Add snippets for ndb cache doc

appengine/ndb/cache/README.md

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+## App Engine Datastore NDB Cache Samples
+
+This contains snippets used in the NDB cache documentation, demonstrating
+various operation on ndb caches.
+
+<!-- auto-doc-link -->
+These samples are used on the following documentation page:
+
+> https://cloud.google.com/appengine/docs/python/ndb/cache
+
+<!-- end-auto-doc-link -->

appengine/ndb/cache/snippets.py

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+# Copyright 2016 Google Inc. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+def set_in_process_cache_policy(ctx, func):
+    ctx.set_cache_policy(func)
+
+
+def set_memcache_policy(ctx, func):
+    ctx.set_memcache_policy(func)
+
+
+def bypass_in_process_cache_for_account_entities(ctx):
+    ctx.set_cache_policy(lambda key: key.kind() != 'Account')
+
+
+def set_datastore_policy(ctx, func):
+    ctx.set_datastore_policy(func)
+
+
+def set_memcache_timeout_policy(ctx, func):
+    ctx.set_memcache_timeout_policy(func)
+
+
+def clear_cache(ctx):
+    ctx.clear_cache()

appengine/ndb/cache/snippets_test.py

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+# Copyright 2016 Google Inc. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from google.appengine.ext import ndb
+import snippets
+
+
+def test_set_in_process_cache_policy(testbed):
+    context = ndb.get_context()
+
+    def policy(key):
+        return 1 == 1
+
+    snippets.set_in_process_cache_policy(context, policy)
+    assert policy == context.get_cache_policy()
+
+
+def test_set_memcache_policy(testbed):
+    context = ndb.get_context()
+
+    def policy(key):
+        return 1 == 2
+
+    snippets.set_memcache_policy(context, policy)
+    assert policy == context.get_memcache_policy()
+
+
+def test_bypass_in_process_cache_for_account_entities(testbed):
+    context = ndb.get_context()
+    assert context.get_cache_policy() == context.default_cache_policy
+    snippets.bypass_in_process_cache_for_account_entities(context)
+    assert context.get_cache_policy() != context.default_cache_policy
+
+
+def test_set_datastore_policy(testbed):
+    context = ndb.get_context()
+
+    def policy(key):
+        return key is None
+
+    snippets.set_datastore_policy(context, policy)
+    assert context.get_datastore_policy() == policy
+
+
+def test_set_memcache_timeout_policy(testbed):
+    context = ndb.get_context()
+
+    def policy(key):
+        return 1
+
+    snippets.set_memcache_timeout_policy(context, policy)
+    assert context.get_memcache_timeout_policy() == policy
+
+
+def test_clear_cache(testbed):
+    context = ndb.get_context()
+    snippets.clear_cache(context)