csv.reader / writer - 为什么迭代 CSV 对象会导致 writer 没有输出？

Question

I found this example of a simple CSV read / write .

This code reads and writes the CSV file, sorting on the 7th column.

import csv
with open('input.csv', newline='') as csvfile:
    rdr = csv.reader(csvfile)
    l = sorted(rdr, key=lambda x: x[6], reverse=True)
with open('output.csv', 'w') as csvout:
    wrtr = csv.writer(csvout)
    wrtr.writerows(l)

The code above outputs a CSV file into output.csv

If I need to do some additional processing other than sorting, I might do it by iterating through the rows of the CSV object. The simplest example exhibiting the problem is just iterating the rows and printing them:

import csv
with open('input.csv', newline='') as csvfile:
    rdr = csv.reader(csvfile)
    for arow in rdr:
        print(arow)
    l = sorted(rdr, key=lambda x: x[6], reverse=True)
with open('output.csv', 'w') as csvout:
    wrtr = csv.writer(csvout)
    wrtr.writerows(l)

The code above prints the CSV rows to the screen as expected. However, the output.csv file is then empty ( 0 bytes)

Why does iteration through the rows cause no output from csv.writer? Is there some type of persistent state in the CSV object that has to be reset?

Answer 1

As @Barmar correctly noted:

rdr is an iterator. You can only loop through an iterator once. Convert it to a list if you want to process it multiple times.

Working code:

import csv
with open('input.csv', newline='') as csvfile:
    rdr = csv.reader(csvfile)
    mylist = list(rdr)
    for arow in mylist:
        print(arow)
    l = sorted(mylist, key=lambda x: x[6], reverse=True)
with open('output.csv', 'w') as csvout:
    wrtr = csv.writer(csvout)
    wrtr.writerows(l)