How is the iterator protocol defined in Python?

How is the iterator protocol in Python defined?

In Python, an iterator (Iterator) is an object that implements the Iterator Protocol (Iterator Protocol). The iterator protocol is a standard specification that defines the behavior of iterator objects. Objects that implement the iterator protocol can access elements one by one by using the iter() and next() functions.

The iterator protocol contains two methods:

1. __iter__(): Returns the iterator object itself. This method enables the iterator object to be used within a for loop.
2. __next__(): Returns the next element of the iterator. If there is no next element, a StopIteration exception should be thrown.

Let us understand the implementation of the iterator protocol through specific code examples.

class MyIterator:
    def __init__(self, data):
        self.data = data
        self.index = 0
    
    def __iter__(self):
        return self
    
    def __next__(self):
        if self.index >= len(self.data):
            raise StopIteration
        value = self.data[self.index]
        self.index += 1
        return value

# 创建一个可迭代对象
my_list = [1, 2, 3, 4, 5]
my_iterator = MyIterator(my_list)

# 使用for循环迭代
for item in my_iterator:
    print(item)

# 手动迭代
while True:
    try:
        item = next(my_iterator)
        print(item)
    except StopIteration:
        break

In the above example, we implemented a custom iterator class MyIterator. It receives a list as input data and returns the next element in the list each time the __next__() method is called. When iterating to the end of the list, a StopIteration exception will be thrown to inform the end of the iteration.

When using the MyIterator object, we can iterate the elements through the for loop, or we can use the next() function to iterate manually until it is thrown A StopIteration exception occurred.

It should be noted that only objects that implement the iterator protocol can be iterated. An object is iterable if it implements the __iter__() method and returns an iterator object. We can use the iter() function to get the iterator of this iterable object. In the above code example, we implemented the __iter__() method in the MyIterator class and returned the self object, making MyIteratorThe object itself becomes an iterable object.

In short, the iterator protocol is a specification that defines the behavior of iterator objects. In Python, we can implement iterator objects by implementing the __iter__() and __next__() methods to access elements one by one. The implementation of this iterator protocol provides us with a convenient, efficient and flexible iteration operation method.

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