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Not via YAML. But your program itself can have the default value baked in - which is more or less equivalent to it being in Pulumi.yaml. For example `const region = config.get(“region”) || “us-west-2”.` 

Definitely considering offering more options for hierarchical config - see <https://github.com/pulumi/pulumi/issues/2307|https://github.com/pulumi/pulumi/issues/2307>. 