Extend Jinja Environment to allow Haystack data classes in ConditionalRouter

[Amnah199]

Related Issues

• fixes Allow for Haystack native data types as outputs to ConditionalRouter #8517

Proposed Changes:

Currently, to use Haystack data classes as an output type in ConditionalRouter, users must set unsafe=True. This limitation arises because SandboxedEnvironment does not support the safe evaluation of Python objects, and extending ast.literal_eval to handle custom data classes is neither practical nor secure.

As a solution, this PR introduces NativeSandboxedEnvironment, a custom Jinja2 environment that tries to combine the sandboxing capabilities of SandboxedEnvironment with the native type rendering of NativeEnvironment. I tried to whitelist Haystack data classes in the is_safe_attribute method, allowing them to be safely rendered as native Python objects. Everything else should pass through default sandbox security rules.

❌ However, currently everything is being rendered as a NativeTemplate which possibly causes a security vulnerability.

Highlights:

• Subclass both SandboxedEnvironment and NativeEnvironment → NativeSandboxedEnvironment.

• Override from_string and template_class so the environment uses a “native” template but still enforces the sandbox.

• Use is_safe_attribute to whitelist Haystack data classes.

How did you test it?

Ran the tests

Notes for the reviewer

As I am still evaluating the vulnerabilities of the above approach, an alternative solution could be to check the output_type. If it corresponds to a Haystack data class, the NativeSandboxedEnvironment could be used specifically for rendering Haystack data classes, while the SandboxedEnvironment is retained for all other cases to enhance overall safety.

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I documented my code
• I ran pre-commit hooks and fixed any issue