How-To Guides

How to Perform Arithmetic Using eval as a Resolver

Sometimes it is necessary to perform arithmetic based on settings from your app’s config. You can register Python’s builtins.eval function as a resolver to perform simple computations.

First, register the builtins.eval function as a new resolver:

>>> from omegaconf import OmegaConf
>>> OmegaConf.register_new_resolver("eval", eval)

Now, define a config and perform some arithmetic using the eval resolver:

yamlpython

>>> yaml_data = """
... ten_squared: ${eval:'10 ** 2'}
... """
>>> cfg = OmegaConf.create(yaml_data)
>>> assert cfg.ten_squared == 100

You can use nested interpolation to perform computation that involves other values from your config:

yamlpython

>>> yaml_data = """
... side_1: 5
... side_2: 6
... rectangle_area: ${eval:'${side_1} * ${side_2}'}
... """
>>> cfg = OmegaConf.create(yaml_data)
>>> assert cfg.rectangle_area == 30

To pass string data to eval, you’ll need to use a nested pair of quotes:

yamlpython

>>> yaml_data = """
... cow_say: moo
... three_cows: ${eval:'3 * "${cow_say}"'}
... """
>>> cfg = OmegaConf.create(yaml_data)
>>> assert cfg.three_cows == "moomoomoo"

The double quotes around "${cow_say}" guarantee that eval will interpret "moo" as a string instead of as a variable moo. See Escaping in interpolation strings for more information.

For more complicated logic, you should consider defining a specialized resolver to encapsulate the computation, rather than relying on the general capabilities of eval. Follow the examples from the Custom resolvers docs.