Pydantic and singledispatch
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I was reading about pydantic-singledispatch from Giddeon's blog and found it very intersting. I'm getting ready to implement pydantic on my static site...

Date: May 3, 2023

I was reading about pydantic-singledispatch <https://www.gidware.com/reducing-complexity-with-pydantic-singledispatch/> from Giddeon’s blog and found it very intersting. I’m getting ready to implement pydantic on my static site generator markata <https://markata.dev/>, and I think there are so uses for this idea, so I want to try it out.

The Idea
────────

Let’s set up some pydantic settings. We will need separate Models for each environment that we want to support for this to work. The whole idea is to use functools.singledispatch and type hints to provide unique execution for each environment. We might want something like a path_prefix in prod for environments like GithubPages that deploy to /<name-of-repo> while keeping the root at / in dev.

Settings Model
──────────────

Here is our model for our settings. We will create a CommonSettings model that will be used by all environments. We will also create a DevSettings model that will be used in dev and ProdSettings that will be used in prod. We will use env as the discriminator so pydantic knows which model to use.

```{.python
from typing import Literal, Union

import pydantic
from pydantic import Field
from rich import print
from typing_extensions import Annotated


class CommonSettings(pydantic.BaseSettings):
    """Common settings for all environments"""
    debug: bool = False
    secret_key: str = "secret"
    algorithm: str = "HS256"
    access_token_expire_minutes: int = 60


class DevSettings(CommonSettings):
    """Settings for dev"""
    env: Literal["dev"]


class ProdSettings(CommonSettings):
    """Settings for prod"""
    env: Literal["prod"]


class Settings(pydantic.BaseSettings):
    """Settings for all environments"""
    __root__: Annotated[Union[DevSettings, ProdSettings], Field(discriminator="env")]

    class Config:
        env_prefix = "APP_"



# Create our settings
settings = Settings(__root__={"env": "dev"}).__root__
# or
settings = Settings.parse_obj({"env": "dev"}).__root__
print(settings)
```

```{.console
DevSettings(debug=False, secret_key='secret', algorithm='HS256', access_token_expire_minutes=60, env='dev')
```

Singledispatch
──────────────

Now let’s create our where_am_i function. We will use functools.singledispatch to provide a unique execution for each environment. It will leverage type hints to provide a unique execution for each environment.

```{.python
from functools import singledispatch

@singledispatch
def where_am_i(obj):
   '''
   Where am I?
   '''

@where_am_i.register
def dev(obj: DevSettings):
   '''
   Where am I?
   '''
   print('I am in dev')

@where_am_i.register
def prod(obj: ProdSettings):
   '''
   Where am I?
   '''
   print('I am in prod')
```

Output
──────

Let’s call our eample function where_am_i with our settings and see the results.

```{.python
where_am_i(settings)
```

results in

```{.console
I am in dev
```

Let’s check prod

```{.python
where_am_i(Settings.parse_obj({'env': 'prod'}).__root__)
```

results in

```{.console
I am in prod
```

Environment Variables
─────────────────────

So far one down side to the TaggedUnion technique is that I am unable to pull env from environment variables. I’m sure there is a way around this with a different model design. Maybe following exactly what Giddeon did.

```{.python
os.environ.clear()
os.environ['APP_ENV'] = 'prod'
where_am_i(Settings().__root__)
```

results in

```
ValidationError: 1 validation error for Settings
__root__
  field required (type=value_error.missing)
<darkmark.darkmark.DarkMark object at 0x7fcc58bec5d0>
```

FIN
───

I’m really digging pydantic lately and excited to get it built into markata <https://markata.dev/>. Not 100% sure if I have a