BaseSettings Singleton option?

See original GitHub issue

I’ve noticed a pattern come up in several of our projects and thought maybe (but I’m really not sure) pydantic could ship support for it.

# config.py
from functools import lru_cache
from pydantic import BaseSettings


class Config(BaseSettings):
    a: str
    b: str
    c: str

@lru_cache()
def read() -> Config:
    return Config()

# a.py
from . import config
conf = config.read

# b.py
from . import config
conf = config.read

Would it be a good library addition if we could do something like:

# config.py
from functools import lru_cache
from pydantic import BaseSettings


class Config(BaseSettings):
    a: str
    b: str
    c: str

    class Config:
        singleton = True

# a.py
from . import Config
conf = Config()

# b.py
from . import config
conf = Config()

I definitely appreciate that pydantic should not try to take care of application-level features like dependency injection and object caching. However, for the special case of BaseSettings this just seems like a practical feature. Thoughts?

Maybe there is already a more general solution here that is readily available that i missed, allowing e.g.:

# config.py
from pydantic import BaseSettings

@singleton # exists?
class Config(BaseSettings):
    a: str
    b: str
    c: str

But I haven’t come across that yet. And if such a thing is from another third party lib. I still feel then that there is value in pydantic shipping an out of box solution, if its what users would want most of the time.

Issue Analytics

State:
Created 4 years ago
Reactions:4
Comments:6 (1 by maintainers)

Top GitHub Comments

2reactions

samuelcolvincommented, Jun 11, 2019

I completely agree settings should be a singleton, however I’ve never had any problem creating it as a singleton which is then used throughout the application.

This is probably because most applications I build are either based on aiohttp or use aiohttp’s approach of a single “app” context.

See for example here. This approach also works well for testing where you generally want a very different settings instance which can easily be changed on a per-test basis without reloading modules.

My suspicion is that you’ve made design mistakes if you find yourself trying to initialise settings more than once. Probably best you fix the root mistake or take care of the workaround outside pydantic.

1reaction

tl24commented, Oct 31, 2022

I usually put these in a factory function with lru_cache

from functools import lru_cache

@lru_cache(maxsize=1)
def get_settings():
   return AppSettings()

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