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 实际得到输出的结果很大程度上取决于num等于什么，但为了理解这一特定的代码行，必须分解各个函数。 
 
 首先，内部有两个使用map函数使用字符串创建的可迭代地图。 
 
 第一个是从变量num到0的所有数字，不包括在内。 所以如果num是4，那么这张地图会有num和1个。 
 
 第二张地图从2开始计数至num + 1 ，不包括在内。 所以如果num是4，那么它会在地图上是2,3和4。 
 
 接下来，这两个地图都用空格连接起来，这样每个地图都会变成一个在每个值之间都有空格的字符串。 前面例子中的第一张地图是"4 3 2 1" ，第二张地图是"2 3 4" 。 +然后将这两个字符串连接在一起，以便它们成为单个字符串，这可以从我们的示例"4 3 2 12 3 4" 。 
 
 最后，len找到这个组合字符串的长度，并将该值保存到string_width ，在我们的示例中该值为12，因为那是多少个字符（它计数空格）。 

What actually gets output is very dependent on what num equals, but to understand this specific line of code you must break down the individual functions.
 
First, internally there are two iterable maps created with strings using the map function.
 
The first is all the numbers from the variable num counting down to 0, non inclusive. So if num was 4, this map would have 4, 3, 2, and 1 in it.
 
The second map starts at 2 and counts up to num + 1, non inclusive. So if num was 4, it would be 2, 3, and 4 in the map.
 
Next, both of the maps are joined with spaces so each one becomes a string with spaces in between each of its values. The first map from the previous example would be "4 3 2 1" and the second would be "2 3 4". The + then concatenates these two strings together so they become a single string, which would be from our examples "4 3 2 12 3 4".
 
Finally, len finds the length of this combined string and that value is saved into string_width, which in our example would be 12, because that is how many characters there are (it counts spaces).