Add _from_json functionality for pybamm.sign (pybamm-team#4517)

* Add _from_json functionality for Sign + test

* Update changelog

---------

Co-authored-by: Eric G. Kratz <kratman@users.noreply.github.com>

CHANGELOG.md

@@ -21,6 +21,7 @@
 
 ## Bug Fixes
 
+- Added `_from_json()` functionality to `Sign` which was erroneously omitted previously. ([#4517](https://github.com/pybamm-team/PyBaMM/pull/4517))
 - Fixed bug where IDAKLU solver failed when `output variables` were specified and an extrapolation event is present. ([#4440](https://github.com/pybamm-team/PyBaMM/pull/4440))
 
 ## Breaking changes

src/pybamm/expression_tree/unary_operators.py

@@ -212,7 +212,8 @@ def __init__(self, child):
 
     @classmethod
     def _from_json(cls, snippet: dict):
-        raise NotImplementedError()
+        """See :meth:`pybamm.UnaryOperator._from_json()`."""
+        return cls(snippet["children"][0])
 
     def diff(self, variable):
         """See :meth:`pybamm.Symbol.diff()`."""

tests/unit/test_expression_tree/test_unary_operators.py

@@ -138,9 +138,20 @@ def test_sign(self):
         )
 
         # Test from_json
-        with pytest.raises(NotImplementedError):
-            # signs are always scalar/array types in a discretised model
-            pybamm.Sign._from_json({})
+        c = pybamm.Multiplication(pybamm.Variable("a"), pybamm.Scalar(3))
+        sign_json = {
+            "name": "sign",
+            "id": 5341515228900508018,
+            "domains": {
+                "primary": [],
+                "secondary": [],
+                "tertiary": [],
+                "quaternary": [],
+            },
+            "children": [c],
+        }
+
+        assert pybamm.sign(c) == pybamm.Sign._from_json(sign_json)
 
     def test_floor(self, mocker):
         a = pybamm.Symbol("a")