> for i in 100:
...   print(i)
...
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable
>
# int整型不是iterable，即int整型不是可以迭代的
# 我们自定义一个容器MyList用来存放数据，可以通过add方法向其中添加数据
> class MyList(object):
...   def __init__(self):
...       self.container = []
...   def add(self, item):
...       self.container.append(item)
...
> mylist = MyList()
> mylist.add(1)
> mylist.add(2)
> mylist.add(3)
> for num in mylist:
...   print(num)
...
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'MyList' object is not iterable
>
# MyList容器的对象也是不能迭代的

In [50]: from collections import Iterable
In [51]: isinstance([], Iterable)
Out[51]: True
In [52]: isinstance({}, Iterable)
Out[52]: True
In [53]: isinstance('abc', Iterable)
Out[53]: True
In [54]: isinstance(mylist, Iterable)
Out[54]: False
In [55]: isinstance(100, Iterable)
Out[55]: False

> class MyList(object):
...   def __init__(self):
...       self.container = []
...   def add(self, item):
...       self.container.append(item)
...   def __iter__(self):
...       """返回一个迭代器"""
...       # 我们暂时忽略如何构造一个迭代器对象
...       pass
...
> mylist = MyList()
> from collections import Iterable
> isinstance(mylist, Iterable)
True
>
# 这回测试发现添加了__iter__方法的mylist对象已经是一个可迭代对象了

> li = [11, 22, 33, 44, 55]
> li_iter = iter(li)
> next(li_iter)
11
> next(li_iter)
22
> next(li_iter)
33
> next(li_iter)
44
> next(li_iter)
55
> next(li_iter)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
StopIteration
>

In [56]: from collections import Iterator
In [57]: isinstance([], Iterator)
Out[57]: False
In [58]: isinstance(iter([]), Iterator)
Out[58]: True
In [59]: isinstance(iter("abc"), Iterator)
Out[59]: True

class MyList(object):
  """自定义的一个可迭代对象"""
  def __init__(self):
    self.items = []
  def add(self, val):
    self.items.append(val)
  def __iter__(self):
    myiterator = MyIterator(self)
    return myiterator
class MyIterator(object):
  """自定义的供上面可迭代对象使用的一个迭代器"""
  def __init__(self, mylist):
    self.mylist = mylist
    # current用来记录当前访问到的位置
    self.current = 0
  def __next__(self):
    if self.current < len(self.mylist.items):
      item = self.mylist.items[self.current]
      self.current += 1
      return item
    else:
      raise StopIteration
  def __iter__(self):
    return self
if __name__ == '__main__':
  mylist = MyList()
  mylist.add(1)
  mylist.add(2)
  mylist.add(3)
  mylist.add(4)
  mylist.add(5)
  for num in mylist:
    print(num)

class FibIterator(object):
  """斐波那契数列迭代器"""
  def __init__(self, n):
    """
    :param n: int, 指明生成数列的前n个数
    """
    self.n = n
    # current用来保存当前生成到数列中的第几个数了
    self.current = 0
    # num1用来保存前前一个数，初始值为数列中的第一个数0
    self.num1 = 0
    # num2用来保存前一个数，初始值为数列中的第二个数1
    self.num2 = 1
  def __next__(self):
    """被next()函数调用来获取下一个数"""
    if self.current < self.n:
      num = self.num1
      self.num1, self.num2 = self.num2, self.num1+self.num2
      self.current += 1
      return num
    else:
      raise StopIteration
  def __iter__(self):
    """迭代器的__iter__返回自身即可"""
    return self
if __name__ == '__main__':
  fib = FibIterator(10)
  for num in fib:
    print(num, end=" ")

li = list(FibIterator(15))
print(li)
tp = tuple(FibIterator(6))
print(tp)