Slightly speed up the contains keyword.

Saves some unnecessary repeated validator re-creation while validating
arrays.

In a quick benchmark (added here) and on my local machine (an M2 Mini)
this goes from:

    ```
    baseline: Mean +- std dev: 3.55 us +- 0.04 us
    beginning: Mean +- std dev: 3.37 ms +- 0.02 ms
    middle: Mean +- std dev: 3.37 ms +- 0.03 ms
    end: Mean +- std dev: 3.36 ms +- 0.02 ms
    invalid: Mean +- std dev: 3.40 ms +- 0.02 ms
    ```
to:

    ```
    baseline: Mean +- std dev: 4.27 us +- 0.05 us
    beginning: Mean +- std dev: 2.65 ms +- 0.01 ms
    middle: Mean +- std dev: 2.66 ms +- 0.02 ms
    end: Mean +- std dev: 2.67 ms +- 0.02 ms
    invalid: Mean +- std dev: 2.70 ms +- 0.02 ms
    ```

on the included example (synthetic of course, but not ridiculously so).

(The lack of difference in timing for how far into the array we get
before finding a match seems interesting but probably requires a
benchmark with a more interesting subschema we're matching on).

CHANGELOG.rst

@@ -1,3 +1,8 @@
+v4.21.1
+=======
+
+* Slightly speed up the ``contains`` keyword by removing some unnecessary validator (re-)creation.
+
 v4.21.0
 =======
 

jsonschema/_keywords.py

@@ -95,8 +95,10 @@ def contains(validator, contains, instance, schema):
     min_contains = schema.get("minContains", 1)
     max_contains = schema.get("maxContains", len(instance))
 
+    contains_validator = validator.evolve(schema=contains)
+
     for each in instance:
-        if validator.evolve(schema=contains).is_valid(each):
+        if contains_validator.is_valid(each):
             matches += 1
             if matches > max_contains:
                 yield ValidationError(

jsonschema/benchmarks/contains.py

@@ -0,0 +1,28 @@
+"""
+A benchmark for validation of the `contains` keyword.
+"""
+
+from pyperf import Runner
+
+from jsonschema import Draft202012Validator
+
+schema = {
+    "type": "array",
+    "contains": {"const": 37},
+}
+validator = Draft202012Validator(schema)
+
+size = 1000
+beginning = [37] + [0] * (size - 1)
+middle = [0] * (size // 2) + [37] + [0] * (size // 2)
+end = [0] * (size - 1) + [37]
+invalid = [0] * size
+
+
+if __name__ == "__main__":
+    runner = Runner()
+    runner.bench_func("baseline", lambda: validator.is_valid([]))
+    runner.bench_func("beginning", lambda: validator.is_valid(beginning))
+    runner.bench_func("middle", lambda: validator.is_valid(middle))
+    runner.bench_func("end", lambda: validator.is_valid(end))
+    runner.bench_func("invalid", lambda: validator.is_valid(invalid))