chore: deprecate PredefinedPipeline templates and PipelineTemplate (#10475)

* chore: deprecate PredefinedPipeline

* move warning to PipelineTemplate

---------

Co-authored-by: Julian Risch <julian.risch@deepset.ai>

Author: Amanbig

haystack/core/pipeline/template.py

@@ -5,6 +5,7 @@
 from enum import Enum
 from pathlib import Path
 from typing import Any
+from warnings import warn
 
 from jinja2 import PackageLoader, TemplateSyntaxError, meta
 from jinja2.sandbox import SandboxedEnvironment
@@ -75,6 +76,12 @@ def __init__(self, template_content: str):
 
         :param template_content: The raw template source to use in the template.
         """
+        msg = (
+            "`PipelineTemplate` and `PredefinedPipeline` are deprecated. "
+            "They will be removed in Haystack version 2.25. "
+            "Pipeline YAML files should be used instead."
+        )
+        warn(msg, DeprecationWarning, stacklevel=4)
         env = SandboxedEnvironment(
             loader=PackageLoader("haystack.core.pipeline", "predefined"), trim_blocks=True, lstrip_blocks=True
         )

releasenotes/notes/deprecate-predefined-pipeline-1a2b3c4d.yaml

@@ -0,0 +1,5 @@
+---
+deprecations:
+  - |
+    Deprecated ``PipelineTemplate``, ``PredefinedPipeline`` and its options (like ``PredefinedPipeline.CHAT_WITH_WEBSITE``).
+    These templates will be removed in Haystack 2.25. Users should switch to using Pipeline YAML files.

test/core/pipeline/test_templates.py

@@ -47,6 +47,11 @@ def test_from_predefined(self):
         tpl = PipelineTemplate.from_predefined(PredefinedPipeline.INDEXING)
         assert len(tpl.template_content)
 
+    def test_from_predefined_emits_deprecation_warning(self, monkeypatch):
+        monkeypatch.setenv("OPENAI_API_KEY", "")
+        with pytest.warns(DeprecationWarning, match="PipelineTemplate.*deprecated.*2\\.25"):
+            PipelineTemplate.from_predefined(PredefinedPipeline.INDEXING)
+
     #  Building a pipeline directly using all default components specified in a predefined or custom template.
     def test_build_pipeline_with_default_components(self, monkeypatch):
         monkeypatch.setenv("OPENAI_API_KEY", "fake_key")