How to Mock a Cache Key Schema Version Bump in Python

Show how to test a cache key schema bump by mocking the class-level version attribute with unittest.mock.

Easy Python 3.9+ Aug 9, 2026 Caching & Redis 13 views 0 copies

mock caching unittest testing

Python code

37 lines

Python 3.9+

from unittest import mock

class VersionCache:
    SCHEMA_VERSION = 1

    def __init__(self, key_prefix="cache"):
        self.key_prefix = key_prefix

    def build_key(self, resource_id):
        return f"{self.key_prefix}:schema-v{self.SCHEMA_VERSION}:{resource_id}"

    def bump_schema(self):
        # Simulated schema bump — increments the cache key version
        self.SCHEMA_VERSION += 1


def main():
    cache = VersionCache()

    # Before schema bump
    print("Before bump:", cache.build_key("user-123"))

    # Bump the schema version
    with mock.patch.object(VersionCache, "SCHEMA_VERSION", 2):
        cache.SCHEMA_VERSION = 2  # simulate the bump within this mock context
        print("During mock:", cache.build_key("user-123"))

    # After mock ends, schema is back to original unless actually bumped
    print("After mock:", cache.build_key("user-123"))

    # Real bump
    cache.bump_schema()
    print("After real bump:", cache.build_key("user-123"))


if __name__ == "__main__":
    main()

Output

stdout

Before bump: cache:schema-v1:user-123
During mock: cache:schema-v2:user-123
After mock: cache:schema-v1:user-123
After real bump: cache:schema-v2:user-123

How it works

The VersionCache class holds a class-level SCHEMA_VERSION that is used when building cache keys. By patching VersionCache.SCHEMA_VERSION inside a mock.patch.object context, you temporarily change the key version without modifying the actual class state. After the context exits, the mock is removed and the original value returns. A real bump_schema() method increments the version permanently, simulating a cache invalidation event in production. This pattern lets you test that old and new key namespaces differ without performing an actual bump.

Common mistakes

Patching the instance attribute instead of the class attribute, which won't affect `build_key` if it reads from the class.
Forgetting to restore the mock context properly, leaving the version changed outside the test.
Assuming a mock on a class attribute persists after the `with` block ends.

Variations

Use `unittest.mock.patch.object(VersionCache, 'SCHEMA_VERSION', 2)` with a decorator on a test function.
Replace the class attribute with a module-level constant and patch it using `patch('module.CONSTANT', 2)`.

Real-world use cases

Testing that a new cache namespace is used after deploying a schema change without waiting for the real rollout.
Verifying that cache keys are versioned correctly when multiple app versions run simultaneously.
Writing regression tests that simulate a cache format upgrade before actually releasing the bump in production.

How to Mock a Cache Key Schema Version Bump in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

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How it works

Common mistakes

Variations

Real-world use cases

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