feat: Adding StringJoiner (#8357)

* Adding StringJoiner

* Release notes

* Remove typing

* Remove unused import

* Try to fix header

* Fix one test

* Add to docs, move test to behavioral pipeline test

* Undo changes

* Fix test

* Update haystack/components/joiners/string_joiner.py

Co-authored-by: Stefano Fiorucci <stefanofiorucci@gmail.com>

* Update haystack/components/joiners/string_joiner.py

Co-authored-by: Stefano Fiorucci <stefanofiorucci@gmail.com>

* Provide usage example

* Apply suggestions from code review

Co-authored-by: Stefano Fiorucci <stefanofiorucci@gmail.com>

---------

Co-authored-by: Stefano Fiorucci <stefanofiorucci@gmail.com>
Co-authored-by: Silvano Cerza <3314350+silvanocerza@users.noreply.github.com>

README.md

@@ -24,7 +24,7 @@ pip install haystack-ai
 
 Install from the `main` branch to try the newest features:
 ```sh
-pip install git+https://github.com/deepset-ai/haystack.git@main 
+pip install git+https://github.com/deepset-ai/haystack.git@main
 ```
 
 Haystack supports multiple installation methods including Docker images. For a comprehensive guide please refer

docs/pydoc/config/joiners_api.yml

@@ -1,7 +1,7 @@
 loaders:
   - type: haystack_pydoc_tools.loaders.CustomPythonLoader
     search_path: [../../../haystack/components/joiners]
-    modules: ["document_joiner", "branch", "answer_joiner"]
+    modules: ["document_joiner", "branch", "answer_joiner", "string_joiner"]
     ignore_when_discovered: ["__init__"]
 processors:
   - type: filter

haystack/components/joiners/__init__.py

@@ -5,5 +5,6 @@
 from .answer_joiner import AnswerJoiner
 from .branch import BranchJoiner
 from .document_joiner import DocumentJoiner
+from .string_joiner import StringJoiner
 
-__all__ = ["DocumentJoiner", "BranchJoiner", "AnswerJoiner"]
+__all__ = ["DocumentJoiner", "BranchJoiner", "AnswerJoiner", "StringJoiner"]

haystack/components/joiners/string_joiner.py

@@ -0,0 +1,59 @@
+# SPDX-FileCopyrightText: 2022-present deepset GmbH <info@deepset.ai>
+#
+# SPDX-License-Identifier: Apache-2.0
+
+from typing import List
+
+from haystack import component, logging
+from haystack.core.component.types import Variadic
+
+logger = logging.getLogger(__name__)
+
+
+@component
+class StringJoiner:
+    """
+    Component to join strings from different components to a list of strings.
+
+    ### Usage example
+
+    ```python
+    from haystack.components.joiners import StringJoiner
+    from haystack.components.builders import PromptBuilder
+    from haystack.core.pipeline import Pipeline
+
+    from haystack.components.generators.chat import OpenAIChatGenerator
+    from haystack.dataclasses import ChatMessage
+
+    string_1 = "What's Natural Language Processing?"
+    string_2 = "What is life?"
+
+    pipeline = Pipeline()
+    pipeline.add_component("prompt_builder_1", PromptBuilder("Builder 1: {{query}}"))
+    pipeline.add_component("prompt_builder_2", PromptBuilder("Builder 2: {{query}}"))
+    pipeline.add_component("string_joiner", StringJoiner())
+
+    pipeline.connect("prompt_builder_1.prompt", "string_joiner.strings")
+    pipeline.connect("prompt_builder_2.prompt", "string_joiner.strings")
+
+    print(pipeline.run(data={"prompt_builder_1": {"query": string_1}, "prompt_builder_2": {"query": string_2}}))
+
+    >> {"string_joiner": {"strings": ["Builder 1: What's Natural Language Processing?", "Builder 2: What is life?"]}}
+    ```
+    """
+
+    @component.output_types(strings=List[str])
+    def run(self, strings: Variadic[str]):
+        """
+        Joins strings into a list of strings
+
+        :param strings:
+            strings from different components
+
+        :returns:
+            A dictionary with the following keys:
+            - `strings`: Merged list of strings
+        """
+
+        out_strings = list(strings)
+        return {"strings": out_strings}

releasenotes/notes/add-string-joiner-a7754e6bff9332ea.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - |
+    Added component StringJoiner to join strings from different components to a list of strings.

