Read a file line by line in Python

Prerequisites:

Python provides inbuilt functions for creating, writing and reading files. There are two types of files that can be handled in python, normal text files and binary files (written in binary language, 0s and 1s). In this article, we are going to study about reading line by line from a file.

Reading line by line

Using readlines()

readlines() is used to read all the lines at a single go and then return them as each line a string element in a list. This function can be used for small files, as it reads the whole file content to the memory, then split it into separate lines. We can iterate over the list and strip the newline '\n' character using strip() function.



Example:

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# Python code to
# demonstrate readlines()
  
L = ["Geeks\n", "for\n", "Geeks\n"]
  
# writing to file
file1 = open('myfile.txt', 'w')
file1.writelines(L)
file1.close()
  
# Using readlines()
file1 = open('myfile.txt', 'r')
Lines = file1.readlines()
  
count = 0
# Strips the newline character
for line in Lines:
    print(line.strip())
    print("Line{}: {}".format(count, line.strip()))

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Output:

Line1: Geeks
Line2: for
Line3: Geeks

Using readline()

readline() function reads a line of the file and return it in the form of the string. It takes a parameter n, which specifies the maximum number of bytes that will be read. However, does not reads more than one line, even if n exceeds the length of the line. It will be efficient when reading a large file because instead of fetching all the data in one go, it fetches line by line. readline() returns the next line of the file which contains a newline character in the end. Also, if end of the file is reached, it will return an empty string.

Example:

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# Python program to
# demonstrate readline()
  
L = ["Geeks\n", "for\n", "Geeks\n"]
  
# Writing to a file
file1 = open('myfile.txt', 'w')
file1.writelines((L))
file1.close()
  
# Using readline()
file1 = open('myfile.txt', 'r')
count = 0
  
while True:
    count += 1
  
    # Get next line from file
    line = file1.readline()
  
    # if line is empty
    # end of file is reached
    if not line:
        break
    print("Line{}: {}".format(count, line.strip()))
  
file1.close()

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Output:

Line1 Geeks
Line2 for
Line3 Geeks

Using for loop

An iterable object is returned by open() function while opening a file. This final way of reading in a file line-by-line includes iterating over a file object in a for loop. Doing this we are taking advantage of a built-in Python functionality that allows us to iterate over the file object implicitly using a for loop in combination of using the iterable object. This approach takes fewer lines of code, which is always the best practice worthy of following.

Example:

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# Python program to
# demonstrate reading files
# using for loop
  
L = ["Geeks\n", "for\n", "Geeks\n"]
  
# Writing to file
file1 = open('myfile.txt', 'w')
file1.writelines(L)
file1.close()
  
# Opening file
file1 = open('myfile.txt', 'r')
count = 0
  
# Using for loop
print("Using for loop")
for line in file1:
    count += 1
    print("Line{}: {}".format(count, line.strip()))
  
# Closing files
file1.close()

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Output:

Using for loop
Line1: Geeks
Line2: for
Line3: Geeks

With statement

In the above approaches, every time the file is opened it is needed to be closed explicitly. If one forgets to close the file, it may introduce several bugs in the code, i.e. many changes in files do not go into effect until the file is properly closed. To prevent this with statement can be used. with statement in Python is used in exception handling to make the code cleaner and much more readable. It simplifies the management of common resources like file streams. Observe the following code example on how the use of with statement makes code cleaner. There is no need to call file.close() when using with statement. The with statement itself ensures proper acquisition and release of resources.

Example:

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# Python program to
# demonstrate with
# statement
  
L = ["Geeks\n", "for\n", "Geeks\n"]
  
  
# Writing to file
with open("myfile.txt", "w") as fp:
    fp.writelines(L)
  
  
# using readlines()
count = 0
print("Using readlines()")
  
with open("myfile.txt") as fp:
    Lines = fp.readlines()
    for line in Lines:
        count += 1
        print("Line{}: {}".format(count, line.strip()))
  
  
# Using readline()
count = 0
print("\nUsing readline()")
  
with open("myfile.txt") as fp:
    while True:
        count += 1
        line = fp.readline()
  
        if not line:
            break
        print("Line{}: {}".format(count, line.strip()))
  
  
# Using for loop
count = 0
print("\nUsing for loop")
  
with open("myfile.txt") as fp:
    for line in fp:
        count += 1
        print("Line{}: {}".format(count, line.strip()))

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Output:

Using readlines()
Line1: Geeks
Line2: for
Line3: Geeks

Using readline()
Line1: Geeks
Line2: for
Line3: Geeks

Using for loop
Line1: Geeks
Line2: for
Line3: Geeks


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