Python File Handling

Till now, we were taking the input from the console and writing it back to the console to interact with the user.

Sometimes, it is not enough to only display the data on the console. The data to be displayed may be very large, and only a limited amount of data can be displayed on the console since the memory is volatile, it is impossible to recover the programmatically generated data again and again.

The file handling plays an important role when the data needs to be stored permanently into the file. A file is a named location on disk to store related information. We can access the stored information (non-volatile) after the program termination.

The file-handling implementation is slightly lengthy or complicated in the other programming language, but it is easier and shorter in Python.

In Python, files are treated in two modes as text or binary. The file may be in the text or binary format, and each line of a file is ended with the special character.

Hence, a file operation can be done in the following order.

• Open a file
• Read or write - Performing operation
• Close the file

Opening a file

Python provides an open() function that accepts two arguments, file name and access mode in which the file is accessed. The function returns a file object which can be used to perform various operations like reading, writing, etc.

Syntax:

file object = open(<file-name>, <access-mode>, <buffering>)   

The files can be accessed using various modes like read, write, or append. The following are the details about the access mode to open a file.

Let's look at the simple example to open a file named "file.txt" (stored in the same directory) in read mode and printing its content on the console.

Example

#opens the file file.txt in read mode  
fileptr = open("file.txt","r")  
  
if fileptr:  
    print("file is opened successfully")  

Output:

<class '_io.TextIOWrapper'>
file is opened successfully

In the above code, we have passed filename as a first argument and opened file in read mode as we mentioned r as the second argument. The fileptr holds the file object and if the file is opened successfully, it will execute the print statement

The close() method

Once all the operations are done on the file, we must close it through our Python script using the close() method. Any unwritten information gets destroyed once the close() method is called on a file object.

We can perform any operation on the file externally using the file system which is the currently opened in Python; hence it is good practice to close the file once all the operations are done.

The syntax to use the close() method is given below.

Syntax

fileobject.close() 

Consider the following example.

# opens the file file.txt in read mode  
fileptr = open("file.txt","r")  
  
if fileptr:  
    print("file is opened successfully")  
  
#closes the opened file  
fileptr.close()

After closing the file, we cannot perform any operation in the file. The file needs to be properly closed. If any exception occurs while performing some operations in the file then the program terminates without closing the file.

We should use the following method to overcome such type of problem.

try:
   fileptr = open("file.txt")
   # perform file operations
finally:
   fileptr.close()

The with statement

The with statement was introduced in python 2.5. The with statement is useful in the case of manipulating the files. It is used in the scenario where a pair of statements is to be executed with a block of code in between.

The syntax to open a file using with the statement is given below.

with open(<file name>, <access mode>) as <file-pointer>:  
    #statement suite   

The advantage of using with statement is that it provides the guarantee to close the file regardless of how the nested block exits.

It is always suggestible to use the with statement in the case of files because, if the break, return, or exception occurs in the nested block of code then it automatically closes the file, we don't need to write the close() function. It doesn't let the file to corrupt.

Consider the following example.

Example

with open("file.txt",'r') as f:  
    content = f.read();  
    print(content)  

Writing the file

To write some text to a file, we need to open the file using the open method with one of the following access modes.

w: It will overwrite the file if any file exists. The file pointer is at the beginning of the file.

a: It will append the existing file. The file pointer is at the end of the file. It creates a new file if no file exists.

Consider the following example.

Example

# open the file.txt in append mode. Create a new file if no such file exists.
fileptr = open("file2.txt", "w")

# appending the content to the file
fileptr.write('''Python is the modern day language. It makes things so simple.
It is the fastest-growing programing language''')

# closing the opened the file
fileptr.close()

Output:

File2.txt

Python is the modern-day language. It makes things so simple. It is the fastest growing programming language.

Snapshot of the file2.txt

We have opened the file in w mode. The file1.txt file doesn't exist, it created a new file and we have written the content in the file using the write() function.

Example 2

#open the file.txt in write mode.  
fileptr = open("file2.txt","a")
  
#overwriting the content of the file  
fileptr.write(" Python has an easy syntax and user-friendly interaction.")  
    
