如何编写新的csv文件作为另一个的扩展名？

Question

I want to write a new value to a csv file "hello.csv" to each row. It's the adjusted time zone for each row.

 with open('hello.csv', 'r', encoding="latin-1") as csvfile:
          readCSV = csv.reader(csvfile, delimiter=',')
          list1 = list(readCSV)
          for j in list1:
              dtstr=j[1] #string
              hours, minutes = [int(t) for t in tstr.split(':')]
              dt = datetime.strptime(dtstr, '%Y-%m-%d %H:%M:%S')         +timedelta(hours=hours+4, minutes=minutes)
              with open('helloout.csv', 'w', encoding="latin-1") as outfile:
                    outCSV = csv.writer(outfile,delimiter=',')
                    newline = j + [dt]
                    outCSV.writerow(newline)

When I use this code I become a csv file "helloout.csv" with only one row. It's the last row of "hello.csv" with the new time in a new column. I think it's overwriting the rows but I have no idea why?

Answer 1

Access modes govern the type of operations possible in the opened file. It refers to how the file will be used once its opened. In order to append a new line your existing file, you need to open the file in append mode , by setting "a" or "ab" as the mode.

When you open with "a" mode , the write position will always be at the end of the file (an append). There are other permutations of the mode argument for updating (+), truncating (w) and binary (b) mode but starting with just "a" is your best. If you want to seek through the file to find the place where you should insert the line, use 'r+'.

The following code append a text in the existing file:

with open("index.txt", "a") as myfile:
    myfile.write("text appended")

You can also use file access_mode "a+" for Open for reading and writing. The file is created if it does not exist. The stream is positioned at the end of the file . The initial file position for reading is at the beginning of the file, but output is appended to the end of the file.

with open("index.txt", "a+") as myfile:
    myfile.write("New text appended")

How to append new data onto a new line?

You can use "\\n" while writing data to file.

with open("index.txt", "a") as myfile:
    myfile.write("First Line\n")
    myfile.write("Second Line\n")

Your code:

with open('hello.csv', 'r', encoding="latin-1") as csvfile:
          readCSV = csv.reader(csvfile, delimiter=',')
          list1 = list(readCSV)
          for j in list1:
              dtstr=j[1] #string
              hours, minutes = [int(t) for t in tstr.split(':')]
              dt = datetime.strptime(dtstr, '%Y-%m-%d %H:%M:%S')         +timedelta(hours=hours+4, minutes=minutes)
              with open('helloout.csv', 'a', encoding="latin-1") as outfile:
                    outCSV = csv.writer(outfile,delimiter=',')
                    newline = j + [dt]
                    outCSV.writerow(newline)