Python program to delete a specific line in a text file [duplicate]

Question

I have a text file Thailand_Rectangle2_National Parks.txt with the following lines.

1
2
3
4
5
dy 0.5965
7

Now, I want to delete the 6th line in this text file.

For that, I am using the following python code.

f = open("C:/Users/Sreeraj/Desktop/Thailand_Rectangle2_National Parks.txt","r")
lines = f.readlines()

So, I saved all the lines of this text file in 'lines'.

line6 = lines[5]
t = line6[:2]
f.close() 

So, now, I have 't' = "dy" . Now,

if t == "dy":
    f = open("C:/Users/Sreeraj/Desktop/Thailand_Rectangle2_National Parks.txt","w")
    for line in lines:
        if lines[5] != line6:
            f.write(line)

f.close()

So, if the condition 't' = "dy" satisfies, then I will open this text file for writing and I will print all the lines except line6 (which means line6 is deleted from this text file).

Unfortunately, I am getting the lines in this text file as blank, which means no lines are printed as outputs.

But, I want the lines in the text file as given below.

1
2
3
4
5
7

How can I solve this issue ?

I want to use only Python programming to solve this issue; since this is a small task of a major work.

Answer 1

Your problem is that lines[5] will always be equal to line6. You never modified the sixth line in lines, so line6 and lines[5] are still equal. Thus, the condition lines[5] != line6 will always fail.

If you want to always remove the sixth line from your file, you can use enumerate. For example:

with open("file.txt", "r") as infile:
    lines = infile.readlines()

with open("file.txt", "w") as outfile:
    for pos, line in enumerate(lines):
        if pos != 5:
            outfile.write(line)

Answer 2

The error in your code was already pointed out, but instead of comparing the content of each line, I recommend you simply compare the line number or use startswith. Otherwise you are doing a lot of unneeded string comparisons, which can be costly.

Other improvements could be to handle your file using with, opening the file only once and allowing to delete multiple lines at once.

# 'r+' allows you to read and write to a file
with open("test.txt", "r+") as f:
    # First read the file line by line
    lines = f.readlines()

    # Go back at the start of the file
    f.seek(0)

    # Filter out and rewrite lines
    for line in lines:
        if not line.startswith('dy'):
            f.write(line)

    # Truncate the remaining of the file
    f.truncate()