Parsing environment variables in an YAML file using PyYAML library

loader.py

"""A simple yaml loader

    Load the content of an yaml file into a python dict. Environment variable can be specified with ${VAR_NAME}. A
    default string value can be specified with ${VAR_NAME:DEFAULT}. Able to process multiple occurrences.

    requirement: PyYAML
    run: python loader.py
"""

import os
import re
from typing import Any

import yaml
from yaml.parser import ParserError

_var_matcher = re.compile(r"\${([^}^{]+)}")
_tag_matcher = re.compile(r"[^$]*\${([^}^{]+)}.*")


def _path_constructor(_loader: Any, node: Any):
    def replace_fn(match):
        envparts = f"{match.group(1)}:".split(":")
        return os.environ.get(envparts[0], envparts[1])
    return _var_matcher.sub(replace_fn, node.value)


def load_yaml(filename: str) -> dict:
    yaml.add_implicit_resolver("!envvar", _tag_matcher, None, yaml.SafeLoader)
    yaml.add_constructor("!envvar", _path_constructor, yaml.SafeLoader)
    try:
        with open(filename, "r") as f:
            return yaml.safe_load(f.read())
    except (FileNotFoundError, PermissionError, ParserError):
        return dict()


if __name__ == "__main__":
    print(load_yaml("sample.yaml"))

sample.yaml

# A sample YAML file
version: 1

node1:
  - field: ${VAR1}
  - field: some ${VAR2} and some
  - field: test ${VAR1} some text'${VAR2}' some other values
  - field: test with default ${VAR3:123}
  - field: ${VAR3:}${VAR4}

test_loader.py

"""Test for yaml loader

    A simple pytest script to test loading environment variables in an yaml file

    requirement: pytest
    run: pytest -s
"""
import os

import pytest

from loader import load_yaml


def test_loader():
    """Test fails if loader fails to parse environment variables"""
    os.environ.update({"VAR1": "var1", "VAR2": "var2"})
    data = load_yaml("sample.yaml")
    assert "var1" in data["node1"][0]["field"]
    assert "var2" in data["node1"][1]["field"]
    assert all(x in data["node1"][2]["field"] for x in ["var1", "var2"])
    assert "123" in data["node1"][3]["field"]
    assert "" == data["node1"][4]["field"]