Context Managers in Python

Learn how context managers help Python programs manage resources safely and automatically.

What is a Context Manager?

A context manager is used to manage resources such as files, database connections and other operations that need proper setup and cleanup.

Python commonly uses the with statement to work with context managers.

Context Manager Flow

The with Statement

The with statement is the most common way to use a context manager in Python.

with open("students.txt", "r") as file:

    data = file.read()

    print(data)

After the with block finishes, Python automatically handles the file resource.

Simple File Example

with open("students.txt", "w") as file:

    file.write("Rahul")

print("File operation completed")

Output:

File operation completed

The file is automatically closed after the with block.

Without a Context Manager

Without using with, we have to manually close the file.

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

data = file.read()

print(data)

file.close()

Forgetting to close a resource can cause problems, especially when working with many resources.

with vs Manual Resource Handling

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

data = file.read()

file.close()

with open("data.txt", "r") as file:

    data = file.read()

Creating a Custom Context Manager

A custom context manager can be created using the __enter__() and __exit__() methods.

class Demo:

    def __enter__(self):

        print("Entering context")

        return self

    def __exit__(self, exc_type, exc_value, traceback):

        print("Exiting context")


with Demo():

    print("Inside context")

Output:

Entering context
Inside context
Exiting context

__enter__ Method

The __enter__() method runs when Python enters the with block.

class Demo:

    def __enter__(self):

        print("Context started")

        return self

    def __exit__(self, exc_type, exc_value, traceback):

        print("Context ended")


with Demo():

    print("Working")

The __enter__() method is used for setup operations.

__exit__ Method

The __exit__() method runs when Python leaves the with block.

It is commonly used for cleanup operations.

Context Manager with Exception

class Demo:

    def __enter__(self):

        print("Context started")

        return self

    def __exit__(self, exc_type, exc_value, traceback):

        print("Context ended")


with Demo():

    print("Inside context")

    raise ValueError("Something went wrong")

The __exit__() method still gets a chance to run when an exception occurs inside the with block.

Example 👀👨‍🏫

File handling is one of the most common practical uses of context managers.

with open("students.txt", "r") as file:

    for line in file:

        print(line.strip())

Python automatically handles the file resource when the with block finishes.

Common Uses of Context Managers

Advantages of Context Managers

Best Practices

Common Mistakes

• Forgetting to close resources when not using a context manager.
• Using a custom context manager when a built-in context manager is enough.
• Putting unrelated operations inside the with block.
• Not understanding the purpose of __enter__ and __exit__.

Practice Programs

1. Read a text file using the with statement.
2. Write data to a file using a context manager.
3. Create a custom context manager using __enter__() and __exit__().
4. Create a context manager that prints messages when entering and leaving a block.
5. Create a context manager that handles a resource safely when an exception occurs.