Python Core | VoidX Academy

17. Python File Handling

Data Management

Python File Handling

File handling is an essential part of any web application or data processing script. Python provides built-in methods to create, read, update, and delete files.

Working with files involves two main steps: opening the file, and then performing an operation (reading or writing). It is also critical to close the file when you are done to free up system resources.

The open() Function

The key function for working with files in Python is the open() function. The open() function takes two parameters: filename, and mode.

There are four different methods (modes) for opening a file:

"r" - Read: Default value. Opens a file for reading, error if the file does not exist.
"a" - Append: Opens a file for appending, creates the file if it does not exist.
"w" - Write: Opens a file for writing, overwrites existing data, creates the file if it does not exist.
"x" - Create: Creates the specified file, returns an error if the file exists.

Reading a File

To read a file, pass the name of the file to the open() function. The file object has a read() method for reading the content of the file.

# Assuming 'demo.txt' exists with the text "Hello World"
f = open("demo.txt", "r")
print(f.read())
f.close()

OutputHello World

💡Best Practice: You should always close your files. In some cases, due to buffering, changes made to a file may not show until you close the file.

The 'with' Statement

A better and more Pythonic way to open a file is by using the with statement. This ensures that the file is properly closed after its suite finishes, even if an exception is raised along the way.

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

# The file is automatically closed here!

OutputHello World

Reading Lines

You can return one line by using the readline() method. By calling it twice, you read the first two lines.

with open("demo.txt", "r") as f:
    print(f.readline())  # Prints line 1
    print(f.readline())  # Prints line 2

OutputLine 1 Content Line 2 Content

To loop through the lines of the file, you can easily loop over the file object itself. This is highly memory-efficient for reading massive files.

with open("demo.txt", "r") as f:
    for line in f:
        print(line.strip()) # strip() removes the trailing newline

OutputLine 1 Content Line 2 Content

Writing to an Existing File

To write to an existing file, you must add a parameter to the open() function: "a" for append, or "w" for write (overwrite).

# Append adds to the end of the file
with open("demo.txt", "a") as f:
    f.write("\nThis line is appended.")

# Write entirely overwrites the file content
with open("demo.txt", "w") as f:
    f.write("Woops! I have deleted the content!")

Output(File operation completed)

Creating a New File

To create a new file in Python, use the open() method, with one of the following parameters: "x", "a", or "w".

# "x" will create the file, but crash if it already exists
try:
    f = open("newfile.txt", "x")
    f.write("Initial content")
    f.close()
    print("File created!")
except FileExistsError:
    print("File already exists.")

OutputFile created!

Deleting a File

To delete a file, you must import the OS module, and run its os.remove() function. Attempting to remove a file that does not exist will cause an error.

import os

if os.path.exists("newfile.txt"):
    os.remove("newfile.txt")
    print("File deleted.")
else:
    print("The file does not exist.")

OutputFile deleted.

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17. Python File Handling

Data Management

Python File Handling

File handling is an essential part of any web application or data processing script. Python provides built-in methods to create, read, update, and delete files.

The open() Function

The key function for working with files in Python is the open() function. The open() function takes two parameters: filename, and mode.

There are four different methods (modes) for opening a file:

"r" - Read: Default value. Opens a file for reading, error if the file does not exist.
"a" - Append: Opens a file for appending, creates the file if it does not exist.
"w" - Write: Opens a file for writing, overwrites existing data, creates the file if it does not exist.
"x" - Create: Creates the specified file, returns an error if the file exists.

Reading a File

To read a file, pass the name of the file to the open() function. The file object has a read() method for reading the content of the file.

# Assuming 'demo.txt' exists with the text "Hello World"
f = open("demo.txt", "r")
print(f.read())
f.close()

OutputHello World

💡Best Practice: You should always close your files. In some cases, due to buffering, changes made to a file may not show until you close the file.

The 'with' Statement

A better and more Pythonic way to open a file is by using the with statement. This ensures that the file is properly closed after its suite finishes, even if an exception is raised along the way.

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

# The file is automatically closed here!

OutputHello World

Reading Lines

You can return one line by using the readline() method. By calling it twice, you read the first two lines.

with open("demo.txt", "r") as f:
    print(f.readline())  # Prints line 1
    print(f.readline())  # Prints line 2

OutputLine 1 Content Line 2 Content

To loop through the lines of the file, you can easily loop over the file object itself. This is highly memory-efficient for reading massive files.

with open("demo.txt", "r") as f:
    for line in f:
        print(line.strip()) # strip() removes the trailing newline

OutputLine 1 Content Line 2 Content

Writing to an Existing File

To write to an existing file, you must add a parameter to the open() function: "a" for append, or "w" for write (overwrite).

# Append adds to the end of the file
with open("demo.txt", "a") as f:
    f.write("\nThis line is appended.")

# Write entirely overwrites the file content
with open("demo.txt", "w") as f:
    f.write("Woops! I have deleted the content!")

Output(File operation completed)

Creating a New File

To create a new file in Python, use the open() method, with one of the following parameters: "x", "a", or "w".

# "x" will create the file, but crash if it already exists
try:
    f = open("newfile.txt", "x")
    f.write("Initial content")
    f.close()
    print("File created!")
except FileExistsError:
    print("File already exists.")

OutputFile created!

Deleting a File

To delete a file, you must import the OS module, and run its os.remove() function. Attempting to remove a file that does not exist will cause an error.

import os

if os.path.exists("newfile.txt"):
    os.remove("newfile.txt")
    print("File deleted.")
else:
    print("The file does not exist.")

OutputFile deleted.

Knowledge Check

Ready to test your understanding of 17. Python File Handling?

16. Python Try/Except

18. Python RegEx