Python中的Map方法 [英] Map method in python

问题描述

class FoodExpert:
    def init(self):
        self.goodFood = []
    def addGoodFood(self, food):
        self.goodFood.append(food)
    def likes(self, x):
        return x in self.goodFood
    def prefers(self, x, y):
        x_rating = self.goodFood.index(x)
        y_rating = self.goodFood.index(y)
        if x_rating > y_rating:
            return y
        else:
            return x

在声明了此类之后，我编写了以下代码:

After declaring this class , I wrote this code :

>>> f = FoodExpert()
>>> f.init()
>>> map(f.addGoodFood, ['SPAM', 'Eggs', 'Bacon', 'Rat', 'Spring Surprise'])
[None, None, None, None, None]

>>> f.goodFood
['SPAM', 'Eggs', 'Bacon', 'Rat', 'Spring Surprise']

我无法理解map函数是如何在后台运行的，为什么它返回包含所有None的列表，但是当我检查f.goodFood时，元素是否已添加到列表中?

I am unable to understand how the map function is working behind the hood , why is it returning a list with all None , but when I check f.goodFood the elements have been added there ?

推荐答案

map在可迭代对象上应用函数，并返回一个新列表，其中该函数应用于每个项目.

map applies a function on an iterable and returns a new list where the function was applied on each item.

在您的情况下，它显示None，因为f.addGoodFood函数什么都不返回.

In your case, it shows None because f.addGoodFood function returns nothing.

出于测试目的，请按以下方式更改addGoodFood:

For testing purposes change addGoodFood this way:

def addGoodFood(self, food):
    self.goodFood.append(food)
    return "test"

并查看:

>>> map(f.addGoodFood, ['SPAM', 'Eggs', 'Bacon', 'Rat', 'Spring Surprise'])
['test', 'test', 'test', 'test', 'test']

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