Support Serializing To Env Var Dict

[NiclasvanEyk]

I saw that you support serializing a BaseSettings instance to cli arguments, and wondered if something like this could also be supported for environment values.

The are probably lots of details to consider:

• should revealing SecretStr require an explicit flag similar to sqlalchemy.URL.render_as_string?
• should primitives like numbers be automatically converted to strings, since env vars values are always strings?
• Is this best done as a method on EnvSettingsSource, a global function, or a utility class like CliApp?
• probably more I cannot think of right now

I mostly opened this issue to start the discussion and see if there you'd be open for this functionality.

Motivation

Mostly infrastructure as code. E.g. when constructing Lambdas or Fargate tasks the AWS CDK, you can pass a dictionary of environment variables. Would be really cool if one could configure the env vars type safe in CDK at deploy time, then serialize from environment variables. Probably similar use cases exist for other IaC tools like Pulumi.

Example

from pydantic import SecretStr, BaseModel, Field
from pydantic_settings import BaseSettings, EnvSettingsSource

class Nested(BaseModel):
    nested_property: str

class MySettings(BaseSettings):
    regular_property: int
    secret_property: SecretStr
    aliased_property: str = Field(alias="aliased")
    nested_object: Nested

settings = MySettings(
    regular_property=123,
    secret_property="s3cr3t",
    aliased_property="i am aliased",
    nested_object=Nested(nested_property="i am nested")
)

env_var_dict = somehow_serialize_to_env_var_dict(settings)
"""
{
    "REGULAR_PROPERTY": "123",
    "SECRET_PROPERTY": "s3cr3t",
    "ALIASED": "i am aliased",
    "NESTED_OBJECT__NESTED_PROPERTY": "i am nested"
}
"""

if we'd then set environment variables based on env_var_dict

import os

for key, value in env_var_dict.values():
    os.environ[key] = value

assert MySettings() == settings

the generated settings based on env vars would be the same as the "manually defined" ones.