August 17, 2018

Remove blank lines from file in python

By Mohammed Abualrob Algorithms and Data Structures, Code Snippets 1 Comment

Introduction

Removing blank lines from a file requires two steps. First, we need to loop through the file lines. Second, check if the line is empty. In Python, there are different ways to perform the loop and check.

Let us summarize that…

Loop method

There are multiple styles to iterate through file lines. Here are some examples…

# Style 1
with open('empty.txt','r') as f:
	for line in f:
		do

# Style 2
with open('empty.txt','r') as f:
	for line in f.readlines():
		do

# Style 3
for line in open('empty.txt'):
	do

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# Style 1

with open('empty.txt','r') as f:

for line in f:

do

# Style 2

with open('empty.txt','r') as f:

for line in f.readlines():

do

# Style 3

for line in open('empty.txt'):

do

Check for empty lines

Checking for blanks can be achieved in various ways. Take a look…

# Before checking we strip leading and trailing characters
# Check if the line is not empty
with open("empty.txt", 'r') as inp, open('out.txt', 'w') as out:
    for line in inp:
        if line.strip():
            out.write(line)

# Before checking we strip only trailing characters
# Check if the line is not empty
with open("empty.txt", 'r') as inp, open('out.txt', 'w') as out:
    for line in inp:
        if line.rstrip():
            out.write(line)

# Check if the length is greater than zero
with open("empty.txt", 'r') as inp, open('out.txt', 'w') as out:
    for line in inp:
        if len(line.strip()) > 0:
            out.write(line)

# Check if the line is not a new line
with open("empty.txt", 'r') as inp, open('out.txt', 'w') as out:
    for line in inp:
        if line != '\n':
            out.write(line)

# Check if the line is not a space using a built in function
with open("empty.txt", 'r') as inp, open('out.txt', 'w') as out:
    for line in inp:
        if not line.isspace():
            out.write(line)

# Check if the line is not white space using regular expressions
import re
with open("empty.txt", 'r') as inp, open('out.txt', 'w') as out:
    for line in inp:
        if re.search('\S', line):
            out.write(line)

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# Before checking we strip leading and trailing characters

# Check if the line is not empty

with open("empty.txt", 'r') as inp, open('out.txt', 'w') as out:

for line in inp:

if line.strip():

out.write(line)

# Before checking we strip only trailing characters

# Check if the line is not empty

with open("empty.txt", 'r') as inp, open('out.txt', 'w') as out:

for line in inp:

if line.rstrip():

out.write(line)

# Check if the length is greater than zero

with open("empty.txt", 'r') as inp, open('out.txt', 'w') as out:

for line in inp:

if len(line.strip()) > 0:

out.write(line)

# Check if the line is not a new line

with open("empty.txt", 'r') as inp, open('out.txt', 'w') as out:

for line in inp:

if line != '\n':

out.write(line)

# Check if the line is not a space using a built in function

with open("empty.txt", 'r') as inp, open('out.txt', 'w') as out:

for line in inp:

if not line.isspace():

out.write(line)

# Check if the line is not white space using regular expressions

import re

with open("empty.txt", 'r') as inp, open('out.txt', 'w') as out:

for line in inp:

if re.search('\S', line):

out.write(line)

One liner implementations

We can utilize Python elegance and write one liner implementations. Check out these…

# Open out.txt for writing
# Read empty.txt into a string
# Replace white space with new line characters
import re
open('out.txt','w').write(
    re.sub('\n\s*\n','\n', 
        open('empty.txt').read()))

# Get all lines from empty.txt and join them
# into one string then write to out.txt
open('out.txt','w').write(
    ''.join(
        l for l in open(
            'empty.txt') if l.strip()))

# Same as above but in this case the check
# is different. l[:-1] means the whole line 
# except the last character. New lines get
# removed this way. Note that this may not 
# work in case of blank lines with white space
open('out.txt','w').write(
    ''.join(
        l for l in open(
            'empty.txt') if l[:-1]))

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# Open out.txt for writing

# Read empty.txt into a string

# Replace white space with new line characters

import re

open('out.txt','w').write(

re.sub('\n\s*\n','\n',

open('empty.txt').read()))

# Get all lines from empty.txt and join them

# into one string then write to out.txt

open('out.txt','w').write(

''.join(

l for l in open(

'empty.txt') if l.strip()))

# Same as above but in this case the check

# is different. l[:-1] means the whole line

# except the last character. New lines get

# removed this way. Note that this may not

# work in case of blank lines with white space

open('out.txt','w').write(

''.join(

l for l in open(

'empty.txt') if l[:-1]))

That is all for today. Thanks for visiting. Please use the comments section below for feedback

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About Author

Mohammed Abualrob

Software Engineer @ Cisco

One Comment

Hengen

I tried all of them but never could delete the blank lines in the last line.
February 21, 2021 Reply

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