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=============
How-To Guides
=============
.. contents::
:local:
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 :ref:`resolver<resolvers>`
to perform simple computations.
.. _builtins.eval: https://docs.python.org/3/library/functions.html#eval
First, register the ``builtins.eval`` function as a new resolver:
.. doctest::
>>> from omegaconf import OmegaConf
>>> OmegaConf.register_new_resolver("eval", eval)
Now, define a config and perform some arithmetic using the ``eval`` resolver:
.. doctest::
>>> yaml_data = """
... ten_squared: ${eval:'10 ** 2'}
... """
>>> cfg = OmegaConf.create(yaml_data)
>>> assert cfg.ten_squared == 100
You can use :ref:`nested interpolation<nested-interpolation>` to perform computation that involves other values from your config:
.. doctest::
>>> yaml_data = """
... side_1: 5
... side_2: 6
... rectangle_area: ${eval:'${side_1} * ${side_2}'}
... """
>>> cfg = OmegaConf.create(yaml_data)
>>> assert cfg.rectangle_area == 30
To pass string data to ``eval``, you'll need to use a nested pair of quotes:
.. doctest::
>>> cfg = OmegaConf.create({
... "cow_say": "moo",
... "three_cows": """${eval:'3 * "${cow_say}"'}"""
... })
>>> assert cfg.three_cows == "moomoomoo"
The double quotes around ``"${cow_say}"`` guarantee that ``eval`` will
interpret ``"moo"`` as a string instead of as a variable ``moo``. See
:ref:`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 :ref:`custom_resolvers` docs.
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