--config-value with a JSON array, number, or string crashes with an internal error

Passing a config value that's valid JSON but not an object crashes with
an internal error instead of a config error. Each exits with status 1:

`--config-value cfg`	Result
`'[]'`	`AttributeError: 'list' object has no attribute 'items'`
`'1'`	`AttributeError: 'int' object has no attribute 'items'`
`'"x"'`	`AttributeError: 'str' object has no attribute 'items'`

python flow.py --config-value cfg '[]' show

Cause

JSON parsing in metaflow/user_configs/config_options.py (~line 388)
succeeds, then ConfigValue in
metaflow/user_configs/config_parameters.py (~line 85) calls .items()
on the result without checking its type.

Expected

A clear config error saying the value for cfg must be a JSON object,
consistent with how other malformed config values are reported.
null currently works through a separate None path and should keep
working as it does today.

Proposed fix

After json.loads, reject any non-None result that isn't a dict with a
config error, preserving the existing null behavior. Add parametrized
tests for arrays, numbers, strings, and null.

Since this touches user_configs/ (Core Runtime), I wanted to confirm
the approach before opening a PR.

Environment

OS: macOS Tahoe 26.5.2
Python: 3.12.14
Metaflow: master (8b500daa)

@eyosi1239

Hi @eyosi1239 , I'd like to take this issue up.

@suhanemathur

Thanks for the interest, @suhanemathur! I found this while testing and
already have the approach and tests worked out locally, so I'd like to
take it. Per CONTRIBUTING.md I'm just waiting on maintainer confirmation
before opening a PR. @saikonen would you be okay with me picking this
up?

seems like a reasonable bug fix. When tackling this, be sure to only cover the JSON path, and maybe not specifically mention JSON in the error messages as this can also result from parsing other types with custom parsers.

Note that bare str configs are also valid:

from metaflow import FlowSpec, step, Config, environment
import os

class ConfigTestFlow(FlowSpec):
    barecfg = Config('barecfg', default_value="", plain=True)

    @step
    def start(self):
        print("BareCFG: ", self.barecfg)
        self.next(self.end)

    @step
    def end(self):
        pass

if __name__ == '__main__':
    ConfigTestFlow()

python configflow.py --config-value barecfg "a bare testvalue" run

@saikonen

opened #3391 with the fix. @saikonen could you take a look and approve the workflow run when you get a chance?

Simplify config type errors and assertions after review

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--config-value with a JSON array, number, or string crashes with an internal error #3390

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--config-value with a JSON array, number, or string crashes with an internal error #3390

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suhanemathur commented on Sep 29, 2026

eyosi1239 commented on Sep 29, 2026

saikonen commented on Sep 29, 2026

eyosi1239 commented on Sep 29, 2026

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