This dummy example splits a string that it was given, then if one of those values is in the callers context it saves those in self.context, and has an output function to assemble it all together. Obviously this example is not very useful, but it shows how a library could do this as a class or function without the user having to pass in locals().

  import inspect
  class MyXString:
      """ will split on whitespace and replace any string that is a variable name 
    with the result of str(variable)"""
        def __init__(self, string):
            self.string = string
            caller_locals = inspect.currentframe().f_back.f_locals
            self.context = {}
            for key in  set(string.split()):
                if key in caller_locals:
                    self.context[key] = caller_locals[key]
  
        def output(self):
            output = self.string
            for k,v in self.context.items():
                output = output.replace(k,str(v))
            return output