Avoid pointless work for objects

Calculating the maxProperties lets us avoid a pointless and heavy filter in many cases, which fixes #55.

Author: Zac-HD

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 
+#### 0.16.1 - 2020-06-15
+- Performance improvement for `object` schemas with `additionalProperties: false` (issue #55)
+
 #### 0.16.0 - 2020-06-07
 - Performance improvement for schemas with non-validation keys (such as `description`)
 - Errors from e.g. invalid schemas are deferred from import time to become failing tests

src/hypothesis_jsonschema/__init__.py

@@ -3,7 +3,7 @@
 The only public API is `from_schema`; check the docstring for details.
 """
 
-__version__ = "0.16.0"
+__version__ = "0.16.1"
 __all__ = ["from_schema"]
 
 from ._from_schema import from_schema

src/hypothesis_jsonschema/_canonicalise.py

@@ -394,13 +394,12 @@ def canonicalish(schema: JSONType) -> Dict[str, Any]:
         schema.pop("items", None)
     if (
         "properties" in schema
-        and "maxProperties" in schema
         and not schema.get("patternProperties")
         and schema.get("additionalProperties") == FALSEY
     ):
-        schema["maxProperties"] = min(
-            schema["maxProperties"], len(schema["properties"])
-        )
+        max_props = schema.get("maxProperties", math.inf)
+        assert isinstance(max_props, (int, float))
+        schema["maxProperties"] = min(max_props, len(schema["properties"]))
     if "object" in type_ and schema.get("minProperties", 0) > schema.get(
         "maxProperties", math.inf
     ):

tests/test_canonicalise.py

@@ -292,6 +292,7 @@ def test_canonicalises_to_expected(schema, expected):
             {
                 "properties": {"foo": {"enum": [False, True]}},
                 "additionalProperties": {"not": {}},
+                "maxProperties": 1,
             },
         ),
         (