In python, why is len () writing an object in parentheses, and lower () writing the object in front, not in parentheses?

I have encountered some minor problems in learning codeacademy. Please kindly advise! I have encountered some minor problems in learning code academy. Please kindly advise! Reply: a common design routine in python is to use a global method to set up instances of different classes to call a method of the same name for different instances, such as 'repr (o) 'called' o. _ repr _ () ', 'iter (o)' calls 'o. _ iter _ () ', 'len (o)' calls 'o. _ len _ () ', but 'o. lower () 'only makes sense to the string, and there is no need to design a global lower method to call it. Len () is the Built-in Functions of Python. For details, search "Built-in Functions" in the help document ". The second lower () is a method of the object. The method of use is naturally the object. lower (). One is a system function, and the other is an object method.