Custom Filters

Filters are used to transform values inside Jinja2 expressions. See Jinja2’s general description or the list of built-in filters.

The following custom filters are available from uwtools

env

Inserts the value of the specified environment variable. For example, given config.yaml

cycle: "{{ 'CYCLE' | env }}"

and the presence of environment variable CYCLE with value 2025021312:

$ uw config realize -i config.yaml --output-format yaml
cycle: '2025021312'

Note that the name of the environment variable must be quoted in the Jinja2 expression so that it is recognized as a string, not a Python variable name.

The env filter can be combined with Custom Tags to obtain appropriately typed YAML values. For example, give config.yaml

leadtime: !int "{{ 'LEADTIME' | env }}"

and the presence of environment variable LEADTIME with value 6:

$ uw config realize -i config.yaml --output-format yaml
leadtime: 6

Note that 6 here is a YAML integer, not a string, and will be instantiated in Python as an int.