feat: Add classifier function push builder (#497)

## Summary
- add a project-level classifier builder for code-backed saved functions
- register classifiers through the existing function push registry with
`type_="classifier"`
- add focused coverage for normal and lazy-load function registry
registration

## Validation
- `UV_CACHE_DIR=/private/tmp/uv-cache
UV_PYTHON_INSTALL_DIR=/private/tmp/uv-python uv run --group test pytest
src/braintrust/test_framework2.py -k classifier`
- `UV_CACHE_DIR=/private/tmp/uv-cache
UV_PYTHON_INSTALL_DIR=/private/tmp/uv-python uv run --group test python
-m py_compile src/braintrust/framework2.py
src/braintrust/test_framework2.py`

Author: Qard

py/src/braintrust/framework2.py

@@ -544,6 +544,65 @@ def create(
             return p
 
 
+class ClassifierBuilder:
+    """Builder to create a classifier in Braintrust."""
+
+    def __init__(self, project: "Project"):
+        self.project = project
+        self._task_counter = 0
+
+    def create(
+        self,
+        *,
+        handler: Callable[..., Any],
+        name: str | None = None,
+        slug: str | None = None,
+        description: str | None = None,
+        parameters: Any = None,
+        returns: Any = None,
+        if_exists: IfExists | None = None,
+        metadata: Metadata | None = None,
+        tags: Sequence[str] | None = None,
+    ) -> CodeFunction:
+        """Creates a classifier.
+
+        Args:
+            handler: The function that is called when the classifier is used.
+            name: The name of the classifier.
+            slug: A unique identifier for the classifier.
+            description: The description of the classifier.
+            parameters: The classifier's input schema, as a Pydantic model.
+            returns: The classifier's output schema, as a Pydantic model.
+            if_exists: What to do if the classifier already exists.
+            metadata: Custom metadata to attach to the classifier.
+            tags: A list of tags for the classifier.
+        """
+        self._task_counter += 1
+        if name is None or len(name) == 0:
+            if handler.__name__ and handler.__name__ != "<lambda>":
+                name = handler.__name__
+            else:
+                name = f"Classifier {self._task_counter}"
+        if slug is None or len(slug) == 0:
+            slug = slugify.slugify(name)
+
+        f = CodeFunction(
+            project=self.project,
+            handler=handler,
+            name=name,
+            slug=slug,
+            type_="classifier",
+            description=description,
+            parameters=parameters,
+            returns=returns,
+            if_exists=if_exists,
+            metadata=metadata,
+            tags=tags,
+        )
+        self.project.add_code_function(f)
+        return f
+
+
 class Project:
     """A handle to a Braintrust project."""
 
@@ -553,6 +612,7 @@ def __init__(self, name: str):
         self.prompts = PromptBuilder(self)
         self.parameters = ParametersBuilder(self)
         self.scorers = ScorerBuilder(self)
+        self.classifiers = ClassifierBuilder(self)
 
         self._publishable_code_functions: list[CodeFunction] = []
         self._publishable_prompts: list[CodePrompt] = []

py/src/braintrust/test_framework2.py

@@ -5,7 +5,8 @@
 
 import pytest
 
-from .framework2 import ProjectIdCache, projects
+from .framework import _set_lazy_load
+from .framework2 import ProjectIdCache, global_, projects
 
 
 # Check if pydantic is available
@@ -145,6 +146,50 @@ def test_llm_scorer_with_metadata(self):
         assert scorer.name == "llm-scorer"
 
 
+class TestClassifierFunctionPush:
+    """Tests for classifier function push registration."""
+
+    def test_code_classifier_registers_as_code_function(self):
+        project = projects.create("test-project")
+
+        def classify(output: str):
+            return {"name": "category", "id": output}
+
+        classifier = project.classifiers.create(
+            handler=classify,
+            name="test-classifier",
+        )
+
+        assert classifier.type_ == "classifier"
+        assert classifier.name == "test-classifier"
+        assert classifier.slug == "test-classifier"
+        assert project._publishable_code_functions == [classifier]
+
+    def test_lazy_code_classifier_registration_uses_functions_registry(self):
+        global_.functions.clear()
+        global_.prompts.clear()
+        global_.parameters.clear()
+
+        try:
+            with _set_lazy_load(True):
+                project = projects.create("test-project")
+
+                def classify(output: str):
+                    return {"name": "category", "id": output}
+
+                classifier = project.classifiers.create(
+                    handler=classify,
+                    name="test-classifier",
+                )
+
+                assert global_.functions == [classifier]
+                assert global_.functions[0].type_ == "classifier"
+        finally:
+            global_.functions.clear()
+            global_.prompts.clear()
+            global_.parameters.clear()
+
+
 class TestCodeFunctionTags:
     """Tests for CodeFunction tags support."""