Python: Fix FunctionResult.__str__ rendering falsy values (0, False, 0.0) as empty string

[sohumt123]

Motivation and Context

FunctionResult.__str__ starts with a truthiness check (if self.value:) where a None-check was intended. Any kernel function that legitimately returns a falsy scalar — 0, 0.0, or False — is rendered as an empty string wherever str(FunctionResult) is used, most visibly when the result is interpolated into a prompt template via {{plugin.function ...}} (CodeBlock.render_code).

Example: with a plugin whose functions return 5 - 5 and 3 % 2 == 0, the template

The result is: {{math.subtract a=$x b=$y}} and even={{math.is_even a=$z}}

renders as The result is: and even= instead of The result is: 0 and even=False — the prompt sent to the model is silently corrupted with no warning or error. The same conflation affects logging and any other consumer of str(FunctionResult).

Description

• Change the guard in FunctionResult.__str__ from if self.value: to if self.value is not None: so falsy-but-meaningful values (0, 0.0, False) render correctly.
• Add explicit empty guards for the list and dict branches, which are now reachable with empty containers: [] renders as "" (previously it would index self.value[0]), and {} renders as "" (previously list(self.value.values())[-1] would raise FunctionResultError).
• Behavior for value=None and value="" is unchanged (both still render as ""), preserving the existing test_function_result_str_empty_value contract.
• Tests (written first, failing on main): parametrized unit tests for 0, 0.0, False, "", [], {} in test_function_result.py, plus a KernelPromptTemplate render test in test_kernel_prompt_template.py asserting The result is: 0 and even=False.

Verified locally: uv run --frozen pytest tests/unit/functions/test_function_result.py tests/unit/functions/test_kernel_function_from_method.py tests/unit/prompt_template tests/unit/template_engine → 729 passed; ruff check/ruff format --check/mypy clean on the touched files.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[Copilot]

Pull request overview

Fixes FunctionResult.__str__ so legitimate falsy return values (e.g., 0, 0.0, False) are rendered correctly instead of being treated as “empty”, preventing silent prompt corruption when results are interpolated into templates.

Changes:

• Update FunctionResult.__str__ to use a None check (is not None) rather than a truthiness check.
• Add safe handling for empty list and dict values now that they pass the None guard.
• Add unit coverage for falsy values and an integration-style prompt-template render assertion.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
python/semantic_kernel/functions/function_result.py	Correct __str__ guard to preserve falsy scalar values; add empty list/dict guards to avoid indexing errors.
python/tests/unit/functions/test_function_result.py	Add parametrized tests covering falsy scalars and empty containers for FunctionResult.__str__.
python/tests/unit/prompt_template/test_kernel_prompt_template.py	Add a render test proving falsy function outputs are preserved in template interpolation.

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[sohumt123]

@microsoft-github-policy-service agree