Add Combine StrEnum With Pydantic For Union Type as a Python TIL

python/combine-strenum-with-pydantic-for-union-type.md

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+# Combine StrEnum With Pydantic For Union Type
+
+I want a [Pydantic](https://pydantic.dev/docs/validation/latest/get-started/)
+model that I can use to verify the shape and values of user-provided config
+file. Let's say, for now, the only config value supported by the file is
+`storage_format` and it can be one of three values (`json`, `sqlite`,
+`inmemory`).
+
+I can start with a
+[`StrEnum`](https://docs.python.org/3/library/enum.html#enum.StrEnum) class to
+represent the set of valid values for the storage format field.
+
+```python
+from enum import StrEnum
+from typing import Literal
+
+class StorageFormat(StrEnum):
+    SQLITE = "sqlite"
+    JSON = "json"
+    INMEMORY = "inmemory"
+
+    @classmethod
+    def default(cls) -> Literal[StorageFormat.SQLITE]:
+        return cls.SQLITE
+```
+
+Notice I've also provided a class method called `default` for serving up a
+fallback default value.
+
+Here is the corresponding Pydantic `CliConfig` model which uses the
+`StorageFormat` string enum class as the type signature for its `storage_format`
+field.
+
+```python
+from pydantic import BaseModel
+
+class CliConfig(BaseModel):
+    storage_format: StorageFormat = StorageFormat.default()
+```
+
+For `storage_format` to be valid in the context of `CliConfig`, it must be one
+of the values defined by that `StrEnum`. I get auto-complete, I get type
+checking, and I can validate a config file I've just read into memory (in this
+case, using
+[`model_validate_json`](https://pydantic.dev/docs/validation/dev/api/pydantic/base_model/#pydantic.BaseModel.model_validate_json)).
+
+```python
+def _read_config(self) -> CliConfig:
+    if self.exists():
+        raw_json = self.config_file.read_text()
+        return CliConfig.model_validate_json(raw_json)
+
+    return CliConfig()
+```