Negate a Router condition

[FrancescoSaverioZuppichini]

Hi people,

What is the right syntax to negate a router condition?

condition = " something == 'hey' and ...."
# how to negate it?

no_condition = f"not ({condition})" -> doesn't work
no_condition = f"! ({condition})" -> doesn't work

Thanks a lot

[julian-risch]

Hi @FrancescoSaverioZuppichini the condition is in Jinja2 syntax and to correctly negate it, you need not to be in the condition that is rendered. Here is more information about Jinja2 syntax: https://jinja.palletsprojects.com/en/stable/templates/ and here in Haystack documentation: https://docs.haystack.deepset.ai/docs/jinja-templates

If you are interested, here are the lines of code in Haystack where the condition in Jinja2 syntax is first rendered and then evaluated:

haystack/haystack/components/routers/conditional_router.py

Lines 324 to 326 in e3d4e9e

	rendered = t.render(**kwargs)
	if not self._unsafe:
	rendered = ast.literal_eval(rendered)

The step of rendering the condition is a simple step or more complex depending on the condition as the condition only needs to be valid Jinja2 syntax and might contain multiple functions.

For example, the following can be easily negated

routes = [
    {
        "condition": "{{messages[-1].meta.finish_reason == 'function_call'}}",
        "output": "{{streams}}",
        "output_type": list[int],
        "output_name": "streams",
    },
    {
        "condition": "{{True}}",
        "output": "{{query}}",
        "output_type": str,
        "output_name": "query",
    },  # catch-all condition
]
router = ConditionalRouter(routes)

However, the following is more complex:

routes = [
    {
        "condition": "{{streams|length > 2}}",
        "output": "{{streams}}",
        "output_name": "enough_streams",
        "output_type": list[int],
    },
    {
        "condition": "{{streams|length <= 2}}",
        "output": "{{streams}}",
        "output_name": "insufficient_streams",
        "output_type": list[int],
    },
]
router = ConditionalRouter(routes)

[FrancescoSaverioZuppichini]

Thanks a lot for the reply! My issue is that if I put not (<CONDITION>) it tries to find a pipeline with name "not (<CONDITION>)" and it doesn't negate it; looks like it tries to use not ... as prefix of the pipeline node

[julian-risch]

Can you give an example? For example,
"condition": "{{messages[-1].meta.finish_reason == 'function_call'}}" works and the following should too
"condition": "{{not messages[-1].meta.finish_reason == 'function_call'}}"

[FrancescoSaverioZuppichini]

imagine having multiple conditions A and B etc, not in front will only negate the first one, you need not (CONDITIONS)