for循环评估其表达式列表多少次? [英] How many times does a for loop evaluate its expression list?

问题描述

我有以下代码:

list1 = [4,1,2,6]
for elem in sorted(list1):
    #some task

我想知道Python使用sorted()方法对列表进行多少次排序?是每次迭代还是一次?

I was wondering how many times does Python sort the list using the sorted() method? Is it for each iteration or just once?

换句话说，下面的代码更有效吗?

In other words is the following code more efficient?

list1 = [4,1,2,6]
list1 = sorted(list1)
for elem in list1:
    #some task 

推荐答案

在两个代码示例中，列表仅排序一次.

In both code examples, the list is sorted only once.

for循环头文件中in运算符右侧的表达式在循环开始之前仅被评估一次.您可以在文档:

The expression on the right of the in operator in a for-loop header is evaluated just once, before looping commences. You can read about this in the documentation:

for_stmt ::=  "for" target_list "in" expression_list ":" suite
              ["else" ":" suite]

表达式列表被评估一次；它应该产生一个可迭代的 对象.

The expression list is evaluated once; it should yield an iterable object.

实际上，第二个代码示例效率更高，因为没有不必要的分配.

In fact, the second code example is less efficient because there is an unnecessary assignment.

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