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6. Context Managers & with Statement

6.1 Understanding Context Managers

A context manager is an object that defines setup and teardown actions for a block of code. It is used with the with statement to handle resources like files, database connections, locks, etc.

Why use context managers?

• Automatically close/release resources (even on exceptions)

• Cleaner code (no manual try-finally)

• Prevents resource leaks

Most common example – file handling

Python

# Old way (error-prone) f = open("data.txt", "w") try: f.write("Hello") finally: f.close() # Modern & safe way with open("data.txt", "w") as f: f.write("Hello") # auto-closed when block ends (even if error)

What happens behind the scenes:

1. open() returns a context manager object

2. enter() is called → returns the object (f)

3. Code inside with block runs

4. exit() is called → closes the file (even if exception occurs)

6.2 Writing Custom Context Managers (__enter__, exit)

You can create your own context manager by defining a class with enter and exit methods.

Example – Timing context manager

Python

import time class Timer: def enter(self): self.start = time.perf_counter() return self # returned value assigned to 'as' variable def exit(self, exc_type, exc_value, traceback): self.end = time.perf_counter() self.elapsed = self.end - self.start print(f"Execution time: {self.elapsed:.4f} seconds") # Return True to suppress exception, False to let it propagate return False # Usage with Timer() as t: time.sleep(1.5) print("Doing some work...") # Output: # Doing some work... # Execution time: 1.5012 seconds

Another example – Temporary directory change

Python

import os from pathlib import Path class ChangeDirectory: def init(self, path): self.new_path = path self.old_path = os.getcwd() def enter(self): os.chdir(self.new_path) return self def exit(self, *args): os.chdir(self.old_path) with ChangeDirectory(Path.home() / "Downloads"): print(os.getcwd()) # now in Downloads print(os.getcwd()) # back to original

Tip: exit receives exception info (exc_type, exc_value, traceback). Return True to suppress exception, False (default) to re-raise it.

6.3 @contextmanager Decorator

Writing full classes can be verbose. The @contextmanager decorator from contextlib lets you write context managers using a generator function — much simpler.

Syntax

Python

from contextlib import contextmanager @contextmanager def my_context(): # setup code try: yield value # value assigned to 'as' variable finally: # teardown code (always runs)

Example – Temporary value change

Python

from contextlib import contextmanager @contextmanager def temp_setting(obj, attr, new_value): old_value = getattr(obj, attr) setattr(obj, attr, new_value) try: yield finally: setattr(obj, attr, old_value) class Config: debug = False cfg = Config() with temp_setting(cfg, "debug", True): print(cfg.debug) # True print(cfg.debug) # False (restored)

Real-world example – Database transaction

Python

@contextmanager def transaction(db): db.begin() try: yield db db.commit() except Exception: db.rollback() raise finally: db.close() # Usage with transaction(my_db) as db: db.execute("INSERT INTO users ...")

6.4 Common Use Cases: File Handling, Database Connections, Timing

1. File Handling (built-in)

Python

with open("log.txt", "a") as f, open("error.txt", "a") as err: f.write("Info log\n") err.write("Error occurred\n") # Both files auto-closed

2. Database Connections (using contextlib + library)

Python

from contextlib import contextmanager import sqlite3 @contextmanager def sqlite_conn(db_path): conn = sqlite3.connect(db_path) try: yield conn conn.commit() except Exception: conn.rollback() raise finally: conn.close() with sqlite_conn("mydb.db") as conn: conn.execute("INSERT INTO users (name) VALUES (?)", ("Anshuman",))

3. Timing / Performance Measurement

Python

from contextlib import contextmanager import time @contextmanager def timer(description=""): start = time.perf_counter() yield elapsed = time.perf_counter() - start print(f"{description} took {elapsed:.4f} seconds") with timer("Data processing"): time.sleep(1.2) # heavy computation here # Output: Data processing took 1.2008 seconds

4. Thread Lock / Resource Management

Python

from threading import Lock from contextlib import contextmanager lock = Lock() @contextmanager def locked(): lock.acquire() try: yield finally: lock.release() with locked(): # critical section print("Thread-safe code here")

Mini Project – Suppressing Specific Exceptions

Python

from contextlib import contextmanager, suppress @contextmanager def ignore_errors(*exceptions): try: yield except exceptions: pass with ignore_errors(ZeroDivisionError, ValueError): print(10 / 0) # no crash print("This won't run") print("Program continues...")

This completes the full Context Managers & with Statement section — now you can manage resources safely and elegantly in any Python program!

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