详解Python中for循环是如何工作的

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

>>> for i in ("zhang", "san", 30):
...  print(i)
...
zhang
san
30

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

>>> for i in {"a","b","c"}:
...  print(i)
...
b
a
c

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

>>> for line in open("requirement.txt"):
...  print(line, end="")
...
Fabric==1.12.0
Markdown==2.6.7

>>> 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