Question

I have the following code which is intended to remove specific lines of a file. When I run it, it prints the two filenames that live in the directory, then deletes all information in them. What am I doing wrong? I’m using Python 3.2 under Windows.

import os

files = [file for file in os.listdir() if file.split(".")[-1] == "txt"]

for file in files:
    print(file)
    input = open(file,"r")
    output = open(file,"w")

    for line in input:
        print(line)
        # if line is good, write it to output

    input.close()
    output.close()

Answer 1

If your file fits in memory, the easiest solution is to open the file for reading, read its contents to memory, close the file, open it for writing and write the filtered output back:

with open(file_name) as f:
    lines = list(f)
# filter lines
with open(file_name, "w") as f:      # This removes the file contents
    f.writelines(lines)

Since you are not intermangling read and write operations, the advanced file modes like "r+" are unnecessary here, and only compicate things.

If the file does not fit into memory, the usual approach is to write the output to a new, temporary file, and move it back to the original file name after processing is finished.