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fix: prompt-builder - jinja2 template set vars still shows required #9932

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fix: prompt-builder - jinja2 template set vars still shows required #9932

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136b7e0
fix: prompt-builder - jinja2 template set vars still shows required
srini047 Oct 24, 2025
f58094e
Merge branch 'main' into jinja2-set-vars-incorrect
srini047 Oct 24, 2025
492073e
Merge branch 'main' into jinja2-set-vars-incorrect
srini047 Oct 24, 2025
c78de27
Merge branch 'main' into jinja2-set-vars-incorrect
srini047 Oct 25, 2025
b102f58
Merge branch 'main' into jinja2-set-vars-incorrect
srini047 Oct 27, 2025
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10 changes: 8 additions & 2 deletions haystack/components/builders/prompt_builder.py
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Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,7 @@

from typing import Any, Literal, Optional, Union

from jinja2 import meta
from jinja2 import meta, nodes
from jinja2.sandbox import SandboxedEnvironment

from haystack import component, default_to_dict, logging
Expand Down Expand Up @@ -178,7 +178,13 @@ def __init__(
# infer variables from template
ast = self._env.parse(template)
template_variables = meta.find_undeclared_variables(ast)
variables = list(template_variables)

assigned_variables = set()
for node in ast.find_all((nodes.Assign, nodes.For)):
if hasattr(node.target, "name"):
assigned_variables.add(node.target.name)

variables = list(template_variables - assigned_variables)
Comment on lines +182 to +187
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@sjrl sjrl Oct 27, 2025 •
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Thanks for the addition, but this doesn't quite cover all the cases we would like. Let's use the following instead which also handles tuple and list unpacking when using set.

Also the nodes.For case doesn't seem to be needed. We don't pick up for-loop variables like doc in your test.

Suggested change
assigned_variables = set()
for node in ast.find_all((nodes.Assign, nodes.For)):
if hasattr(node.target, "name"):
assigned_variables.add(node.target.name)
variables = list(template_variables - assigned_variables)
assigned_variables = set()
for node in ast.find_all((nodes.Assign,)):
if isinstance(node.target, nodes.Name):
assigned_variables.add(node.target.name)
elif isinstance(node.target, (nodes.List, nodes.Tuple)):
for name_node in node.target.items:
if isinstance(name_node, nodes.Name):
assigned_variables.add(name_node.name)
variables = list(template_variables - assigned_variables)

variables = variables or []
self.variables = variables

Expand Down
22 changes: 22 additions & 0 deletions test/components/builders/test_prompt_builder.py
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Expand Up @@ -337,3 +337,25 @@ def test_warning_no_required_variables(self, caplog):
with caplog.at_level(logging.WARNING):
_ = PromptBuilder(template="This is a {{ variable }}")
assert "but `required_variables` is not set." in caplog.text

def test_template_assigned_variables_from_required_inputs(self) -> None:
template = """{% if existing_documents is not none %}
{% set existing_doc_len = existing_documents|length %}
{% else %}
{% set existing_doc_len = 0 %}
{% endif %}
{% for doc in docs %}
<document reference="{{loop.index + existing_doc_len}}">
{{ doc.content }}
</document>
{% endfor %}
"""

builder = PromptBuilder(template=template, required_variables="*")
docs = [Document(content="Doc 1"), Document(content="Doc 2")]

res = builder.run(docs=docs, existing_documents=None)

Comment on lines +357 to +358
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Let's also explicitly test the found variables

Suggested change
res = builder.run(docs=docs, existing_documents=None)
builder = PromptBuilder(template=template, required_variables="*")
assert set(builder.variables) == {"docs", "existing_documents"}

assert "<document reference=" in res["prompt"]
assert "Doc 1" in res["prompt"]
assert "Doc 2" in res["prompt"]
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Let's also add a few more tests to test tuple and list unpacking

    def test_variables_correct_with_tuple_assignment(self):
        template = """{% if existing_documents is not none %}
{% set x, y = (existing_documents|length, 1) %}
{% else %}
{% set x, y = (0, 1) %}
{% endif %}
x={{ x }}, y={{ y }}
"""
        builder = PromptBuilder(template=template, required_variables="*")
        assert set(builder.variables) == {"existing_documents"}
        res = builder.run(existing_documents=None)
        assert "x=0, y=1" in res["prompt"]

    def test_variables_correct_with_list_assignment(self):
            template = """{% if existing_documents is not none %}
{% set x, y = [existing_documents|length, 1] %}
{% else %}
{% set x, y = [0, 1] %}
{% endif %}
x={{ x }}, y={{ y }}
"""
            builder = PromptBuilder(template=template, required_variables="*")
            assert set(builder.variables) == {"existing_documents"}
            res = builder.run(existing_documents=None)
            assert "x=0, y=1" in res["prompt"]

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