Deleting a line from a Python text file

Question

While I was learning how to work with files in Python, I had a question: How can you delete a line from a file that contains a specific word. I wrote the following code:

arr = []
try:
    with open("test.txt") as file:
        arr = file.readlines()
except FileNotFoundError:
    print("File not found!")

word = "five"
try:
    with open("test.txt", "w") as file:
        for row in arr:
            if word not in row:
                file.write(row)
except FileNotFoundError:
    print("File not found!")

But I would like to know if it is possible to do this without writing all the lines in one array, because the file can sometimes be very large and there can be a lack of memory.

Asked By: Bohdan

||

Source

Answer 1

You can write as you read instead of storing

Consider the following:

try:
    with open("test.txt") as file:
        with open("new_test.txt", "w") as new_file:
            for row in file:
                if word not in row:
                    new_file.write(row)
except FileNotFoundError:
    print("File not found!")

Answered By: Achille G

Answer 2

This should do the trick:

try:
    with open("test.txt") as file:
        arr = file.readlines()
except IOError:
    from sys import exit as exit_the_progrem
    try:
        exit_the_progrem(__status=-1)
    except:
        import sys
        exiter = sys
        del sys
        exiter.exit()
finally:
    try:
        condtion = True
        arr = ['hello', 'world']  ## or comment?
        locl_i = len(arr)
        for i, elem in enumerate(reversed(arr), start=1):
            locl_i -= i
            if condtion:  # example
                del i
                if not locl_i:
                    del locl_i
                del elem
                del arr[locl_i]
        print(arr)
        with open('out_f.xtxt', 'w') as of:
            of.write(arr)
    except Exception as __this_thing_went_wrong:
        print(__this_thing_went_wrong)
        from sys import exit as exit_the_progrem
        success = 123
        exit_the_progrem(~success)

Answered By: rv.kvetch

Answer 3

You just need to do the following:

open it in read and write mode.

Read it and filter the line out

move the cursor to the begin of the file

write it back

remove anything than is after it

with open("test.txt", "r+") as f: 
    output = filter(lambda line: word not in line, f.readlines()) 
    f.seek(0) 
    f.write(''.join(output)) 
    f.truncate()

To improve memory usage, just read the file line by line using f.readline() and adjust the seek dinamically.

with open("test.txt", "r+") as f: 
    latest_written_position = f.tell() 
    line = f.readline() 
    reading_position = f.tell() 
    while line: 
        if word not in line: 
            f.seek(latest_written_position) 
            f.write(line) 
            latest_written_position = f.tell() 
            f.seek(reading_position) 
        line = f.readline() 
        reading_position = f.tell() 
    f.seek(latest_written_position) 
    f.truncate()

Answered By: Axeltherabbit

Answer 4

reading a file line by line and writing it to the same file, without storing all the lines in memory:

word = "five"
try:
    with open("test.txt", "r") as file_in:
         with open("test.txt", "w") as file_out:
              for line in file_in:
                   if word not in line:
                       file_out.write(line)
except FileNotFoundError:
    print("File not found!")

Answered By: Mohammed Chaaraoui

Deleting a line from a Python text file

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