+    # `Ellipsis` and `EllipsisType` should be directly imported from this
+    # module by related code that needs them; hopefully that will stay confined
+    # to `lsst.daf.butler.registry`.  Putting these in __all__ is bad for
+    # Sphinx, and probably more confusing than helpful overall.
+    from enum import Enum
+
+    class EllipsisType(Enum):
+        Ellipsis = "..."
+
+    Ellipsis = EllipsisType.Ellipsis
+
+else:
+    EllipsisType = type(Ellipsis)
+    Ellipsis = Ellipsis