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In our last tutorial, we have introduced you to some important file input/output operations that we could perform. In this tutorial, we are going to explain how to read the content of an existing file. But before we read the content of an existing file present on the disk, we must open it. Although we have already discussed how to open a file in Python but for those who have not read out last article, here is a recap.

• Calling the open() function to open a file to read


In order to open a file to perform the read operation, we could call the open() function of the io module, which is the default module for performing file operations. Using the open() function, we could provide location of a file stored on the disk and in return(if the file is found), the open() function opens the file in a mode of your choice, to let you perform the specific operation.

Let us take a look at a general syntax of open() function to open a file.



open('filename', mode);



• filename(a string value), is the name of file on which we are going to open and perform the file operation.
• mode(a string value), is a single or multiple file modes in which we are going to open a file to perform the file operations.

Let's read about the required file mode value, to read the content of a file.





• Reading a file


Let's look at the mode required to read a file.



File Mode	Description
r	Searches for the file and opens it in read mode only(if the file is found). This is also a default value.








• Methods to read to a file


let us take a look at the methods available to all the file objects, which we could use to read the content of a file.



Methods	Description
read(n)	The read(n) method reads all the characters from the file in a string(until the EOF(end of file) is reached) and returns the string value. The n parameter is optional, but when specified, the read method reads only the n number of bytes from the file.
readline(n)	This method reads a single line from a file and returns it. The n parameter is optional, but when specified, the read method reads only the n number of bytes from the file.
readlines(n)	This method reads a list of lines from the file and returns the list. The n parameter is optional, but when specified, the read method reads only the n number of bytes from the file.






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• Reading a file using read() method


Let us see a program which uses the open() function to search for a particular file in the current directory(the same directory where we are going to store our upcoming Python program), and then we are going to read the searched file by using the read() method. Let's say the name of this file is File.txt

File.txt

Hello there!
How are you doing? 
May you have a blessed day!




A program to read a file present on disk

# Python - Reading the content of a file using the built-in open() function and the read() method


# Number of bytes to read
bytes_to_read = 60


# Calling the open() function to open a file named File.txt in read mode
file = open('File.txt', 'r')


# Calling the read() method for file objects, to read all its content.
print(file.read())


# Calling the close() method for file objects, to close the file after we have read it.
file.close()




Output




Hello there!
How are you doing? 
May you have a blessed day!





Program Analysis


As you may see in the output of the code, the program looks for a file named File.txt in the directory where this Python program is stored and if the file is found, the program reads the characters present in the file and display it on the screen.






• Another example of reading a file using read() method


Let us another program to read a file using the read() method, but this time we are also going to pass a value to the n parameter of read() method. Let's say the name of this file is File2.txt

File2.txt

Good day!
Keep smiling 
Dont'give up!




A program to read a file present on disk

# Python - Reading the content of a file using the built-in open() function.


# Number of bytes to read
bytes_to_read = 5


# Calling the open() function to open a file named File2.txt in read mode
file = open('File2.txt', 'r')

# Calling the read() method for file objects, to read first 5 bytes
print('read(5) : ', file.read(bytes_to_read))



# Calling the read() method for file objects, to read its all leftover unread content.
print('read() : ', file.read())


# Calling the close() method for file objects, to close the file after we have read it.
file.close()




Output




read(5) :  Good 
read() :  day!
Keep smiling 
Dont'give up!





Program Analysis


As you may see in the output of the code, the program looks for a file named File2.txt in the directory where this Python program is stored.
• The program first calls the read() method(passing 5 as argument), which reads only the first 5 bytes from the file and display it.
• Next, it calls the read() method(passing no arguments), which reads all the leftover unread characters from the file and display it on the screen.




Note :

Calling the read() method on a file which we have already closed by calling the close() method, will throw a NameError in runtime, hence it should be avoided.






• Reading a file using readline() method


Let us another program to read a file using the readline() method, which only reads a single line at a time, unless we have passed an integer value as an argument while calling it and in that case it will only read the number of bytes corresponding to the integer value passed to it. Let's say the name of the file to read is File.txt

File.txt

Hello there!
How are you doing? 
May you have a blessed day!




A program to read a file present on disk

# Python - Reading the content of a file using the built-in open() function and the read() method


# Number of bytes to read
bytes_to_read = 60


# Calling the open() function to open a file named File.txt in read mode
file = open('File.txt', 'r')


# Calling the readline() method for file objects, to read a single line
print('readline() : ', file.readline())


# Calling the readline() method for file objects, to read the next 4 bytes in a file
print(readline(4) : ', file.readline(4))


# Calling the readline() method for file objects, to read the next single line
print(readline() : ', file.readline())


# Calling the close() method for file objects, to close the file after we have read it.
file.close()




Output




readline() : Hello there!

readline(4) : How 
readline() : are you doing? 





Program Analysis


As you may see in the output of the code, the program looks for a file named File.txt in the directory where this Python program is stored and if the file is found, it reads in the following sequence -
• readline(), reads a single line.
• readline(4), reads the first 4 bytes of the next line.
• readline(), reads the remaining characters in the same line, which weren't read by the previous call to readline() method.







• Reading a file using readlines() method

Let us another program to read a file using the readlines() method, which reads all the lines in a file and returns a list of lines, unless we have passed an integer value as an argument while calling it and in that case it will only read the number of bytes corresponding to the integer value passed to it. Let's read the same file, as we did in the last program - File.txt

File.txt

Hello there!
How are you doing? 
May you have a blessed day!

A program to read a file present on disk

# Python - Reading the content of a file using the built-in open() function and the read() method


# Number of bytes to read
bytes_to_read = 60


# Calling the open() function to open a file named File.txt in read mode
file = open('File1.txt', 'r')


# Calling the readline() method for file objects, to read a single line
print('readlines() : ', file.readlines())


# Calling the readline() method for file objects, to read the next 4 bytes in a file
print('readlines(4) : ', file.readlines(4))

# Calling the readline() method for file objects, to read the next single line
print('readlines() : ', file.readlines())


# Calling the close() method for file objects, to close the file after we have read it.
file.close()

Output

readlines() : ['Hello there!\n', 'How are you doing? \n', 'May you have a blessed day!']
readlines(4) : []
readlines() : []

Program Analysis

As you may see in the output of the code, the program looks for a file named File.txt in the directory where this Python program is stored and if the file is found, it reads in the following sequence -

• readlines(), reads all the lines in the file at once and returns a list of lines, which is printed.
• readline(4), returns an empty string because there is nothing left to read anymore.
• readline(), returns an empty string because there is nothing left to read anymore.

Note: We can even use the r+(read plus update mode), which allows us to not just read a file but also modify its content. For more on it, you will have to read our article, how to modify a file in Python.

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