A custom YAML tag for referencing environment variables in YAML files. The content of the env var will be resolved to one of YAML's implicit types (bool, float, int, timestamp, null, value). The content of the env var is always assumed to be a Scalar node, so sequence and mappings are ignored and simply passed through as a string.

example.yml

foo: !ENV FOO
bar: !ENV [BAR]
baz: !ENV [BAZ, ~]
fbb: !ENV [FOO, BAR, BAZ, ~]

pyyaml_env_tag.py

# A Python clone of https://github.com/jirutka/yaml-env-tag

import os
import yaml
from typing import Any


def construct_env_tag(loader: yaml.Loader, node: yaml.Node) -> Any:
    """Assign value of ENV variable referenced at node."""
    default = None
    if isinstance(node, yaml.nodes.ScalarNode):
        vars = [loader.construct_scalar(node)]
    elif isinstance(node, yaml.nodes.SequenceNode):
        child_nodes = node.value
        if len(child_nodes) > 1:
            # default is resvoled using YAML's (implicit) types.
            default = loader.construct_object(child_nodes[-1])
            child_nodes = child_nodes[:-1]
        # Env Vars are resolved as string values, ignoring (implicit) types.
        vars = [loader.construct_scalar(child) for child in child_nodes]
    else:
        raise yaml.constructor.ConstructorError(None, None,
            f'expected a scalar or sequence node, but found {node.id}',
            node.start_mark)

    for var in vars:
        if var in os.environ:
            value = os.environ[var]
            # Resolve value to value of type using YAML's implicit resolvers
            tag = loader.resolve(yaml.nodes.ScalarNode, value, (True, False))
            return loader.construct_object(yaml.nodes.ScalarNode(tag, value))

    return default


yaml.add_constructor('!ENV', construct_env_tag)


if __name__ == '__main__':
    with open('example.yml', 'r') as f:
        data = yaml.load(f, Loader=yaml.Loader)
    print(data)

This is a Python clone of the ruby lib yaml-env-tag.

See some alternative implementations at pyyaml-tags and python_yaml_environment_variables.py, each of which insert env vars in a template string.

A fully tested version of this now lives at waylan/pyyaml-env-tag and is available to install from PyPI.