Python初学者教程：for循环和迭代器

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>>>languages = ["C", "C++", "Perl", "Python"] 
>>> for x in languages:
...     print (x)
... 
C
C++
Perl
Python

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>>>list=[1,2,3,4]
>>> it = iter(list)    # 创建迭代器对象
>>> print (next(it))   # 输出迭代器的下一个元素
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>>> print (next(it))
2
>>>

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for x in y:
    z(x,y)

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for i in range(4):
    print i

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0
1
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3

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help(range)

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for s in "hello":
    print s

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h
e
l
l
o

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for x in [None,3,4.5,"foo",lambda : "moo",object,object()]:
    print "{0}  ({1})".format(x,type(x))

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None  (<type 'NoneType'>)
3  (<type 'int'>)
4.5  (<type 'float'>)
foo  (<type 'str'>)
<function <lambda> at 0x7feec7fa7578>  (<type 'function'>)
<type 'object'>  (<type 'type'>)
<object object at 0x7feec7fcc090>  (<type 'object'>)

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for x in (None,3,4.5,"foo",lambda : "moo",object,object()):
    print "{0}  ({1})".format(x,type(x))

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None  (<type 'NoneType'>)
3  (<type 'int'>)
4.5  (<type 'float'>)
foo  (<type 'str'>)
<function <lambda> at 0x7feec7fa7578>  (<type 'function'>)
<type 'object'>  (<type 'type'>)
<object object at 0x7feec7fcc090>  (<type 'object'>)

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d = {
  'apples' : 'tasty',
  'bananas' : 'the best',
  'brussel sprouts' : 'evil',
  'cauliflower' : 'pretty good'
}

for sKey in d:
  print "{0} are {1}".format(sKey,d[sKey])

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brussel sprouts are evil
apples are tasty
cauliflower are pretty good
bananas are the best

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'__iter__' in dir([1,2,3])           # returns True
'__iter__' in dir((1,2,3))           # returns True
'__iter__' in dir(range(3))      # returns True
'__iter__' in dir(3)                   # returns False

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for i in 3:
    print i

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TypeError: 'int' object is not iterable

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my_list = [1,2,3]
for i in my_list:
    print i

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1
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3

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my_iterator = my_list.__iter__()

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while True:
    print my_iterator.next()  

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这打印出这样的东西：

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Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
 StopIteration

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class MyNumbers:
  def __iter__(self):
    self.a = 1
    return self
 
  def __next__(self):
    x = self.a
    self.a += 1
    return x
 
myclass = MyNumbers()
myiter = iter(myclass)
 
print(next(myiter))
print(next(myiter))
print(next(myiter))
print(next(myiter))
print(next(myiter))

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1
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5

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class MyNumbers:
  def __iter__(self):
    self.a = 1
    return self
 
  def __next__(self):
    if self.a <= 20:
      x = self.a
      self.a += 1
      return x
    else:
      raise StopIteration
 
myclass = MyNumbers()
myiter = iter(myclass)
 
for x in myiter:
  print(x)

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