feat: add required flag for prompt builder inputs (#7553)

haystack/components/builders/prompt_builder.py

@@ -1,4 +1,4 @@
-from typing import Any, Dict
+from typing import Any, Dict, List, Optional
 
 from jinja2 import Template, meta
 
@@ -10,8 +10,9 @@ class PromptBuilder:
     """
     PromptBuilder is a component that renders a prompt from a template string using Jinja2 templates.
 
-    The template variables found in the template string are used as input types for the component and are all optional.
-    If a template variable is not provided as an input, it will be replaced with an empty string in the rendered prompt.
+    The template variables found in the template string are used as input types for the component and are all optional,
+    unless explicitly specified. If an optional template variable is not provided as an input, it will be replaced with
+    an empty string in the rendered prompt.
 
     Usage example:
     ```python
@@ -21,17 +22,23 @@ class PromptBuilder:
     ```
     """
 
-    def __init__(self, template: str):
+    def __init__(self, template: str, required_variables: Optional[List[str]] = None):
         """
         Constructs a PromptBuilder component.
 
         :param template: A Jinja2 template string, e.g. "Summarize this document: {documents}\\nSummary:"
+        :param required_variables: An optional list of input variables that must be provided at all times.
         """
         self._template_string = template
         self.template = Template(template)
+        self.required_variables = required_variables or []
         ast = self.template.environment.parse(template)
         template_variables = meta.find_undeclared_variables(ast)
+
         for var in template_variables:
+            if var in self.required_variables:
+                component.set_input_type(self, var, Any)
+            else:
                 component.set_input_type(self, var, Any, "")
 
     def to_dict(self) -> Dict[str, Any]:
@@ -54,4 +61,9 @@ class PromptBuilder:
         :returns: A dictionary with the following keys:
             - `prompt`: The updated prompt text after rendering the prompt template.
         """
+        missing_variables = [var for var in self.required_variables if var not in kwargs]
+        if missing_variables:
+            missing_vars_str = ", ".join(missing_variables)
+            raise ValueError(f"Missing required input variables in PromptBuilder: {missing_vars_str}")
+
         return {"prompt": self.template.render(kwargs)}

releasenotes/notes/add-required-flag-for-prompt-builder-inputs-f5d3ffb3cb7df8d0.yaml

@@ -0,0 +1,4 @@
+---
+enhancements:
+  - |
+    Enhanced PromptBuilder to specify and enforce required variables in prompt templates.

test/components/builders/test_prompt_builder.py

@@ -33,3 +33,13 @@ def test_run_with_missing_input():
     builder = PromptBuilder(template="This is a {{ variable }}")
     res = builder.run()
     assert res == {"prompt": "This is a "}
+
+
+def test_run_with_missing_required_input():
+    builder = PromptBuilder(template="This is a {{ foo }}, not a {{ bar }}", required_variables=["foo", "bar"])
+    with pytest.raises(ValueError, match="foo"):
+        builder.run(bar="bar")
+    with pytest.raises(ValueError, match="bar"):
+        builder.run(foo="foo")
+    with pytest.raises(ValueError, match="foo, bar"):
+        builder.run()