Using PyYaml load env vars

yaml_load_env.py

#
# load yaml by env variables
#
import yaml, os, re

pattern = re.compile( '^\$\{(.*?)(:(.*))?\}$' )
yaml.add_implicit_resolver ( "!env", pattern )
def env_construction(loader, node):
    value = loader.construct_scalar(node)
    env_var_name, _, default = pattern.match(value).groups()
    env_var = os.getenv(env_var_name, default)
    if not env_var:
        raise ValueError('the environment variable: {} is not existed.'.format(env_var_name))
    return env_var
yaml.add_constructor('!env', env_construction)

pattern2 = re.compile('^\$(.*?)(:(.*))?$')
yaml.add_implicit_resolver('!env2', pattern2)
def env2_construction(loader, node):
    value = loader.construct_scalar(node)
    env_var_name, _, default = pattern2.match(value).groups()
    env_var = os.getenv(env_var_name, default)
    if not env_var:
        raise ValueError('the environment variable: {} is not existed.'.format(env_var_name))
    return env_var
yaml.add_constructor('!env2', env2_construction)