September 2, 2018

Python read file line by line

By Mohammed Abualrob Code Snippets 0 Comments

Introduction

Welcome to a new code snippet post. Today, we are going to demonstrate how to read a text file line by line in Python. There are various ways to do that, however we are going to show the recommended method specially when dealing with large files.

Let us get started…

Using with statement

A recommended way to read a (large) file is to use (with) statement. For more information about (with) statement and context managers, you may check the references. Using (with) statement when reading a text file has a couple of advantages. We do not need to worry about closing the file handle as it closes automatically. Also, if something goes wrong, the exception is taken care of for us. Finally, the entire file is not read into memory but rather read on demand line by line. In case of large files, this becomes handy. Take a look…

# Open the file for reading
with open('data.txt') as lines:
   # Read the file line by line
   # note that the entire file
   # is not loaded into memory
   for line in lines:
      # Remove whitespace and print
      print(line.strip())

# Open the file for reading

with open('data.txt') as lines:

# Read the file line by line

# note that the entire file

# is not loaded into memory

for line in lines:

# Remove whitespace and print

print(line.strip())

Note that the following code block does something similar, however the entire file is loaded into memory which is not an efficient method in case of large files

# Open the file for reading
with open('data.txt') as file:
   # Read the entire file into memory
   lines = file.readlines()
   # Print each line
   for line in lines:
      # Remove white space and print
      print(line.strip())

# Open the file for reading

with open('data.txt') as file:

# Read the entire file into memory

lines = file.readlines()

# Print each line

for line in lines:

# Remove white space and print

print(line.strip())

Using fileinput module

File input module can be used to read a text file line by line in an efficient manner. Take a look at the following code snippet. The file lines are read sequentially…

# Import file input module
import fileinput
# Read the file line by line
for line in fileinput.input(['data.txt']):
   # Remove whitespace and print
   print(line.strip())

# Import file input module

import fileinput

# Read the file line by line

for line in fileinput.input(['data.txt']):

# Remove whitespace and print

print(line.strip())

Manual method

We can also implement our own reading logic. We can use a while loop to read the text file line by line as follows…

# Get the file handle
file = open('data.txt')
# Initialize line to the first line
line = file.readline()
# While there are lines in the file read
while line:
    # Remove whitespace and print
    print(line.strip())
    # Read next ine
    line = file.readline()

# Close file handle
file.close()

# Get the file handle

file = open('data.txt')

# Initialize line to the first line

line = file.readline()

# While there are lines in the file read

while line:

# Remove whitespace and print

print(line.strip())

# Read next ine

line = file.readline()

# Close file handle

file.close()

References

That is all for today. Thanks for visiting.

8 BIT AVENUE

Python read file line by line

Introduction

Using with statement

Using fileinput module

Manual method

References

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Mohammed Abualrob

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