python magic method __reduce __ () Magical

A, __ reduce __ () Introduction

When defining the extension type (ie type of use of Python C language API implementation), if you want to pickle them, you have to tell Python how to pickle them. __reduce__  After being defined, when the object is Pickle will be called. It either returns a string that represents the global name, Pyhton will find it and pickle, or return a tuple. The tuple contains 2-5 elements, comprising: a callable objects, when the call for reconstructing an object; a parameter element, used for the callable; is passed to  __setstate__  state (optional); generating a elements in this list is iterated pickle (optional); iterator generating a dictionary elements are pickle (optional)

Second, the use case

class ServerError (UserWarning):
     # UserWarning user code generated warning 
    DEF error (Self):
         # C language interface, error information into a string pickle 
        return Self. __reduce__ () [. 1 ]

# Returns (<class '__main __. ServerError '>, ( ' error')), it takes a slice

DEF instance_name (name):
     the try :
         IF  not isinstance (name, str):
             The raise ServerError ( " name must be a string " )
     the except ServerError AS E:
        error=e.error()[0]
        print(error)

instance_name ( 123 )
 # action is: can customize the error message, the composition is returned to the front end json format