Add Annotate Return Type On Enum Function as a Python TIL

README.md

@@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/).
 
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-_1854 TILs and counting..._
+_1855 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -1076,6 +1076,7 @@ If you've learned something here, support my efforts writing daily TILs by
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 - [Access SQLite Result Values By Name With Row Factory](python/access-sqlite-result-values-by-name-with-row-factory.md)
 - [Access Variables Outside Loop Scope](python/access-variables-outside-loop-scope.md)
+- [Annotate Return Type On Enum Function](python/annotate-return-type-on-enum-function.md)
 - [Another Way To Mark Keyword-Only Dataclass Fields](python/another-way-to-mark-keyword-only-dataclass-fields.md)
 - [Argument Defaults Are Evaluated When Function Is Defined](python/argument-defaults-are-evaluated-when-function-is-defined.md)
 - [Assert Is Only A Development Check](python/assert-is-only-a-development-check.md)

python/annotate-return-type-on-enum-function.md

@@ -0,0 +1,61 @@
+# Annotate Return Type On Enum Function
+
+I defined the following `StrEnum` class to capture the canonical set of valid
+values for a _storage format_ in
+[`py-vmt`](https://github.com/jbranchaud/py-vmt). It includes a `default`
+function that can be called to get what the system considers the default storage
+format.
+
+```python
+class StorageFormat(StrEnum):
+    SQLITE = "sqlite"
+    JSON = "json"
+
+    @staticmethod
+    def default():
+        return StorageFormat.SQLITE
+```
+
+This works, but what is missing is a type annotation for the return value of
+`default`.
+
+I'd like to use `StorageFormat` as the return type annotation. However, the
+annotations are eagerly evaluated at the time the function definition is being
+processed. The `StorageFormat` class is not fully evaluated at that point and so
+isn't available.
+
+Instead, what I can do is quote `StorageFormat` as a string to make it a
+_forward reference_.
+
+```python
+class StorageFormat(StrEnum):
+    SQLITE = "sqlite"
+    JSON = "json"
+
+    @staticmethod
+    def default() -> "StorageFormat":
+        return StorageFormat.SQLITE
+```
+
+The string type annotation will be lazily evaluated at a later time when
+`StorageFormat` fully exists. This is behavior that is slated to be deprecated.
+And it is no longer needed as of [Python 3.14 which lazily evaluates all
+annotations by default
+now](https://docs.python.org/3/reference/compound_stmts.html#annotations).
+
+If I want to be more specific with the above annotation, I can import [`Literal`
+from `typing`](https://docs.python.org/3/library/typing.html#typing.Literal) and
+then wrap a specific enum value.
+
+```python
+class StorageFormat(StrEnum):
+    SQLITE = "sqlite"
+    JSON = "json"
+
+    @staticmethod
+    def default() -> "Literal[StorageFormat.SQLITE]":
+        return StorageFormat.SQLITE
+```
+
+Now it will specifically enforce `SQLITE` rather than accepting `SQLITE` or
+`JSON`.