Python:列表理解背后的机制

问题描述

在for循环上下文中使用列表理解或in关键字时，即:

When using list comprehension or the in keyword in a for loop context, i.e:

for o in X:
    do_something_with(o)

或

l=[o for o in X]

• in背后的机制如何工作?
• 它调用X中的哪些函数\方法?
• 如果X可以遵循多种方法，那么优先级是多少?
• 如何编写高效的X，以便快速理解列表?
• How does the mechanism behind in works?
• Which functions\methods within X does it call?
• If X can comply to more than one method, what's the precedence?
• How to write an efficient X, so that list comprehension will be quick?

推荐答案

答案是正确的.

for在for循环和列表推导中都在X上调用iter().如果X具有__iter__方法或__getitem__方法，则iter()将返回可迭代.如果两者都实现，则使用__iter__.如果没有，您将得到TypeError: 'Nothing' object is not iterable.

for, both in for loops and list comprehensions, calls iter() on X. iter() will return an iterable if X either has an __iter__ method or a __getitem__ method. If it implements both, __iter__ is used. If it has neither you get TypeError: 'Nothing' object is not iterable.

这实现了__getitem__:

class GetItem(object):
    def __init__(self, data):
        self.data = data

    def __getitem__(self, x):
        return self.data[x]

用法:

>>> data = range(10)
>>> print [x*x for x in GetItem(data)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

这是实现__iter__的示例:

class TheIterator(object):
    def __init__(self, data):
        self.data = data
        self.index = -1

    # Note: In  Python 3 this is called __next__
    def next(self):
        self.index += 1
        try:
            return self.data[self.index]
        except IndexError:
            raise StopIteration

    def __iter__(self):
        return self

class Iter(object):
    def __init__(self, data):
        self.data = data

    def __iter__(self):
        return TheIterator(data)

用法:

>>> data = range(10)
>>> print [x*x for x in Iter(data)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

如您所见，您既需要实现一个迭代器，又需要返回迭代器的__iter__.

As you see you need both to implement an iterator, and __iter__ that returns the iterator.

您可以将它们组合在一起:

You can combine them:

class CombinedIter(object):
    def __init__(self, data):
        self.data = data

    def __iter__(self):
        self.index = -1
        return self

    def next(self):
        self.index += 1
        try:
            return self.data[self.index]
        except IndexError:
            raise StopIteration

用法:

>>> well, you get it, it's all the same...

但是，一次只能有一个迭代器.好的，在这种情况下，您可以这样做:

But then you can only have one iterator going at once.OK, in this case you could just do this:

class CheatIter(object):
    def __init__(self, data):
        self.data = data

    def __iter__(self):
        return iter(self.data)

但这是作弊，因为您只是在重用list的__iter__方法.一种更简单的方法是使用yield，并将__iter__用作生成器:

But that's cheating because you are just reusing the __iter__ method of list.An easier way is to use yield, and make __iter__ into a generator:

class Generator(object):
    def __init__(self, data):
        self.data = data

    def __iter__(self):
        for x in self.data:
            yield x

这是我推荐的方式.简单高效.

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