Reading a file in Python programming language

Python is a popular programming language that is widely used in advanced analytics and data science. One of the most basic yet essential tasks in data processing is being able to read data from a file, which is why understanding how to read a file in Python is crucial. This tutorial will explore the different approaches available to read files in Python programming language, including using the open() function, 'with' statement, and the  read()  and  readline()  methods. By the end of this tutorial, you should have a solid foundation in reading files in Python that you can use in your data processing workflow.

Opening a file in read mode

• We can read a file in python by opening the file in read mode.
• To open a file in read mode, the access_mode (second argument of the open function) should be for read operation.
• Different access mode values for reading the file in Python :

Python file read methods : 

• Different functions for reading a file in python : 
  • read(): Reads the entire content of a file into a single string.
  • readline(): Reads a single line from a file.
  • readlines(): Reads all lines of a file into a list.
• Steps for Reading a File in Python : 
  • Open a file in read mode using the  open()  method.
  • Use one of the read methods listed above to read the file.
  • Close the file using the  close()  method.

Python read() Method : 

• We can use the read method in python to read a file.
• The syntax of using the  read()  method is :  fileObject.read( size ) 
• The  read()  method takes an optional numeric argument  size  which indicates the number of bytes to be read from the file.
• When the  size  parameter is omitted or is negative, the entire contents of the file will be read and returned.
• The  read()  method returns the contents of the file as a string (in text mode) or bytes object (in binary mode)
• Code Sample : 

file = open("example.txt", "r")
print(file.read())
file.close()

How to read N bytes from the file :

• We can read the first n of bytes from a file by passing the optional integer size parameter.
• This block of code reads the first 10 bytes of a file, using the  read()  method.
• Code Sample : 

with open("example.txt", "r") as file:
   bytes = file.read(10)
   print(bytes)
   
   
   

Python readline() Method : 

• We can also use the  readline()  method in python to read a file.
• The syntax of using the  readline()  method is :  fileObject.readline( size ) 
• The  readline()  method takes an optional numeric argument  size  which indicates the number of bytes from the line to return.
• This method reads the file line by line until the end of the file is reached.
• The  readline()  method returns an empty string when the end of the file has been reached
• Code Sample : 

file = open("example.txt", "r")
print(file.readline())
file.close()

How to read first N lines from a file using readline() :

• The  readline()  method reads the content of the file line by line.
• We can then use a loop in python to iterate over the first n lines of the file .
• The code sample below iterates over the first 15 lines of the file and prints them on the console.
• Code Sample : 

file = open("example.txt", "r")

for i in range(15):
   print(file.readline())

file.close()

How to read entire file using readline()

• This block of code reads the entire file line by line using a while loop, 
• The  readline()  method is called repeatedly until the end of the file is reached.
• We know that the  readline()  method returns an empty string when the end of the file has been reached.
• This can be used to terminate the loop when we reach the end of the loop.
• Code Sample : 

file = open("example.txt", "r")

while True:
   line = file.readline()
   if not line:
       break
   print(line)
   
file.close()

How to read first and the last line using readline() :

• We can use the indexing syntax to read desired line through the  readline()  method.
• This example reads the first and last lines of the file using the  readline()  method.
• Code Sample : 

with open("example.txt", "r") as file:
   first_line = file.readline()
   last_line = file.readlines()[-1]
   
print(first_line)
print(last_line)

Python readlines() method : reading file into list

• The  readlines()  method reads all the lines of a text file into a list, 
• Each line of text becomes an element of the list.
• We can then iterate over the list to get the lines of the text.
• The  readlines()  method takes an optional hint parameter which limits the number of lines that should be returned.
• Code Sample : 

with open("example.txt", "r") as file:
   lines = file.readlines()
   for line in lines:
       print(line)
       
       
       

How to read first N lines from a file using readlines() method :

• We know that  readlines()  method returns a list containing the lines of the text as list elements.
• We can use list slicing operation to get the first n elements of the list.
• This code sample below reads the first 5 lines from a file.
• Code Sample : 

with open("example.txt", "r") as file:
   for line in file.readlines()[:5]:
       print(line)
       
       
       

How to read the last N lines of a file using readlines() method :

• List slicing can also be used to read the last n lines of a file.
• This example reads the last 5 lines from a file using list slicing.
• Code Sample : 

with open("example.txt", "r") as file:
   lines = file.readlines()
   for line in lines[-5:]:
       print(line)
       
       
       

Reading a file using the with statement in python

• The  with  statement is a more elegant way of reading a file, as 
• it takes care of closing the file after the code block has finished executing.
• Code Sample : 

with open("example.txt", "r") as file:
   print(file.read())
   
   
   

Reading and Writing to the same file

• We can pass the ‘r+’ access_mode to the open() method to perform both the read and write operations on the file simultaneously.
• This code reads data from a file and appends an extra line to it, using the  read()  and  write()  methods in combination.
• Code Sample : 

with open("example.txt", "r+") as file:
   data = file.read()
   file.write("\nThis is another line.")
   
   
   

Reading a binary file in Python :

• We can pass the ‘rb’ access mode to the open() function to read a binary file.
• This block of code reads binary data from a file using the `rb` mode.
• It reads the entire file into a bytes object.
• Code Sample : 

with open("example.png", "rb") as file:
   data = file.read()
   print(data)
   
   
   

Reading file in reverse order in python

• The  reversed()  function in python can be used to read the file in reverse order.
• This block of code reads a text file in a reverse order by iterating over the file in reversed order.
• Code Sample : 

with open("example.txt", "r") as file:
   lines = file.readlines()
   for line in reversed(lines):
       print(line.strip())