test/components/joiners/test_string_joiner.py

@@ -0,0 +1,37 @@
+# SPDX-FileCopyrightText: 2022-present deepset GmbH <info@deepset.ai>
+#
+# SPDX-License-Identifier: Apache-2.0
+
+from haystack.core.serialization import component_from_dict, component_to_dict
+from haystack.components.joiners.string_joiner import StringJoiner
+
+
+class TestStringJoiner:
+    def test_init(self):
+        joiner = StringJoiner()
+        assert isinstance(joiner, StringJoiner)
+
+    def test_to_dict(self):
+        joiner = StringJoiner()
+        data = component_to_dict(joiner, name="string_joiner")
+        assert data == {"type": "haystack.components.joiners.string_joiner.StringJoiner", "init_parameters": {}}
+
+    def test_from_dict(self):
+        data = {"type": "haystack.components.joiners.string_joiner.StringJoiner", "init_parameters": {}}
+        string_joiner = component_from_dict(StringJoiner, data=data, name="string_joiner")
+        assert isinstance(string_joiner, StringJoiner)
+
+    def test_empty_list(self):
+        joiner = StringJoiner()
+        result = joiner.run([])
+        assert result == {"strings": []}
+
+    def test_single_string(self):
+        joiner = StringJoiner()
+        result = joiner.run("a")
+        assert result == {"strings": ["a"]}
+
+    def test_two_strings(self):
+        joiner = StringJoiner()
+        result = joiner.run(["a", "b"])
+        assert result == {"strings": ["a", "b"]}

test/core/pipeline/features/pipeline_run.feature

@@ -43,6 +43,7 @@ Feature: Pipeline running
         | that is linear and a component in the middle receives optional input from other components and input from the user |
         | that has a loop in the middle |
         | that has variadic component that receives a conditional input |
+        | that has a string variadic component |
 
     Scenario Outline: Running a bad Pipeline
         Given a pipeline <kind>

test/core/pipeline/features/test_run.py

@@ -13,7 +13,7 @@ from haystack.components.builders import PromptBuilder, AnswerBuilder, ChatPromp
 from haystack.components.preprocessors import DocumentCleaner, DocumentSplitter
 from haystack.components.retrievers.in_memory import InMemoryBM25Retriever
 from haystack.document_stores.in_memory import InMemoryDocumentStore
-from haystack.components.joiners import BranchJoiner, DocumentJoiner, AnswerJoiner
+from haystack.components.joiners import BranchJoiner, DocumentJoiner, AnswerJoiner, StringJoiner
 from haystack.testing.sample_components import (
     Accumulate,
     AddFixedValue,
@@ -2195,3 +2195,32 @@ def that_has_variadic_component_that_receives_a_conditional_input():
             ],
         ),
     ]
+
+
+@given("a pipeline that has a string variadic component", target_fixture="pipeline_data")
+def that_has_a_string_variadic_component():
+    string_1 = "What's Natural Language Processing?"
+    string_2 = "What's is life?"
+
+    pipeline = Pipeline()
+    pipeline.add_component("prompt_builder_1", PromptBuilder("Builder 1: {{query}}"))
+    pipeline.add_component("prompt_builder_2", PromptBuilder("Builder 2: {{query}}"))
+    pipeline.add_component("string_joiner", StringJoiner())
+
+    pipeline.connect("prompt_builder_1.prompt", "string_joiner.strings")
+    pipeline.connect("prompt_builder_2.prompt", "string_joiner.strings")
+
+    return (
+        pipeline,
+        [
+            PipelineRunData(
+                inputs={"prompt_builder_1": {"query": string_1}, "prompt_builder_2": {"query": string_2}},
+                expected_outputs={
+                    "string_joiner": {
+                        "strings": ["Builder 1: What's Natural Language Processing?", "Builder 2: What's is life?"]
+                    }
+                },
+                expected_run_order=["prompt_builder_1", "prompt_builder_2", "string_joiner"],
+            )
+        ],
+    )