python iterators and generators, exceptions

iterator

• An object that can be called by the next() function and keeps returning the next value (until there is no data and a StopIteration is thrown)

• l=[1,2,3];
  print(type(l))
  l = iter (l);
  print(type(l))
  for i in l:
      print(i);
  print(next(l));
  print(next(l));
  print(next(l));

• The iterator internally holds a state, which is used to record the position of the current iteration,
• In order to facilitate the next iteration to get the correct element
• Fibonacci sequence demo iterator, generator

• def fab(max):
      n,a,b=0,0,1
      L=[];
      while n<max:
          #print(b);
          L.append(b);
          a,b=b,a+b;
          n=n+1;
      return L;
  l=fab(10)
  for i in l:
      print(i);

• iterable class

• class Fab(object):
      def __init__(self,max):
          self.max=max;
          self.n,self.a,self.b=0,0,1;
      def __iter__(self):
          return self;
      def __next__(self):
          if self.n<self.max:
              r=self.b;
              self.a,self.b=self.b,self.a+self.b;
              self.n+=1;
              return r;
          raise StopIteration ();
  f=Fab(10);
  print(next(f));
  print(next(f));
  print(next(f));
  print(next(f));
  print(next(f));

yield generator

• def fab(max):
      n,a,b=0,0,1
      while n<max:
          # print(b);
          yield b;
          a,b=b,a+b;
          n=n+1;
  l=fab(10);
  print(next(l));
  print(next(l));
  print(next(l));
  print(next(l));

abnormal

• a=10;
  b=input('Please enter the dividend:')
  try: # statements that may generate exceptions
      b = int(b);
      c = a / b;
      print(c)
  except Exception: #Catch the exception and handle it
      print('An error occurred!')
      print("The program ended normally..");

• Multi-channel capture of error information: subclass error first, parent class error after

• a=10;
  b=input('Please enter the dividend:')
  try:
      b = int(b);
      c = a / b;
      print(c)
  except ZeroDivisionError as zero:#Subclass error
      print('The dividend cannot be 0..Exception information: ',zero)
  except ValueError as v:#Subclass error
      print('Error converting numbers, please input numbers. Exception information: ',v)
  except Exception: #Parent class error
      print('An error occurred!')
  else: # code that will be executed without exception
      print("The program ended normally..")

• finally: the final content to be executed is written in finally
• Role: release resources

• a=10;
  b=input('Please enter the dividend:')
  try:
      b = int(b);
      c = a / b;
      print(c)
  except ZeroDivisionError as zero:#Subclass error
      print('The dividend cannot be 0..Exception information: ',zero)
  except ValueError as v:#Subclass error
      print('Error converting numbers, please input numbers. Exception information: ',v)
  except Exception: #Parent class error
      print('An error occurred!')
  else: # code that will be executed without exception
      print("The program ended normally..");
  finally:
      try:
          of a, b, c
      except Exception as e:
          print (s)
      print("finally: release resources quickly");
  print('Program other code');