Subclassing can override json dumper and loader

Additionally:

* from_string gets a loads parameter
* to_string gets a dumps_parameter
* documentation added
* added more tests

Author: clj

doc/tutorial.rst

@@ -67,3 +67,55 @@ explicitly.
     # or from a list
     >>> patch = [{'op': 'add', 'path': '/baz', 'value': 'qux'}]
     >>> res = jsonpatch.apply_patch(obj, patch)
+
+
+Dealing with Custom Types
+-------------------------
+
+Custom JSON dump and load functions can be used to support custom types such as
+`decimal.Decimal`. The following examples shows how the
+`simplejson <https://simplejson.readthedocs.io/>`_ package, which has native
+support for Python's ``Decimal`` type, can be used to create a custom
+``JsonPatch`` subclass with ``Decimal`` support:
+
+.. code-block:: python
+
+    >>> import decimal
+    >>> import simplejson
+
+    >>> class DecimalJsonPatch(jsonpatch.JsonPatch):
+            @staticmethod
+            def json_dumper(obj):
+                return simplejson.dumps(obj)
+
+            @staticmethod
+            def json_loader(obj):
+                return simplejson.loads(obj, use_decimal=True,
+                                        object_pairs_hook=jsonpatch.multidict)
+
+    >>> src = {}
+    >>> dst = {'bar': decimal.Decimal('1.10')}
+    >>> patch = DecimalJsonPatch.from_diff(src, dst)
+    >>> doc = {'foo': 1}
+    >>> result = patch.apply(doc)
+    {'foo': 1, 'bar': Decimal('1.10')}
+
+Instead of subclassing it is also possible to pass a dump function to
+``from_diff``:
+
+    >>> patch = jsonpatch.JsonPatch.from_diff(src, dst, dumps=simplejson.dumps)
+
+a dumps function to ``to_string``:
+
+    >>> serialized_patch = patch.to_string(dumps=simplejson.dumps)
+    '[{"op": "add", "path": "/bar", "value": 1.10}]'
+
+and load  function to ``from_string``:
+
+    >>> import functools
+    >>> loads = functools.partial(simplejson.loads, use_decimal=True,
+                                  object_pairs_hook=jsonpatch.multidict)
+    >>> patch.from_string(serialized_patch, loads=loads)
+    >>> doc = {'foo': 1}
+    >>> result = patch.apply(doc)
+    {'foo': 1, 'bar': Decimal('1.10')}

jsonpatch.py

@@ -165,6 +165,9 @@ def make_patch(src, dst):
 
 
 class JsonPatch(object):
+    json_dumper = staticmethod(json.dumps)
+    json_loader = staticmethod(_jsonloads)
+
     """A JSON Patch is a list of Patch Operations.
 
     >>> patch = JsonPatch([
@@ -246,19 +249,23 @@ def __ne__(self, other):
         return not(self == other)
 
     @classmethod
-    def from_string(cls, patch_str):
+    def from_string(cls, patch_str, loads=None):
         """Creates JsonPatch instance from string source.
 
         :param patch_str: JSON patch as raw string.
         :type patch_str: str
+        :param loads: A function of one argument that loads a serialized
+                      JSON string.
+        :type loads: function
 
         :return: :class:`JsonPatch` instance.
         """
-        patch = _jsonloads(patch_str)
+        json_loader = loads or cls.json_loader
+        patch = json_loader(patch_str)
         return cls(patch)
 
     @classmethod
-    def from_diff(cls, src, dst, optimization=True, dumps=json.dumps):
+    def from_diff(cls, src, dst, optimization=True, dumps=None):
         """Creates JsonPatch instance based on comparing of two document
         objects. Json patch would be created for `src` argument against `dst`
         one.
@@ -282,15 +289,16 @@ def from_diff(cls, src, dst, optimization=True, dumps=json.dumps):
         >>> new == dst
         True
         """
-
-        builder = DiffBuilder(dumps)
+        json_dumper = dumps or cls.json_dumper
+        builder = DiffBuilder(json_dumper)
         builder._compare_values('', None, src, dst)
         ops = list(builder.execute())
         return cls(ops)
 
-    def to_string(self):
+    def to_string(self, dumps=None):
         """Returns patch set as JSON string."""
-        return json.dumps(self.patch)
+        json_dumper = dumps or self.json_dumper
+        return json_dumper(self.patch)
 
     @property
     def _ops(self):

tests.py

@@ -268,6 +268,34 @@ def test_str(self):
         self.assertEqual(json.dumps(patch_obj), patch.to_string())
 
 
+def custom_types_dumps(obj):
+    def default(obj):
+        if isinstance(obj, decimal.Decimal):
+            return {'__decimal__': str(obj)}
+        raise TypeError('Unknown type')
+
+    return json.dumps(obj, default=default)
+
+
+def custom_types_loads(obj):
+    def as_decimal(dct):
+        if '__decimal__' in dct:
+            return decimal.Decimal(dct['__decimal__'])
+        return dct
+
+    return json.loads(obj, object_hook=as_decimal)
+
+
+class CustomTypesJsonPatch(jsonpatch.JsonPatch):
+    @staticmethod
+    def json_dumper(obj):
+        return custom_types_dumps(obj)
+
+    @staticmethod
+    def json_loader(obj):
+        return custom_types_loads(obj)
+
+
 class MakePatchTestCase(unittest.TestCase):
 
     def test_apply_patch_to_copy(self):
@@ -446,18 +474,33 @@ def test_issue103(self):
         self.assertEqual(res, dst)
         self.assertIsInstance(res['A'], float)
 
-    def test_custom_types(self):
-        def default(obj):
-            if isinstance(obj, decimal.Decimal):
-                return str(obj)
-            raise TypeError('Unknown type')
+    def test_custom_types_diff(self):
+        old = {'value': decimal.Decimal('1.0')}
+        new = {'value': decimal.Decimal('1.00')}
+        generated_patch = jsonpatch.JsonPatch.from_diff(
+            old, new,  dumps=custom_types_dumps)
+        str_patch = generated_patch.to_string(dumps=custom_types_dumps)
+        loaded_patch = jsonpatch.JsonPatch.from_string(
+            str_patch, loads=custom_types_loads)
+        self.assertEqual(generated_patch, loaded_patch)
+        new_from_patch = jsonpatch.apply_patch(old, generated_patch)
+        self.assertEqual(new, new_from_patch)
 
-        def dumps(obj):
-            return json.dumps(obj, default=default)
+    def test_custom_types_subclass(self):
+        old = {'value': decimal.Decimal('1.0')}
+        new = {'value': decimal.Decimal('1.00')}
+        generated_patch = CustomTypesJsonPatch.from_diff(old, new)
+        str_patch = generated_patch.to_string()
+        loaded_patch = CustomTypesJsonPatch.from_string(str_patch)
+        self.assertEqual(generated_patch, loaded_patch)
+        new_from_patch = jsonpatch.apply_patch(old, loaded_patch)
+        self.assertEqual(new, new_from_patch)
 
+    def test_custom_types_subclass_load(self):
         old = {'value': decimal.Decimal('1.0')}
         new = {'value': decimal.Decimal('1.00')}
-        patch = jsonpatch.JsonPatch.from_diff(old, new, dumps=dumps)
+        patch = CustomTypesJsonPatch.from_string(
+            '[{"op": "replace", "path": "/value", "value": {"__decimal__": "1.00"}}]')
         new_from_patch = jsonpatch.apply_patch(old, patch)
         self.assertEqual(new, new_from_patch)