在python中写入文件时没有多余的行

Question

I'm writing python script to read line from a input file and write a unique lines(if the same line is not already in output file) to output file. somehow, my scripts always append the first line of input file to output file even if the same line is already in output file. I can't figure out why this happens. can anyone know why and how do I fix this? thanks,

import  os

input_file= 'input.txt'
output_file = 'output.txt'

fo = open(output_file, 'a+')
flag = False
with open(input_file, 'r') as fi:
    for line1 in fi:
       print line1
       for line2 in fo:
           print line2
           if line2 == line1:
               flag = True
               print('Found Match!!')
               break
       if flag == False:
           fo.write(line1)
       elif flag == True:
           flag == False
       fo.seek(0)
    fo.close()
    fi.close()

Answer 1

When you open a file in append mode, the file object position is at the end of the file. So the first time through, when it reaches for line2 in fo: , there aren't any more lines in fo , so that block is skipped, and flag is still true, so that first line is written to the output file. After that, you do fo.seek(0) , so you are checking against the entire file for subsequent lines.

Answer 2

The answer by kmacinnis is right on as to why your code isn't working; you need to use mode 'r+' instead of 'a+' , or else put fo.seek(0) at the beginning of the for loop instead of the end.

That said, there's a much better way to do this than reading the entire output file for every line of the input file.

def ensure_file_ends_with_newline(handle):
    position = handle.tell()

    handle.seek(-1, 2)
    handle_end = handle.read(1)
    if handle_end != '\n':
        handle.write('\n')

    handle.seek(position)


input_filepath = 'input.txt'
output_filepath = 'output.txt'

with open(input_file, 'r') as infile, open(output_file, 'r+') as outfile:
    ensure_file_ends_with_newline(outfile)

    written = set(outfile)

    for line in infile:
        if line not in written:
            outfile.write(line)
            written.add(line)

Answer 3

Your flag was never set to False.

flag == True is an equality

flag = True is an assignment.

Try the latter.

import  os

input_file= 'input.txt'
output_file = 'output.txt'

fo = open(output_file, 'a+')
flag = False
with open(input_file, 'r') as fi:
    for line1 in fi:
       #print line1
       for line2 in fo:
           #print line2
           if line2 == line1:
               flag = True
               print('Found Match!!')
               print (line1,line2)
               break
       if flag == False:
           fo.write(line1)
       elif flag == True:
           flag = False
       fo.seek(0)