在python中将数据写入CSV文件

Question

My code below checks the validity of postcodes with a regular expression. The postcodes are given in a form of an array. The for loop checks the validity of each postcode with the given regular expression. Now I want to write all the valid postcodes into a csv file. Below is my code:

import csv
import re

regex = r"(GIR\s0AA)|((([A-PR-UWYZ][0-9][0-9]?)|(([A-PR-UWYZ][A-HK-Y][0-9]((BR|FY|HA|HD|HG|HR|HS|HX|JE|LD|SM|SR|WC|WN|ZE)[0-9])[0-9])|([A-PR-UWYZ][A-HK-Y](AB|LL|SO)[0-9])|(WC[0-9][A-Z])|(([A-PR-UWYZ][0-9][A-HJKPSTUW])|([A-PR-UWYZ][A-HK-Y][0-9][ABEHMNPRVWXY]))))\s[0-9][ABD-HJLNP-UW-Z]{2})"

postcodes = ['$%±()()','XX XXX','A1 9A','LS44PL','Q1A 9AA','V1A 9AA','X1A 9BB','LI10 3QP','LJ10 3QP','LZ10 3QP','A9Q 9AA','AA9C 9AA','FY10 4PL','SO1 4QQ','EC1A 1BB','W1A 0AX','M1 1AE','B33 8TH','CR2 6XH','DN55 1PT','GIR 0AA','SO10 9AA','FY9 9AA','WC1A 9AA']


for x in postcodes:
    if(re.findall(regex,x)):
        with open('test2.csv','w',newline='') as fp:
            a = csv.writer(fp)
            a.writerows(x)

The problem with the code is it does not write all the valid postcodes into the csv file, instead it only write the last postcode (WC1A 9AA) in the following format:

W
C
1
A 

9
A
A

I don't know where I am making the mistake. Please help.

Answer 1

There a few issues but the biggest one is the 'w' -- you're wiping out the file each time you write to it! :) Change that to an 'a' for append.

Secondly I'm not sure what you're attempting to do, but if you're trying to write them all on seperate rows

codes = []
for x in postcodes:
    if(re.findall(regex,x)):
       codes.append([x])

with open('test2.csv','w',newline='') as fp:
    a = csv.writer(fp)
    a.writerows(codes)

Answer 2

With the open file command set with flag "w" in the Loop, it delete and creates a new file in every iteration, hence you only getting the last postcode. This should get you the correct result:

with open('test2.csv','w',newline='') as fp:
    for x in postcodes:
        if(re.findall(regex,x)):
            a = csv.writer(fp)
            a.writerows(x)

Yeah, I forgot. "fp.close()" unnessesary with the "with open" statement.

Answer 3

如@jcfollower所建议的那样，在open函数中将'w'更改为'a'模式以附加而不是覆盖每个循环中的文件，或仅将with上下文管理器中的for循环移动到with上下文管理器中。