Advanced Context Managers

Also Known As

TL;DR

Explanation

Context managers implement __enter__ and __exit__. contextlib.contextmanager turns a generator function into a context manager — yield once, cleanup after yield runs on exit. contextlib.suppress suppresses specific exceptions. contextlib.ExitStack dynamically composes multiple context managers. contextlib.asynccontextmanager for async with. Use for: database connections, file handles, locks, temporary directories, mocking, and any resource that needs guaranteed cleanup regardless of exceptions.

Common Misconception

Why It Matters

Common Mistakes

• Not using a context manager for database connections — connections leak if an exception occurs before close().
• contextmanager generator that yields more than once — raises RuntimeError; yield exactly once.
• Not using contextlib.suppress for expected exceptions — try/except/pass where suppress is cleaner.
• ExitStack not used when the number of context managers is dynamic — manual nesting becomes unmanageable.

Code Examples

# Resource leak on exception:
def process_file(path):
    f = open(path)
    data = f.read()    # Exception here?
    f.close()          # Never reached — file handle leaked
    return data

from contextlib import contextmanager, suppress

# Generator-based context manager:
@contextmanager
def db_transaction(conn):
    try:
        yield conn
        conn.commit()
    except Exception:
        conn.rollback()
        raise
    finally:
        conn.close()  # Always runs

# Usage:
with db_transaction(get_connection()) as conn:
    conn.execute('INSERT INTO orders ...')

# Suppress specific exceptions:
with suppress(FileNotFoundError):
    os.unlink('/tmp/lock_file')  # OK if already gone

References

• ↗ https://docs.python.org/3/library/contextlib.html

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