Expressions and variable interpolation¶

confarg can process expressions, which are declared within ${...}. These expressions allow you to deduce configuration values from other parts of the configuration.

Suppose we have a configuration composed of two floating point values:

@dataclass
class Config:
    value1: float
    value2: float

You can set the value of value2 based on an expression relying on value1, for example.

value1: 3.0
value2: ${value1 * 1.5}

$ uv run two_floats.py --config derived_value2.yaml
Config(value1=3.0, value2=4.5)

value1: ${value2 * 1.5} value2: 3.0

```console

It's ok to have expressions relying on variables that are themselves expressions. For example, if we now have three input float values, the following configuration is valid.

```yaml
# config3.yaml
$ uv run two_floats.py --config derived_value1.yaml
Config(value1=4.5, value2=3.0)

```console
If your chain of expression contains a loop, confarg will complain.

```yaml
# config_with_loop.yaml
$ uv run three_floats.py --config chained_expressions.yaml
Config(value1=4.5, value2=3.0, value3=2.0)

value1: ${value2 * 1.5} value2: ${value3 * 1.5} value3: ${value1 * 1.5}

```console
$ # Error: circular reference
$ uv run three_floats.py --config config_with_loop.yaml
...

The expressions can reach across definition boundaries. For example, a configuration file can define value1, and we can define value2 based on a expression over value1 from the command line.

# config4.yaml
value1: 3.0

$ uv run two_floats.py --config cli_expression.yaml --value2 '${value1 * 1.5}'
Config(value1=3.0, value2=4.5)

The opposite also works: we can define value1 as an expression of value2, which is defined late on the command line.

value1: ${value2 * 1.5}

$ uv run two_floats.py --config cli_value.yaml --value2 3.0
Config(value1=4.5, value2=3.0)

Even if value2 is already defined in the configuration file, the final value of value2 is taken into account in the expression.

$ uv run two_floats.py --config derived_value1.yaml
Config(value1=4.5, value2=3.0)
$ uv run two_floats.py --config derived_value1.yaml --value2 6
Config(value1=9.0, value2=6.0)