In Python, why does Len () write the object in parentheses, while lower () writes the object in front rather than in parentheses?

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A common design in Python is to use global methods to set up instances of different classes to invoke a method of the same name for different instances, such as ' Repr (o) ' called ' o.__repr__ () ', ' iter (o) ' called ' o.__iter__ () ', ' Len (o ' Call ' o.__len__ () ', but ' o.lower () ' only has meaning to the string, there is no need to design a global lower method to tune it. Len () is a Python built-in Functions, which searches the help document for "built-in Functions". The second lower () is a method of the object, which is used naturally as an object. Lower (). One is the system function, the other is the object method.

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