for循环在Python中的工作原理详细

>>> for elem in [1,2,3]:
...     print(elem)
...
1
2
3

>>> for c in "abc":
...     print(c)
...
a
b
c

>>> for k in {"age":10, "name":"wang"}:
...     print(k)
...
age
name

>>> class MyRange:
...     def __init__(self, num):
...         self.num = num
...
>>> for i in MyRange(10):
...     print(i)
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'MyRange' object is not iterable

>>> x = [1,2,3]
>>> its = x.__iter__() # x有此方法，说明列表是可迭代对象
>>> its
<list_iterator object at 0x100f32198>

>>> its.__next__()  # its有此方法，说明its是迭代器
1
>>> its.__next__()
2
>>> its.__next__()
3
>>> its.__next__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

class MyRange:
    def __init__(self, num):
        self.i = 0
        self.num = num

    def __iter__(self):
        return self

    def __next__(self):
        if self.i < self.num:
            i = self.i
            self.i += 1
            return i
        else:
            # 达到某个条件时必须抛出此异常，否则会无止境地迭代下去
            raise StopIteration()

for i in MyRange(3):
    print(i)
# 输出
 0
 1
 2