>>> iter([1,2,3])
<listiterator object at 0x026C8970>

>>> iter({1:2, 2:4})
<dictionary-keyiterator object at 0x026CC1B0>

>>> iter(1234)
Traceback (most recent call last):
 File "<pyshell#145>", line 1, in <module>
  iter(1234)
TypeError: 'int' object is not iterable

class String(object):
 def __init__(self, val):
  self.val = val
 def __str__(self):
  return self.val
st = String('sample string')

>>> iter(st)

TypeError: 'String' object is not iterable

class String(object):
  def __init__(self, val):
    self.val = val
  def __str__(self):
    return self.val
  def __iter__(self):
    print "This is __iter__ method of String class"
    return iter(self.val) #self.val is python string so iter() will return it's iterator
>>> st = String('Sample String')
>>> iter(st)
This is __iter__ method of String class
<iterator object at 0x026C8150>

class String(object):
  def __init__(self, val):
    self.val = val
  def __str__(self):
    return self.val
  def __getitem__(self, index):
    return self.val[index]
>>> st = String('Sample String')
>>> iter(st)
<iterator object at 0x0273AC10>

for x in iterable:
  print x

def iterate_while(iterable):
  index = 0
  while(i< len(iterable)):
    print iterable[i]
    i +=1

def simulate_for_loop(iterable):
  it = iter(iterable)
  while(True):
 try:
   print next(it)
 except StopIteration:
   break
>>> simulate_for_loop([23,12,34,56])
23
12
34
56

class Iterator:
  def __init__(self, iterable)
    self.iterable = iterable
  .
  .
  def __iter__(self): #iter should return self if called on iterator
    return self
  def next(self): #Use __next__() in python 3.x
    if condition: #it should raise StopIteration exception if no next element is left to return
      raise StopIteration

class list_iter(object):
  def __init__(self, list_data):
    self.list_data = list_data
    self.index = 0
  def __iter__(self):
    return self
  def next(self):  #Use __next__ in python 3.x
    if self.index < len(self.list_data):
      val = self.list_data[self.index]
      self.index += 1 
      return val
    else:
      raise StopIteration()

class List(object):
  def __init__(self, val):
    self.val = val
  def __iter__(self):
    return list_iter(self.val)
>>> ls = List([1,2,34])
>>> it = iter(ls)
>>> next(it)
1
>>> next(it)
2
>>> next(it)
34
>>> next(it)

Traceback (most recent call last):
 File "<pyshell#254>", line 1, in <module>
  next(it)
 File "<pyshell#228>", line 13, in next
  raise StopIteration()
StopIteration

>>> x = xrange(10)
>>> type(x)
<type 'xrange'>

>>> iter(xrange(10))
<rangeiterator object at 0x0264EFE0>

class xrange_iter(object):
  def __init__(self, num):
    self.num = num
    self.start = 0
  def __iter__(self):
    return self
  def next(self):
    if self.start < self.num:
      val = self.start
      self.start += 1
      return val
    else:
      raise StopIteration()

class my_xrange(object):
  def __init__(self, num):
    self.num = num
  def __iter__(self):
    return xrange_iter(self.num)
>>> for x in my_xrange(10):
 print x,

0 1 2 3 4 5 6 7 8 9