Python Load a yaml configuration file and resolve any environment variables

python_yaml_environment_variables.py

import os
import re
import yaml


def parse_config(path=None, data=None, tag='!ENV'):
    """
    Load a yaml configuration file and resolve any environment variables
    The environment variables must have !ENV before them and be in this format
    to be parsed: ${VAR_NAME}.
    E.g.:

    database:
        host: !ENV ${HOST}
        port: !ENV ${PORT}
    app:
        log_path: !ENV '/var/${LOG_PATH}'
        something_else: !ENV '${AWESOME_ENV_VAR}/var/${A_SECOND_AWESOME_VAR}'

    :param str path: the path to the yaml file
    :param str data: the yaml data itself as a stream
    :param str tag: the tag to look for
    :return: the dict configuration
    :rtype: dict[str, T]
    """
    # pattern for global vars: look for ${word}
    pattern = re.compile('.*?\${(\w+)}.*?')
    loader = yaml.SafeLoader

    # the tag will be used to mark where to start searching for the pattern
    # e.g. somekey: !ENV somestring${MYENVVAR}blah blah blah
    loader.add_implicit_resolver(tag, pattern, None)

    def constructor_env_variables(loader, node):
        """
        Extracts the environment variable from the node's value
        :param yaml.Loader loader: the yaml loader
        :param node: the current node in the yaml
        :return: the parsed string that contains the value of the environment
        variable
        """
        value = loader.construct_scalar(node)
        match = pattern.findall(value)  # to find all env variables in line
        if match:
            full_value = value
            for g in match:
                full_value = full_value.replace(
                    f'${{{g}}}', os.environ.get(g, g)
                )
            return full_value
        return value

    loader.add_constructor(tag, constructor_env_variables)

    if path:
        with open(path) as conf_data:
            return yaml.load(conf_data, Loader=loader)
    elif data:
        return yaml.load(data, Loader=loader)
    else:
        raise ValueError('Either a path or data should be defined as input')

This code has an error because not import re ...

import re

before the 1st line will fix it.

Thanks! Fixed it

Thanks for sharing this code. It works perfectly for my scenario also where the value is fetched from config file. Btw, can I use this code in my project?

Thanks for sharing this code. It works perfectly for my scenario also where the value is fetched from config file. Btw, can I use this code in my project?

@nandhinik glad to know you found this helpful! Of course you can use it. Let me know if I can be of any help.

An improved version of this is in the very small pyaml-env library : https://pypi.org/project/pyaml-env/
(mostly for convenience)