#closing the opened file   
fileptr.close()

Output:

Python is the modern day language. It makes things so simple.
It is the fastest growing programing language Python has an easy syntax and user-friendly interaction.

Snapshot of the file2.txt

We can see that the content of the file is modified. We have opened the file in a mode and it appended the content in the existing file2.txt.

To read a file using the Python script, the Python provides the read() method. The read() method reads a string from the file. It can read the data in the text as well as a binary format.

The syntax of the read() method is given below.

Syntax:

fileobj.read(<count>)  

Here, the count is the number of bytes to be read from the file starting from the beginning of the file. If the count is not specified, then it may read the content of the file until the end.

Consider the following example.

Example

#open the file.txt in read mode. causes error if no such file exists.  
fileptr = open("file2.txt","r")
#stores all the data of the file into the variable content  
content = fileptr.read(10) 
# prints the type of the data stored in the file  
print(type(content))    
#prints the content of the file  
print(content)     
#closes the opened file  
fileptr.close()  

Output:

<class 'str'>
Python is 

In the above code, we have read the content of file2.txt by using the read() function. We have passed count value as ten which means it will read the first ten characters from the file.

If we use the following line, then it will print all content of the file.

content = fileptr.read()
print(content) 

Output:

Python is the modern-day language. It makes things so simple.
It is the fastest-growing programing language Python has easy an syntax and user-friendly interaction.

Read file through for loop

We can read the file using for loop. Consider the following example.

#open the file.txt in read mode. causes an error if no such file exists.  
fileptr = open("file2.txt","r");   
#running a for loop   
for i in fileptr:  
    print(i) # i contains each line of the file   

Output:

Python is the modern day language.

It makes things so simple.

Python has easy syntax and user-friendly interaction.

Read Lines of the file

Python facilitates to read the file line by line by using a function readline() method. The readline() method reads the lines of the file from the beginning, i.e., if we use the readline() method two times, then we can get the first two lines of the file.

Consider the following example which contains a function readline() that reads the first line of our file "file2.txt" containing three lines. Consider the following example.

Example 1: Reading lines using readline() function

#open the file.txt in read mode. causes error if no such file exists.  
fileptr = open("file2.txt","r");   
#stores all the data of the file into the variable content  
content = fileptr.readline()   
content1 = fileptr.readline()
#prints the content of the file  
print(content)   
print(content1)
#closes the opened file  
fileptr.close()  

Output:

Python is the modern day language.

It makes things so simple.

We called the readline() function two times that's why it read two lines from the file.

Python provides also the readlines() method which is used for the reading lines. It returns the list of the lines till the end of file(EOF) is reached.

Example 2: Reading Lines Using readlines() function

#open the file.txt in read mode. causes error if no such file exists.  
fileptr = open("file2.txt","r");   
  
#stores all the data of the file into the variable content  
content = fileptr.readlines()   

#prints the content of the file  
print(content)   
  
#closes the opened file  
fileptr.close()  

Output:

['Python is the modern day language.\n', 'It makes things so simple.\n', 'Python has easy syntax and user-friendly interaction.']

Creating a new file

The new file can be created by using one of the following access modes with the function open().

x: it creates a new file with the specified name. It causes an error a file exists with the same name.

a: It creates a new file with the specified name if no such file exists. It appends the content to the file if the file already exists with the specified name.

w: It creates a new file with the specified name if no such file exists. It overwrites the existing file.

Consider the following example.

Example 1

#open the file.txt in read mode. causes error if no such file exists.  
fileptr = open("file2.txt","x") 
print(fileptr)  
if fileptr:  
    print("File created successfully")

Output:

<_io.TextIOWrapper name='file2.txt' mode='x' encoding='cp1252'>
File created successfully 

File Pointer positions

Python provides the tell() method which is used to print the byte number at which the file pointer currently exists. Consider the following example.

