Python 3.14: `RuntimeError: dictionary changed size during iteration` during `JsonModel` class creation (`schema_for_fields` iterates `cls.__dict__`)

[davidbuck-ortex]

Summary

On Python 3.14, redis-om-python can crash at import/startup time when constructing a JsonModel, raising:

RuntimeError: dictionary changed size during iteration

This occurs during schema generation (__init_subclass__ → redisearch_schema → schema_for_fields) while iterating directly over the live class namespace (cls.__dict__). The code inside the loop mutates FieldInfo objects, but does not change the class dict size; the size change appears to be coming from concurrent class construction/mutation (likely Pydantic model construction still injecting attributes into the class namespace while redis-om iterates it).

I appreciate that you aren't claiming Python 3.14 compatability at the present time, so this was discovered during my attempts to review my app's third-party dependencies for Python 3.14 compatability.

Environment

• Python: 3.14
• redis-om-python: 0.3.5
• Pydantic: 2.12.5
• Django: 5.2.10 app importing a JsonModel at startup (error happens at import time)
• This fails both on AWS Elastic Beanstalk and also locally on MacOS

Stack trace (trimmed)

File ".../redis_cloud/models.py", line 9, in <module>
  class ShortInterest(JsonModel):

File ".../redis_om/model/model.py", line 1916, in __init_subclass__
  cls.redisearch_schema()

File ".../redis_om/model/model.py", line 1985, in redisearch_schema
  schema_parts = [schema_prefix] + cls.schema_for_fields()

File ".../redis_om/model/model.py", line 1995, in schema_for_fields
  for name, field in cls.__dict__.items():
RuntimeError: dictionary changed size during iteration

Code path / failing loop

The failure happens iterating the live cls.__dict__ during model construction:

for name, field in cls.__dict__.items():
    if isinstance(field, FieldInfo):
        if not field.annotation:
            field.annotation = cls.__annotations__.get(name)
        fields[name] = field

The field.annotation = ... assignment mutates the FieldInfo object, but should not add/remove keys on cls.__dict__, so it should not cause a “changed size during iteration” error. The class namespace appears to be modified externally during this phase (likely Pydantic injecting attributes during construction).

I note that in the current version in the repo, this code has moved to aredis_om/model/model.py, but that failing for loop is identical.

Reproduction outline

1. Define a JsonModel in a Django app
2. Import the module at startup
3. __init_subclass__ triggers schema generation
4. Python 3.14 raises RuntimeError: dictionary changed size during iteration when iterating cls.__dict__