code: def envvar_constructor(loader, node):
      
      value = loader.construct_python_unicode(node)
      data = value.split(':=', 1)
      if len(data) == 2:
          var, default = data
          return os.environ.get(var, default)
      else:
          return os.environ[value]
docs: Tag constructor to use environment variables in YAML files. Usage:
  
      - !TAG VARIABLE
          raise while loading the document if variable does not exists
      - !TAG VARIABLE:=DEFAULT_VALUE
  
      For instance:
  
          credentials:
              user: !env USER:=root
              group: !env GROUP:= root