# open the file file2.txt in read mode  
fileptr = open("file2.txt","r")  

#initially the filepointer is at 0   
print("The filepointer is at byte :",fileptr.tell())  
  
#reading the content of the file  
content = fileptr.read();  
  
#after the read operation file pointer modifies. tell() returns the location of the fileptr.   
  
print("After reading, the filepointer is at:",fileptr.tell())  

Output:

The filepointer is at byte : 0
After reading, the filepointer is at: 117

Modifying file pointer position

In real-world applications, sometimes we need to change the file pointer location externally since we may need to read or write the content at various locations.

For this purpose, the Python provides us the seek() method which enables us to modify the file pointer position externally.

The syntax to use the seek() method is given below.

Syntax:

<file-ptr>.seek(offset[, from)  

The seek() method accepts two parameters:

offset: It refers to the new position of the file pointer within the file.

from: It indicates the reference position from where the bytes are to be moved. If it is set to 0, the beginning of the file is used as the reference position. If it is set to 1, the current position of the file pointer is used as the reference position. If it is set to 2, the end of the file pointer is used as the reference position.

Consider the following example.

Example

# open the file file2.txt in read mode  
fileptr = open("file2.txt","r")  
  
#initially the filepointer is at 0   
print("The filepointer is at byte :",fileptr.tell())  
  
#changing the file pointer location to 10.  
fileptr.seek(10);  
  
#tell() returns the location of the fileptr.   
print("After reading, the filepointer is at:",fileptr.tell())  

Output:

The filepointer is at byte : 0
After reading, the filepointer is at: 10

Python OS module

Renaming the file

The Python os module enables interaction with the operating system. The os module provides the functions that are involved in file processing operations like renaming, deleting, etc. It provides us the rename() method to rename the specified file to a new name. The syntax to use the rename() method is given below.

Syntax:

rename(current-name, new-name)  

The first argument is the current file name and the second argument is the modified name. We can change the file name bypassing these two arguments.

Example 1:

import os  
  
#rename file2.txt to file3.txt  
os.rename("file2.txt","file3.txt")

Output:

The above code renamed current file2.txt to file3.txt

Removing the file

The os module provides the remove() method which is used to remove the specified file. The syntax to use the remove() method is given below.

remove(file-name) 

Example 1

import os;  
#deleting the file named file3.txt   
os.remove("file3.txt")  

Creating the new directory

The mkdir() method is used to create the directories in the current working directory. The syntax to create the new directory is given below.

Syntax:

mkdir(directory name)

Example 1

import os  
  
#creating a new directory with the name new  
os.mkdir("new")  

The getcwd() method

This method returns the current working directory.

The syntax to use the getcwd() method is given below.

Syntax

os.getcwd()

Example

import os
os.getcwd()

Output:

'C:\\Users\\DEVANSH SHARMA'

Changing the current working directory

The chdir() method is used to change the current working directory to a specified directory.

The syntax to use the chdir() method is given below.

Syntax

chdir("new-directory")  

Example

import os 
# Changing current directory with the new directiory
os.chdir("C:\\Users\\DEVANSH SHARMA\\Documents")
#It will display the current working directory
os.getcwd()

Output:

'C:\\Users\\DEVANSH SHARMA\\Documents'

Deleting directory

The rmdir() method is used to delete the specified directory.

The syntax to use the rmdir() method is given below.

Syntax

os.rmdir(directory name)  

Example 1

import os
#removing the new directory   
os.rmdir("directory_name")  

It will remove the specified directory.

Writing Python output to the files

In Python, there are the requirements to write the output of a Python script to a file.

The check_call() method of module subprocess is used to execute a Python script and write the output of that script to a file.

The following example contains two python scripts. The script file1.py executes the script file.py and writes its output to the text file output.txt.

Example

file.py

temperatures=[10,-20,-289,100]  
def c_to_f(c):  
    if c< -273.15:  
        return "That temperature doesn't make sense!"  
    else:  
        f=c*9/5+32  
        return f  
for t in temperatures:  
    print(c_to_f(t))  

file.py

import subprocess  
  
with open("output.txt", "wb") as f:  
    subprocess.check_call(["python", "file.py"], stdout=f)  

The file related methods

The file object provides the following methods to manipulate the files on various operating systems.