[Python] How does the for loop work?

If you understand it from an iterative level, you may have a deeper understanding of how for works.
First, let’s use dir to see what the two different types of range and str have in common.

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

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

查看这两个的共有属性

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

We focus on the __iter__ attribute. Both of them have this function. If you look at other objects that can be iterated using a for loop, you can find this special method.
The object that implements this method is called iterable.
We pass the object to Python's built-in iter() method, which will return an iterator. The for loop uses this pattern to implement it applicable to all objects.
For example:

>>> iter([1, 2])
<list_iterator object at 0x000001A1141E0668>
>>> iter(range(0, 10))
<range_iterator object at 0x000001A1124C6BB0>
>>> iter("abc")
<str_iterator object at 0x000001A1141E0CF8>
>>>
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。

For example:

>>> t = iter("abc")
>>> next(t)
'a'
>>> next(t)
'b'
>>> next(t)
'c'
>>> next(t)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

When the iterator has no more elements to iterate, an exception will be thrown.
So here I give the definitions of itrable and iterator.
iterable:
Can be passed to iter and return an iteratot object
iterator:
Can be passed to the next function and return the object of the next iterated element, and throw an exception at the end of the iteration.

So, for the example you mentioned, we use iterators to redefine it.

>>> t = iter(range(90, 0, -1))
>>> t
<range_iterator object at 0x000001A1124C6BB0>
>>> next(t)
90
>>> next(t)
89
>>> next(t)
88

I hope to gain something from reading this.

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