What is a context manager and how does the with statement work?

A context manager is an object that automatically performs actions when entering a block of code and when exiting it (even if an error occurs).

The with statement:

The most common example is working with files:

# Without with — you must remember to close the file
file = open("data.txt", "r")
try:
    content = file.read()
finally:
    file.close()

# With with — the file gets closed automatically
with open("data.txt", "r") as file:
    content = file.read()
# The file is already closed

How it works:

• __enter__() — called upon entering the with block. Its return value is assigned to the variable after as.
• __exit__() — called upon exiting the with block, even if an exception was raised.

Creating a custom context manager:

class Timer:
    def __enter__(self):
        import time
        self.start = time.time()
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        import time
        elapsed = time.time() - self.start
        print(f"Execution time: {elapsed:.2f} sec")
        return False  # Don't suppress exceptions

with Timer():
    total = sum(range(1_000_000))
# Execution time: 0.03 sec

Where context managers are used:

• Working with files (open)
• Locks in multithreading (threading.Lock)
• Database connections (automatic commit/rollback)
• Temporary resources (temporary files, network connections)