list - Confused about classes Python -

i bit confused on how classes work in python.

i wanted sort input in natural way , found solution:

def atoi(text):     return int(text) if text.isdigit() else text  def natural_keys(text):     '''     alist.sort(key=natural_keys) sorts in human order     http://nedbatchelder.com/blog/200712/human_sorting.html     (see toothy's implementation in comments)     '''     return [ atoi(c) c in re.split('(\d+)', text) ]  alist=[     "something1",     "something12",     "something17",     "something2",     "something25",     "something29"]  alist.sort(key=natural_keys) 

works charm.

now wanted include solution in class.

class __test():      def atoi(self,text):          return int(text) if text.isdigit() else text      def natural_keys(self,text):          return [ self.atoi(c) c in re.split('(\d+)', text) ]  b = __test()  alist=[     "something1",     "something12",     "something17",     "something2",     "something25",     "something29"]  alist.sort(key=b.natural_keys(alist)) 

which not work since alist given natural keys list

this works charm:

class __test():      def atoi(self,text):          return int(text) if text.isdigit() else text      def natural_keys(self,text):          return [ self.atoi(c) c in re.split('(\d+)', text) ]      def sorta(self,input):          = input[::]          a.sort(key=natural_keys)          return   b = __test()  alist=[     "something1",     "something12",     "something17",     "something2",     "something25",     "something29"]  print b.sorta(alist) 

i can sort of understand have sorta() def since inside class , outside not known.

but why can not call first try? how input natural_keys end list in first try not in second?

another question why can not this:

   def sorta(self,input):         return input.sort(key=natural_keys) 

thx in advance. sorry if wording seems confusing hope mean.