[event trigger decorator] multi-events passing via `config_expr` crashes with TypeError

[valayDave]

The below flow is a simple example to verify that reproduces the crash when values to events parameters is set via config_expr in the @trigger decorator.

"""
Test: Does @trigger(events=config_expr(...)) crash due to ConfigValue immutability?

This reproduces the bug where events_decorator.py tries to mutate a ConfigValue dict:
    event["parameters"] = self.process_parameters(...)
    -> TypeError: ConfigValue is immutable

Run:
    # Should FAIL (multi-event, list-of-dicts config value):
    python test_config_value_immutability.py --config-value config '{"project_name":"test","trigger_events":[{"name":"evt_a","parameters":{"my_param":"my_param"}},{"name":"evt_b"}]}' show
"""
from metaflow import FlowSpec, step, Parameter, Config, config_expr, project, trigger


@project(name=config_expr("config.project_name"))
@trigger(events=config_expr("config.trigger_events"))
class MultiEventConfigTest(FlowSpec):
    config = Config(
        "config",
        default_value={
            "project_name": "test",
            "trigger_events": [
                {
                    "name": "evt_a",
                    "parameters": {"my_param": "my_param"},
                },
                {"name": "evt_b"},
            ],
        },
    )

    my_param = Parameter("my_param", default="", type=str)

    @step
    def start(self):
        print(f"my_param={self.my_param}")
        self.next(self.end)

    @step
    def end(self):
        print("done")


if __name__ == "__main__":
    MultiEventConfigTest()

The error:

  Traceback (most recent call last):
       File "/home/ubuntu/miniconda3/envs/ob-lambdas/lib/python3.13/site-packages/metaflow/cli.py", line 727, in main
         start(auto_envvar_prefix="METAFLOW", obj=state)
         ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       File "/home/ubuntu/miniconda3/envs/ob-lambdas/lib/python3.13/site-packages/metaflow/_vendor/click/core.py", line 829, in __call__
         return self.main(args, kwargs)
                ~~~~~~~~~^^^^^^^^^^^^^^^^^
       File "/home/ubuntu/miniconda3/envs/ob-lambdas/lib/python3.13/site-packages/metaflow/_vendor/click/core.py", line 782, in main
         rv = self.invoke(ctx)
       File "/home/ubuntu/miniconda3/envs/ob-lambdas/lib/python3.13/site-packages/metaflow/cli_components/utils.py", line 65, in invoke
         click.Command.invoke(self, ctx)
         ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
       File "/home/ubuntu/miniconda3/envs/ob-lambdas/lib/python3.13/site-packages/metaflow/_vendor/click/core.py", line 1066, in invoke
         return ctx.invoke(self.callback, ctx.params)
                ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       File "/home/ubuntu/miniconda3/envs/ob-lambdas/lib/python3.13/site-packages/metaflow/_vendor/click/core.py", line 610, in invoke
         return callback(args, kwargs)
       File "/home/ubuntu/miniconda3/envs/ob-lambdas/lib/python3.13/site-packages/metaflow/_vendor/click/decorators.py", line 21, in new_func
         return f(get_current_context(), args, kwargs)
       File "/home/ubuntu/miniconda3/envs/ob-lambdas/lib/python3.13/site-packages/metaflow/cli.py", line 568, in start
         decorators._init_flow_decorators(
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
             ctx.obj.flow,
             ^^^^^^^^^^^^^
         ...<8 lines>...
             ctx.obj.skip_decorators,
             ^^^^^^^^^^^^^^^^^^^^^^^^
         )
         ^
       File "/home/ubuntu/miniconda3/envs/ob-lambdas/lib/python3.13/site-packages/metaflow/decorators.py", line 788, in _init_flow_decorators
         deco.flow_init(
         ~~~~~~~~~~~~~~^
             flow,
             ^^^^^
         ...<6 lines>...
             deco_flow_init_options,
             ^^^^^^^^^^^^^^^^^^^^^^^
         )
         ^
       File "/home/ubuntu/miniconda3/envs/ob-lambdas/lib/python3.13/site-packages/metaflow/plugins/events_decorator.py", line 236, in flow_init
         processed_event = self.process_event(event)
       File "/home/ubuntu/miniconda3/envs/ob-lambdas/lib/python3.13/site-packages/metaflow/plugins/events_decorator.py", line 122, in process_event
         event["parameters"] = self.process_parameters(
         ~~~~~^^^^^^^^^^^^^^
       File "/home/ubuntu/miniconda3/envs/ob-lambdas/lib/python3.13/site-packages/metaflow/user_configs/config_parameters.py", line 190, in __setitem__
         raise TypeError("ConfigValue is immutable")
     TypeError: ConfigValue is immutable

[leeneyad]

Hi, I'm an AI student and I'm interested in fixing this TypeError related to ConfigValue immutability. I've reproduced the crash and would like to work on it. Could you please assign this to me?

[RONAK-AI647]

@valayDave I have opened a pr for this issue .