python leading and trailing underscores in user defined functions -

how can used rt function, understand leading & trailing underscores __and__() available native python objects or wan't customize behavior in specific situations. how can user take advantages of . ex: in below code can use function @ all,

class a(object):   def __rt__(self,r):       return "yes special functions"   a=a() print dir(a) print a.rt('1') # attributeerror: 'a' object has no attribute 'rt' 

but

class room(object):      def __init__(self):      self.people = []    def add(self, person):      self.people.append(person)     def __len__(self):      return len(self.people)    room = room() room.add("igor") print len(room) #prints 1 

python doesn't translate 1 name another. specific operations under covers call __special_method__ if has been defined. example, __and__ method called python hook & operator, because python interpreter explicitly looks method , documented how should used.

in other words, calling object.rt() not translated object.__rt__() anywhere, not automatically.

note python reserves such names; future versions of python may use name specific purpose , existing code using __special_method__ name own purposes break.

from reserved classes of identifiers section:

__*__
system-defined names. these names defined interpreter , implementation (including standard library). current system names discussed in special method names section , elsewhere. more defined in future versions of python. use of __*__ names, in context, not follow explicitly documented use, subject breakage without warning.

you can ignore advice of course. in case, you'll have write code calls method:

class somebaseclass:     def rt(self):         """call __rt__ special method"""         try:             return self.__rt__()         except attributeerror:             raise typeerror("the object doesn't support operation") 

and subclass somebaseclass.

again, python won't automatically call new methods. still need write such code.