[Python]for循环是怎样工作的

>>> dir(range)
[&＃39;__class__&＃39;, &＃39;__contains__&＃39;, &＃39;__delattr__&＃39;, &＃39;__dir__&＃39;, &＃39;__doc__&＃39;, &＃39;__eq__&＃39;, &＃39;__format__&＃39;,   
&＃39;__ge__&＃39;, &＃39;__getattribute__&＃39;, &＃39;__getitem__&＃39;, &＃39;__gt__&＃39;, &＃39;__hash__&＃39;, &＃39;__init__&＃39;, &＃39;__iter__&＃39;,   
&＃39;__le__&＃39;, &＃39;__len__&＃39;, &＃39;__lt__&＃39;, &＃39;__ne__&＃39;, &＃39;__new__&＃39;, &＃39;__reduce__&＃39;, &＃39;__reduce_ex__&＃39;,   
&＃39;__repr__&＃39;, &＃39;__reversed__&＃39;, &＃39;__setattr__&＃39;, &＃39;__sizeof__&＃39;, &＃39;__str__&＃39;, &＃39;__subclasshook__&＃39;,   
&＃39;count&＃39;, &＃39;index&＃39;, &＃39;start&＃39;, &＃39;step&＃39;, &＃39;stop&＃39;]  

>>> dir(str)
[&＃39;__add__&＃39;, &＃39;__class__&＃39;, &＃39;__contains__&＃39;, &＃39;__delattr__&＃39;, &＃39;__dir__&＃39;, &＃39;__doc__&＃39;, &＃39;__eq__&＃39;,   
&＃39;__format__&＃39;, &＃39;__ge__&＃39;, &＃39;__getattribute__&＃39;, &＃39;__getitem__&＃39;, &＃39;__getnewargs__&＃39;, &＃39;__gt__&＃39;,   
&＃39;__hash__&＃39;, &＃39;__init__&＃39;, &＃39;__iter__&＃39;, &＃39;__le__&＃39;, &＃39;__len__&＃39;, &＃39;__lt__&＃39;, &＃39;__mod__&＃39;, &＃39;__mul__&＃39;, 
&＃39;__ne__&＃39;, &＃39;__new__&＃39;, &＃39;__reduce__&＃39;, &＃39;__reduce_ex__&＃39;, &＃39;__repr__&＃39;, &＃39;__rmod__&＃39;, &＃39;__rmul__&＃39;, 
&＃39;__setattr__&＃39;, &＃39;__sizeof__&＃39;, &＃39;__str__&＃39;, &＃39;__subclasshook__&＃39;, &＃39;capitalize&＃39;, &＃39;casefold&＃39;, 
&＃39;center&＃39;, &＃39;count&＃39;, &＃39;encode&＃39;, &＃39;endswith&＃39;, &＃39;expandtabs&＃39;, &＃39;find&＃39;, &＃39;format&＃39;, &＃39;format_map&＃39;,
 &＃39;index&＃39;, &＃39;isalnum&＃39;, &＃39;isalpha&＃39;, &＃39;isdecimal&＃39;, &＃39;isdigit&＃39;, &＃39;isidentifier&＃39;, &＃39;islower&＃39;, 
&＃39;isnumeric&＃39;, &＃39;isprintable&＃39;, &＃39;isspace&＃39;, &＃39;istitle&＃39;, &＃39;isupper&＃39;, &＃39;join&＃39;, &＃39;ljust&＃39;, &＃39;lower&＃39;, 
&＃39;lstrip&＃39;, &＃39;maketrans&＃39;, &＃39;partition&＃39;, &＃39;replace&＃39;, &＃39;rfind&＃39;, &＃39;rindex&＃39;, &＃39;rjust&＃39;, &＃39;rpartition&＃39;, 
&＃39;rsplit&＃39;, &＃39;rstrip&＃39;, &＃39;split&＃39;, &＃39;splitlines&＃39;, &＃39;startswith&＃39;, &＃39;strip&＃39;, &＃39;swapcase&＃39;, &＃39;title&＃39;,
 &＃39;translate&＃39;, &＃39;upper&＃39;, &＃39;zfill&＃39;]

查看这两个的共有属性

>>> set(dir(range)) & set(dir(str))
{&＃39;__hash__&＃39;, &＃39;__eq__&＃39;, &＃39;__contains__&＃39;, &＃39;__iter__&＃39;, &＃39;__getitem__&＃39;, &＃39;count&＃39;, &＃39;__lt__&＃39;, 
&＃39;__dir__&＃39;, &＃39;__le__&＃39;, &＃39;__subclasshook__&＃39;, &＃39;__ge__&＃39;, &＃39;__sizeof__&＃39;, &＃39;__format__&＃39;, &＃39;__len__&＃39;, 
&＃39;__ne__&＃39;, &＃39;__getattribute__&＃39;, &＃39;__delattr__&＃39;, &＃39;__reduce_ex__&＃39;, &＃39;__gt__&＃39;, &＃39;__reduce__&＃39;, 
&＃39;__setattr__&＃39;, &＃39;__doc__&＃39;, &＃39;__class__&＃39;, &＃39;__new__&＃39;, &＃39;__repr__&＃39;, &＃39;__init__&＃39;, &＃39;index&＃39;, &＃39;__str__&＃39;}

我们关注__iter__属性，他们两个都有这个函数，如果你查看其他可以使用for循环迭代的对象，你都可以发现这个特殊方法。
实现了这个方法的对象我们称之为iterable。
我们把对象传给Python内置的iter()方法，会返回一个迭代器，for循环就是使用这个模式来实现适用于所有的对象。
比如:

>>> iter([1, 2])

>>> iter(range(0, 10))

>>> iter("abc")

>>>
iter函数返回的对象我们称之为iterator，iterator只需要做一件事，那就是调用next(iterator)方法，返回下一个元素。

举例:

>>> t = iter("abc")
>>> next(t)
&＃39;a&＃39;
>>> next(t)
&＃39;b&＃39;
>>> next(t)
&＃39;c&＃39;
>>> next(t)
Traceback (most recent call last):
  File "", line 1, in 
StopIteration

当迭代器再没有元素可以迭代时会引发一个异常。
那么在这里我给出itrable和iterator的定义。
iterable:
可以传给iter并返回一个iteratot的对象
iterator:
可以传给next函数并返回下一个迭代元素的对象，并在迭代结束引发一个异常。

因此，对于你提的例子我们使用迭代器来重新定义一下。

>>> t = iter(range(90, 0, -1))
>>> t

>>> next(t)
90
>>> next(t)
89
>>> next(t)
88

希望看完有所收获。

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