earthkitfieldlambda: specify fn as string not working

[bastien-francois]

What happened?

When specifying the parameter fn for the earthkitfieldlambda filter as a string like this:

    - earthkitfieldlambda:
        param: r
        fn: "lambda x, s: x.clone(values=x.values * s)"
        fn_args: [100]

It tries to import it directly:
ValueError: Could not import function lambda x, s: x.clone(values=x.values * s)

This error could be solved by simply modifying init() of EarthkitFieldLambdaFilter:

self.fn = eval(fn) instead of self.fn = self._import_fn(fn) if isinstance(fn, str) else fn

Optimally, the _import_fn() function might be modified to differentiate when the string is in the form "package.module.function" or a Python function "lambda x, s: x.clone(values=x.values * s)".

What are the steps to reproduce the bug?

Applying the filter earthkitfieldlambda with Python function as string does not work properly. Example of a recipe to reproduce the issue:

input:
  join: 
  - pipe:
    - join:
      - grib:
          path: path1
          param: [ t ]
          level: [2]
          typeOfLevel: heightAboveGround
          levtype: sfc
      - grib:
          path: path1
          param: [ r ]
          typeOfLevel: heightAboveGround
          level: [2]
          levtype: sfc
    - earthkitfieldlambda:
        param: r
        fn: "lambda x, s: x.clone(values=x.values * s)"
        fn_args: [100]
    - r_to_d:
        relative_humidity: r
        temperature: t
        dewpoint: 2d

Version

0.1.16

Platform (OS and architecture)

Linux pc190605.dynamic.knmi.nl 6.15.3-100.fc41.x86_64 #1 SMP PREEMPT_DYNAMIC Thu Jun 19 15:09:31 UTC 2025 x86_64 GNU/Linux

Relevant log output

ValueError: Could not import function lambda x, s: x.clone(values=x.values * s)
2025-08-11 08:09:31 ERROR 
💣 Could not import function lambda x, s: x.clone(values=x.values * s)

Accompanying data

No response

Organisation

KNMI

[aaron-hopkinson]

Hi @bastien-francois,

This isn't a bug as such – this particular filter is designed to import the relevant function if a string is passed via the config, rather than allowing the definition of functions through the config (perhaps we could improve the documentation here?).

In theory, we could allow for functions to be defined in the config (though we'd need to sanitise the input and ensure that it's safe, rather than passing it to eval), but I think the preferred option would be for the user to create the code separately in python and then import it through the config (or create a new filter, either as a plugin or contributed directory to this repo).