Source code for preprocessor.config

import os
from typing import TextIO
import re

import yaml


ENV_PATTERN = re.compile(r".*?\${(\w+)}.*?")


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 = ENV_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


def load_config(input_file: TextIO):
    tag = "!env"
    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, ENV_PATTERN, None)
    loader.add_constructor(tag, constructor_env_variables)

    return yaml.load(input_file, Loader=loader)