英文:
sum() function in Python
问题
我对以下语法感到困惑：
def getClassiness(self):
classiness_points = {
"tophat": 2,
"bowtie": 4,
"monocle": 5
}
classiness = sum(classiness_points.get(item, 0) for item in self.items)
return classiness
我理解以下的代码，但两者都是正确的。
def getClassiness(self):
classiness_points = {
"tophat": 2,
"bowtie": 4,
"monocle": 5
}
classiness = 0
for item in self.items:
classiness += classiness_points.get(item, 0)
return classiness
我猜我不理解的是生成器表达式的工作原理。我理解sum()接受一个可迭代对象作为参数并返回元素的总和。我本以为应该是这样的：
classiness = sum(classiness_points.get(item, 0) for item in self.items)
return classiness
更好的写法应该是这样的：
classiness = sum(classiness_points.get(item, 0) for item in self.items)
return classiness
因为我习惯将表达式放在for循环之后。这可能只是语法问题。
英文:
I am confused with the following syntax:
def getClassiness(self):
classiness_points = {
"tophat": 2,
"bowtie": 4,
"monocle": 5
}
classiness = sum(classiness_points.get(item, 0) for item in self.items)
return classiness
I understand the following one, but both are correct.
def getClassiness(self):
classiness_points = {
"tophat": 2,
"bowtie": 4,
"monocle": 5
}
classiness = 0
for item in self.items:
classiness += classiness_points.get(item, 0)
return classiness
I guess what I do not understand is how a generator expression works. I understand that sum() takes an iterable as its argument and returns the sum of the elements. I would have thought of:
classiness = sum(classiness_points.get(item, 0) for item in self.items)
return classiness
better this way,
classiness = sum( for item in self.items: classiness_points.get(item, 0))
return classiness
Because I am used to place the expression after the for loop
It must be just syntax.
答案1
得分: 3
以下是翻译好的部分：
classiness_points = {'tophat': 2, 'bowtie': 4, 'monocle': 5}
items = ['tophat', 'monocle', 'shirt']
生成器的作用几乎与列表推导式相同：
[classiness_points.get(item, 0) for item in items]
# [2, 5, 0]
除了它只在被请求时生成项目，sum 通过迭代生成器的项目来执行这一操作：
sum(classiness_points.get(item, 0) for item in items)
# 7
因此，这与以下代码大致等效：
sum([classiness_points.get(item, 0) for item in items])
不同之处在于，在一种情况下，Python 预先生成一个要传递给 sum 的列表，而在另一种情况下，sum 本身通过迭代生成器的元素来执行操作。
英文:
Assuming this input:
classiness_points = {'tophat': 2, 'bowtie': 4, 'monocle': 5}
items = ['tophat', 'monocle', 'shirt']
What a generator does is almost the same as a list comprehension:
[classiness_points.get(item, 0) for item in items]
# [2, 5, 0]
Except it only produces the items when requested, which sum is doing by iterating over the generator's items.
sum(classiness_points.get(item, 0) for item in items)
# 7
So, this is more or less equivalent to:
sum([classiness_points.get(item, 0) for item in items])
Except in one case, python pre-generates a list to pass to sum. In the other sum is iterating by itself over the generators' elements.
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