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```        if "foo" in outputs:
    TypeError: argument of type 'Output' is not iterable```

you are running into the async nature of outputs. Outputs aren't iterable because they haven't resolved yet.

You need to do the check inside an `apply`:

```stack_reference.output.apply(lambda: i if i // do something```

Ok thanks, can i not extract the list of outputs that are currently there ?

can you elaborate what you mean by extract?

```outputs = stack_reference.outputs.apply(lambda outputs: [output for output in outputs])
pulumi.export("outputs", outputs)```


I would now like to check, if a key exists in this list.

like would this work ?
```def stack_contains_output(stack_reference, output_name):
    return outputs.apply(lambda outputs: True if output_name in outputs else False)```


yes basically, if you're inside the `apply` you can treat the object like a standard object because it's now resolved, so you can check however you'd like

this might help with some additional context: <https://leebriggs.co.uk/blog/2021/05/09/pulumi-apply>