EnvValue

This value node has a string argument which substitutes values from the environment. This is useful for:

• Generating small initialization files
• Ensuring that builders which are python function gets the correct dependencies if the function uses environment values.

import SCons.Node.Python

class EnvValue(SCons.Node.Python.Value):
    """Node class which substitutes variables from the environment"""

    def __init__(self, env, value):
        """Initialiser takes an Environment and a string to be expanded."""
        super().__init__(value)
        self.env = env

    def __str__(self):
        return env.subst(self.value)

Now you can do something like this:

env.Command("target", EnvValue(env, "$MYVAR"), "echo $SOURCE > $TARGET")
env["MYVAR"] = "SomeValue"

There is one scenario in which the results might not be as expected. This is when the builder creates an override environment. In this case the environment used for the value node is not the same as that used for the builder:

env["MYVAR"] = "DefaultValue"
env.Command(
    "target", EnvValue(env, "$MYVAR"), "echo $SOURCE > $TARGET", MYVAR="SomeValue"
)

In this example the command becomes

echo DefaultValue

For a builder which is a python function which references environment values, you might want to add an emitter function which adds an EnvValue as a dependency (I have not tried that yet).