更改列表的最后一个元素也会覆盖倒数第二个元素

Question

I have a csv-file along the following structure:

1 start,end,ID
2 int1,int2,string1
3 int3,int4,string2
4 int5,int6,string3
5 int7,int8,string4

My goal is to create a new csv that writes a unique row for, first, the end value and, second, the start value as an end value.

1 start,end,ID
2 int1,int2,string1
3 ,int1,string1
4 int3,int4,string2
5 ,int3,string2

I attempted this by writing the input csv in a list and iterating through that list. For each row, two new rows are appended in the output list. After appending the second row, respectively, the end value was set to the start value of the input list. The following is the code that I used:

import csv

with open(r"input path") as csv_sbw, open("output path","wb") as csv_new:
    csv_in = csv.reader(csv_sbw)
    csv_out = csv.writer(csv_new)
    fields_out = [[]] #list for the output csv
    fields = list(csv_in) #list for the input csv
    fields_out[0] = fields[0] #headline is taken from the input
    fields[0].append("m_value")
    for row in fields[1:]:
        row.append(1)
        if row[2].isdigit() == False and len(row[2]) == 16 and row[2][0] != 0 and row[0] != '' and row[0] != '0' and row[1] != '0': #invalid rows are skipped
            fields_out.append(row) #first row is appended
            fields_out.append(row) #second row is appended
            fields_out[-1][1] = row[0] #the start value of the last appended row is set as an end value
            fields_out[-1][0] = '' #start field of last appended row is deleted
            fields_out[-1][3] = 0
    csv_out.writerows(fields_out) #output csv is written

Instead of producing a csv along the example described above, I get the following result:

1 start,end,ID,m_value
2 1032,1032,'A',0
3 1032,1032,'A',0
4 613,613,'B',0
5 613,613,'B',0

So, by changing fields[-1] the code seems to overwrite the second last appended row, as well. As far as I understand, appending two values in a row to a list creates two new list elements of which only the last appended value would be returned if I requested list[-1] . How do I prevent the code from overwriting both appended rows and instead let it overwrite only the last appended row?

Answer 1

To restate your objective, the snippet below creates a new csv that

1. writes a row for the original line and,
2. writes a row with the start value and the string.

If that assessment is valid, I usually open one file at a time to minimize my cognitive burden.

with open('input.csv','r') as file_handle:
    file_content = file_handle.read().split('\n')
with open('output.csv','r') as file_handle:
    for index,line in enumerate(file_content):
        if index==0:
            print(line)
            file_handle.write(line)
        else:
            line_as_list = line.split(',')
            print(line_as_list)
            file_handle.write(line)
            print(line_as_list[0], line_as_list[-1])
            file_handle.write(str(line_as_list[0])+","+str(line_as_list[-1]))