class TestRange:
  def __init__(self, num):
    self.num = num
for i in TestRange(10):
  print(i)

# 输出
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'TestRange' object is not iterable

> nums = [13,12,33]
> iter_ret = nums.__iter__() # x有此方法，说明list是可迭代的，而且该方法返回一个迭代器
> iter_ret
<list_iterator object at 0x100f32198>

> iter_ret.__next__()
13
> iter_ret.__next__()
12
> iter_ret.__next__()
33
> iter_ret.__next__()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
StopIteration

class TestRange:
  def __init__(self, _max):
    self.i = 0
    self._max = _max

  def __iter__(self):
    return self

  def __next__(self):
    if self.i < self._max:
      i = self.i
      self.i += 1
      return i
    else:
      # 达到停止条件时，抛出此异常
      raise StopIteration()

# 进行测试
for i in TestRange(3):
  print(i)
# 输出
 0
 1
 2