Skip to content

Context Managers Explained

A context manager is something that sets up a resource when you enter a block and cleans it up when you leave, even if an error occurs. The with statement is how you use them. If you have ever written with open("file.txt") as f:, you have already used one.

Learn Your Way

Read Build Watch Test Review Visualize
You are here Projects Videos Quiz Flashcards Diagrams

Why This Matters

Resources like files, database connections, and network sockets need to be properly closed when you are done with them. Forgetting to close a file can corrupt data. Forgetting to close a database connection can exhaust the connection pool and crash your server. Context managers make cleanup automatic, so you cannot forget.

Visualize It

Watch how with guarantees cleanup, even when an error is raised: Open in Python Tutor

The basic pattern: with and files

Without a context manager, you must remember to close the file yourself:

# WITHOUT a context manager — risky
f = open("data.txt")
content = f.read()
f.close()    # What if an error happens before this line?

With a context manager, cleanup is guaranteed:

# WITH a context manager — safe
with open("data.txt") as f:
    content = f.read()
# f.close() is called automatically, even if an error occurred

The with statement calls f.__enter__() at the start and f.__exit__() at the end, no matter what happens inside the block.

How __enter__ and __exit__ work

Any object can be a context manager if it has two special methods:

class ManagedFile:
    def __init__(self, filename):
        self.filename = filename

    def __enter__(self):
        # Called when entering the `with` block
        self.file = open(self.filename)
        return self.file    # This becomes the `as` variable

    def __exit__(self, exc_type, exc_val, exc_tb):
        # Called when leaving the `with` block — ALWAYS
        self.file.close()
        return False    # False means: do not suppress exceptions


# Usage:
with ManagedFile("data.txt") as f:
    content = f.read()

The three arguments to __exit__ describe any exception that occurred: - exc_type: the exception class (e.g., ValueError), or None if no error - exc_val: the exception instance - exc_tb: the traceback object

If __exit__ returns True, the exception is suppressed (swallowed). If it returns False (the default), the exception propagates normally. Almost always return False.

The easy way: contextlib.contextmanager

Writing a class with __enter__ and __exit__ is verbose. The contextlib module gives you a decorator that turns a generator function into a context manager:

from contextlib import contextmanager

@contextmanager
def managed_file(filename):
    # __enter__: everything before yield
    f = open(filename)
    try:
        yield f    # This value becomes the `as` variable
    finally:
        # __exit__: everything after yield
        f.close()


# Usage — exactly the same:
with managed_file("data.txt") as f:
    content = f.read()

The pattern is: 1. Before yield: setup (like __enter__) 2. yield value: the value assigned by as 3. After yield: cleanup (like __exit__), usually in a finally block

Real-world examples

Database connection

from contextlib import contextmanager
import sqlite3

@contextmanager
def get_db(path):
    conn = sqlite3.connect(path)
    try:
        yield conn
        conn.commit()      # Commit if no errors
    except Exception:
        conn.rollback()    # Rollback on error
        raise              # Re-raise the exception
    finally:
        conn.close()       # Always close


with get_db("app.db") as conn:
    conn.execute("INSERT INTO users (name) VALUES (?)", ("Alice",))
    # If this block raises, the transaction is rolled back
    # If it succeeds, changes are committed
    # Either way, the connection is closed

Temporary directory

import tempfile
import os

# Python's built-in context manager for temp directories
with tempfile.TemporaryDirectory() as tmpdir:
    path = os.path.join(tmpdir, "temp_file.txt")
    with open(path, "w") as f:
        f.write("temporary data")
    # Work with the directory...
# Directory and all contents are deleted automatically

Timing a block of code

from contextlib import contextmanager
import time

@contextmanager
def timer(label):
    start = time.time()
    yield
    elapsed = time.time() - start
    print(f"{label}: {elapsed:.2f}s")


with timer("data processing"):
    # ... expensive work here ...
    total = sum(range(10_000_000))
# Prints: data processing: 0.23s

Suppressing specific exceptions

from contextlib import suppress

# Instead of try/except/pass:
with suppress(FileNotFoundError):
    os.remove("temp.txt")
# If the file does not exist, no error — the exception is silently caught

Thread lock

import threading

lock = threading.Lock()

# The lock is a context manager — it acquires on enter, releases on exit
with lock:
    # Only one thread can be here at a time
    shared_data.append(item)

Nesting context managers

You can nest with statements, or combine them on one line:

# Nested:
with open("input.txt") as src:
    with open("output.txt", "w") as dst:
        dst.write(src.read())

# Combined (Python 3.1+):
with open("input.txt") as src, open("output.txt", "w") as dst:
    dst.write(src.read())

# Parenthesized (Python 3.10+):
with (
    open("input.txt") as src,
    open("output.txt", "w") as dst,
):
    dst.write(src.read())

Common Mistakes

Forgetting that __exit__ always runs: 

# This is fine — __exit__ runs even if you return early
with open("data.txt") as f:
    first_line = f.readline()
    if not first_line:
        return    # File is still closed properly

Suppressing exceptions by accident: 

class BadContextManager:
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.cleanup()
        return True    # DANGER: swallows ALL exceptions silently!

Always return False from __exit__ unless you have a very specific reason to suppress exceptions.

Using a closed resource outside the with block: 

with open("data.txt") as f:
    pass

f.read()    # ValueError: I/O operation on closed file

The resource is only valid inside the with block.

Practice

Further Reading

← Prev Home Next →