282 lines
7.7 KiB
Markdown
282 lines
7.7 KiB
Markdown
# 如何在 Python 中创建迭代器
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> 原文：<https://www.pythonforbeginners.com/basics/how-to-create-an-iterator-in-python>
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迭代器用于以顺序方式访问可迭代对象的元素。我们可以为任何容器对象创建一个迭代器，比如一个 [python 字典](https://www.pythonforbeginners.com/dictionary/how-to-use-dictionaries-in-python/)、列表、元组或集合。在本文中，我们将讨论如何在 python 中创建迭代器。我们还将学习如何通过重写内置方法来创建自定义迭代器。
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## 使用内置方法创建迭代器
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我们可以使用 iter()函数和 __next__()方法创建一个迭代器。iter()函数接受一个容器对象，比如 list、tuple 或 set，并返回一个迭代器，通过它我们可以访问容器对象的元素。
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要使用 iter()函数为任何容器对象创建迭代器，我们只需将对象传递给 iter()函数。该函数创建一个迭代器并返回对它的引用。我们可以使用 iter()函数创建一个迭代器，如下所示。
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```py
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myList = [1, 2, 3, 4, 5, 6, 7]
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myIter = iter(myList)
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print("The list is:", myList)
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print("The iterator is:", myIter)
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```
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输出:
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```py
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The list is: [1, 2, 3, 4, 5, 6, 7]
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The iterator is: <list_iterator object at 0x7f73fed18070>
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```
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在输出中，您可以看到在执行 iter()函数时创建了一个 list_iterator 对象。
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## 如何从迭代器中访问元素？
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要使用序列中的迭代器访问容器对象的元素，我们可以使用如下的 for 循环。
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```py
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myList = [1, 2, 3, 4, 5, 6, 7]
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myIter = iter(myList)
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print("The list is:", myList)
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print("The elements in the iterator are:")
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for i in myIter:
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print(i)
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```
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输出:
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```py
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The list is: [1, 2, 3, 4, 5, 6, 7]
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The elements in the iterator are:
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1
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2
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3
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4
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5
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6
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7
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```
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如果需要逐个访问元素，可以使用 next()函数或 __next()方法。
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为了使用 next()函数遍历迭代器，我们将迭代器作为输入参数传递给函数。它返回迭代器中的下一个元素。next()函数还会记住迭代器上次遍历的索引。当再次调用它时，它返回尚未遍历的下一个元素。这可以在下面的例子中观察到。
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```py
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myList = [1, 2, 3, 4, 5, 6, 7]
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myIter = iter(myList)
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print("The list is:", myList)
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print("The elements in the iterator are:")
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element = next(myIter)
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print(element)
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element = next(myIter)
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print(element)
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element = next(myIter)
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print(element)
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element = next(myIter)
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print(element)
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element = next(myIter)
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print(element)
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element = next(myIter)
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print(element)
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element = next(myIter)
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print(element)
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```
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输出:
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```py
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The list is: [1, 2, 3, 4, 5, 6, 7]
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The elements in the iterator are:
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1
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2
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3
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4
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5
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6
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7
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Process finished with exit code 0
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```
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__next__()方法的工作方式与 next()函数类似。每当在迭代器上调用 __next__()方法时，它都会返回尚未遍历的下一个元素。
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```py
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myList = [1, 2, 3, 4, 5, 6, 7]
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myIter = iter(myList)
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print("The list is:", myList)
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print("The elements in the iterator are:")
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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```
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输出:
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```py
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The list is: [1, 2, 3, 4, 5, 6, 7]
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The elements in the iterator are:
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1
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2
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3
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4
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5
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6
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7
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Process finished with exit code 0
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```
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当没有元素可供遍历，并且我们使用 next()函数或 __next__()方法时，它会引发 StopIteration 异常。
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```py
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myList = [1, 2, 3, 4, 5, 6, 7]
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myIter = iter(myList)
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print("The list is:", myList)
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print("The elements in the iterator are:")
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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```
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输出:
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```py
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Traceback (most recent call last):
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File "/home/aditya1117/PycharmProjects/pythonProject/webscraping.py", line 19, in <module>
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element = myIter.__next__()
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StopIteration
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```
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建议在 [python try except](https://www.pythonforbeginners.com/error-handling/python-try-and-except) 块中使用这些函数来避免异常。
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## 如何在 Python 中创建自定义迭代器
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要创建自定义迭代器，我们可以覆盖设置为 default 的 __iter__()和 __next__()方法。我们将通过下面的例子来理解如何创建一个自定义迭代器。
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假设您想要创建一个迭代器，当我们使用迭代器遍历对象时，它返回列表或元组中每个元素的平方。为此，我们将重写 __iter__()方法和 __next__()方法。
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迭代器的构造函数将接受列表或元组以及其中的元素总数。然后它将初始化迭代器类。因为我们需要跟踪被遍历的最后一个元素，我们将初始化一个索引字段，并将其设置为 0。
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```py
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class SquareIterator:
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def __init__(self, data, noOfElements):
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self.data = data
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self.noOfElements = noOfElements
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self.count = 0
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```
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方法的作用是初始化一个迭代器。__iter__()方法实现如下。
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```py
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class SquareIterator:
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def __init__(self, data, noOfElements):
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self.data = data
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self.noOfElements = noOfElements
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self.count = 0
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def __iter__(self):
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return self
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```
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在重写 __iter__()方法之后，我们将重写 __next__()方法。__next__()方法用于遍历尚未遍历的下一个元素。这里，我们需要返回下一个元素的平方。
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因为我们已经在 __iter__()方法中将索引初始化为-1，所以我们将首先递增索引。之后，我们将返回指定索引处元素的平方。每次调用 __next__()方法时，索引都会递增，指定索引处的元素的平方将作为输出给出。当遍历的元素数量等于列表或元组中元素的总数时，我们将引发 StopIteration 异常。它将停止迭代。__next__()方法可以定义如下。
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```py
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class SquareIterator:
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def __init__(self, data, noOfElements):
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self.data = data
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self.noOfElements = noOfElements
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self.count = 0
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def __iter__(self):
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return self
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def __next__(self):
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if self.count < self.noOfElements:
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square = self.data[self.count] ** 2
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self.count = self.count + 1
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return square
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else:
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raise StopIteration
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```
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创建迭代器的整个程序如下。
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```py
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class SquareIterator:
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def __init__(self, data, noOfElements):
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self.data = data
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self.noOfElements = noOfElements
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self.count = 0
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def __iter__(self):
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return self
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def __next__(self):
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if self.count < self.noOfElements:
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square = self.data[self.count] ** 2
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self.count = self.count + 1
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return square
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else:
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raise StopIteration
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myList = [1, 2, 3, 4, 5, 6, 7]
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myIter = SquareIterator(myList, 7)
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print("The list is:", myList)
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print("The elements in the iterator are:")
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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element = myIter.__next__()
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print(element)
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```
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输出:
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```py
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The list is: [1, 2, 3, 4, 5, 6, 7]
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The elements in the iterator are:
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1
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4
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9
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16
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25
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36
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49
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Process finished with exit code 0
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```
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## 结论
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在本文中，我们研究了在 Python 中创建迭代器的两种方法。在遍历容器对象时，可以使用自定义迭代器对容器对象的元素执行不同的操作。建议实现一些程序来创建迭代器，以便更好地理解这个主题。 |