Problem

After calling the endpoint, the instance db_session was not freed, causing a warning to be emitted.

def endpoint(request):
  data = MySchema(context={"db_session": create_db_session()}).load(request.json)
  return "OK"

The reason is that the MySchema instance is kept in a global LRU cache. Calling endpoint multiple times will eventually release the older db_session instances, though.

Analysis

The @lru_cache(max_size=8) decorator was used on a method, causing instances of self to be cached and outlive their intended scope. This led to unexpected behavior where instances persisted beyond a single web request, retaining references and consuming memory unnecessarily.

This problem was called out by PyCQA/flake8-bugbear#310 but ignored via noqa.

Options

1. Use cachetools.cachedmethod (requires a third party dependency)
2. Implement manually
3. Remove LRU cache

Proposal

The LRU cache was introduced in #1309 (the PR reported 3% performance increase on the test base). I'm not sure the LRU cache actually has that much of an effect in real world scenarios:

1. The cost of two tuple constructions and two dict lookups is small in the first place
2. If more than 2 schema instances are used, max_size is exhausted, leading to cache misses.
   • A cold load causes 3 misses, 0 hits (warm -> 3 hits)
   • A cold dump causes 2 misses, 0 hits (warm -> 3 hits).
3. A more complex schema guarantees 100% misses, as too many instances are involved.

class Child(ma.Schema):
  pass

class Parent(ma.Schema):
  a = ma.fields.Nested(Child)
  b = ma.fields.Nested(Child)
  c = ma.fields.Nested(Child)

ma.Schema._has_processors.cache_clear()
p = Parent()
p.load({"a": {}, "b": {}, "c": {}})
p.load({"a": {}, "b": {}, "c": {}})
ma.Schema._has_processors.cache_info() # CacheInfo(hits=0, misses=24, maxsize=8, currsize=8)