BaseSettings descendants call default_factory on import

[pomo-mondreganto]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

When importing BaseSettings descendant from another file, default_factory functors are always called for uninstantiated models. See the code snippet attached below. def factory() is called once on import, even though B is not even instantiated.

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.8.1
            pydantic compiled: True
                 install path: /usr/local/anaconda3/envs/dirty_3.9/lib/python3.9/site-packages/pydantic
               python version: 3.9.2 | packaged by conda-forge | (default, Feb 21 2021, 05:02:20)  [Clang 11.0.1 ]
                     platform: macOS-11.1-x86_64-i386-64bit
     optional deps. installed: ['typing-extensions']

The first file (kek.py):

from pydantic import BaseSettings, Field

v = 0

def factory():
    print('factory called')
    global v
    v += 1
    return v


class A(BaseSettings):
    pass


class B(A):
    id: int = Field(default_factory=factory)

        
class C(A):
    pass

The second file:

from kek import C

Expected output: empty.

Actual output: factory called.

It's also worth mentioning that BaseModel does not have the described behavior, importing the BaseModel descendants doesn't call default_factory functors.

[MihanixA]

I co-discovered this issue. 100% True

[PrettyWood]

Hi and thanks for reporting
BaseSettings inherits from BaseModel with other defaults for the config including validate_all which is set to True by default.
As explained in the warning section in the doc, to validate the default value of a field, we actually need to call the default_factory.
I guess you could find a workaround in your default factory or set validate_all = False

class A(BaseSettings, validate_all=False):
    pass

We could also allow people to not validate values with default_factory
Hope it helps in the meantime

[pomo-mondreganto]

@PrettyWood

Thank you, that works for now. It's still kind of strange though, I believe it should be possible to validate all values (including the default ones) on initialization. Might be better to hide this functionality under some option (validate_on_init), too.

[PrettyWood]

@pomo-mondreganto you're absolutely right! I don't even remember why I did this 🤷 I opened a PR to remove this side effect! Thanks for reporting

[guhcampos]

I had also missed the warning and had my expectations incorrect.

I would suggest a lazy validation for all fields with a default factory is the reasonably expected way to go. If we are defining a factory it means we will only have the actual data in runtime at some point.

In my case I stumbled into this while writing unit tests: I kept getting validation errors in tests at points in code I was not instantiating a settings object, which got me quite confused until I found out it was being created at import time. I think most of us will agree that running code with side effects in import time is a bad thing.