Don't reorder dicts (particularly schemas) when rendering errors.

Schemas, particularly longer ones, are often written intentionally with
their keywords in some specific human-understandable order.

pprint, which we currently use (for better or worse) for rendering
schemas and instances, supports *not* sorting dicts now that they
maintain their insertion order.

So we now pass that argument, and thereby preserve the order the schema
was written in.

Refs: #243

Author: Julian

docs/errors.rst

@@ -216,8 +216,8 @@ easier debugging.
     3 is not valid under any of the given schemas
 
     Failed validating 'anyOf' in schema['items']:
-        {'anyOf': [{'maxLength': 2, 'type': 'string'},
-                   {'minimum': 5, 'type': 'integer'}]}
+        {'anyOf': [{'type': 'string', 'maxLength': 2},
+                   {'type': 'integer', 'minimum': 5}]}
 
     On instance[1]:
         3

jsonschema/exceptions.py

@@ -27,6 +27,13 @@
 _unset = _utils.Unset()
 
 
+def _pretty(thing: Any, prefix: str):
+    """
+    Format something for an error message as prettily as we currently can.
+    """
+    return indent(pformat(thing, width=72, sort_dicts=False), prefix).lstrip()
+
+
 def __getattr__(name):
     if name == "RefResolutionError":
         warnings.warn(
@@ -109,10 +116,10 @@ def __str__(self) -> str:
             {self.message}
 
             Failed validating {self.validator!r} in {schema_path}:
-                {indent(pformat(self.schema, width=72), prefix).lstrip()}
+                {_pretty(self.schema, prefix=prefix)}
 
             On {instance_path}:
-                {indent(pformat(self.instance, width=72), prefix).lstrip()}
+                {_pretty(self.instance, prefix=prefix)}
             """.rstrip(),
         )
 
@@ -278,10 +285,10 @@ def __str__(self):
         return dedent(
             f"""\
             Unknown type {self.type!r} for validator with schema:
-                {indent(pformat(self.schema, width=72), prefix).lstrip()}
+                {_pretty(self.schema, prefix=prefix)}
 
             While checking instance:
-                {indent(pformat(self.instance, width=72), prefix).lstrip()}
+                {_pretty(self.instance, prefix=prefix)}
             """.rstrip(),
         )
 

jsonschema/tests/test_exceptions.py

@@ -648,6 +648,29 @@ def test_uses_pprint(self):
             validator="maxLength",
         )
 
+    def test_does_not_reorder_dicts(self):
+        self.assertShows(
+            """
+            Failed validating 'type' in schema:
+                {'do': 3, 'not': 7, 'sort': 37, 'me': 73}
+
+            On instance:
+                {'here': 73, 'too': 37, 'no': 7, 'sorting': 3}
+            """,
+            schema={
+                "do": 3,
+                "not": 7,
+                "sort": 37,
+                "me": 73,
+            },
+            instance={
+                "here": 73,
+                "too": 37,
+                "no": 7,
+                "sorting": 3,
+            },
+        )
+
     def test_str_works_with_instances_having_overriden_eq_operator(self):
         """
         Check for #164 which rendered exceptions unusable when a