Python - How does the FOR loop work in python

Question

I have the following code in Python and I am an unsure about how the for loop here is working.

cur.execute("SELECT CUST_ID, COMPANY, LASTNAME, CITY, STATE FROM 
mytable")                            

colnames = [desc[0] for desc in cur.description]      

How does this for work ?

[desc[0] for desc in cur.description]   

What is desc[0 ]?

Answer 1

Based on the code you've given us it seems like desc is an array itself and thus colnames is an array of all the desc[0] you get from each desc in cur.description

desc[0] is the first item in desc as arrays in Python are zero-indexed.

Answer 2

它使用可迭代对象，尤其是下一个方法，它将消耗对象内部的值，直到停止迭代为止，它的作用类似于生成器对象...。

Answer 3

When in doubt of something, try to recreate the thing. Althought, it could be anything iterable, we could assume it's just a list, then:

description = [['first'], ['second', 'third'], [x, y, z], [...], ...]
colnames = [desc[0] for desc in description]

in the output you'll have: ['first', 'second', x, ... ]