Context Managers & with Statement

Also Known As

TL;DR

Explanation

Context managers implement __enter__ (setup, returns the resource) and __exit__ (cleanup, always called) — or use @contextlib.contextmanager with yield. with open('file') as f: closes the file whether the body succeeds or raises. Multiple resources: with open(a) as f, open(b) as g. Custom managers: @contextlib.contextmanager def db_transaction(conn): try: yield conn; conn.commit() except: conn.rollback(). contextlib.suppress(FileNotFoundError) silently ignores specific exceptions. contextlib.ExitStack manages a dynamic number of resources. The PHP equivalent is try/finally blocks or RAII patterns — Python's with is more ergonomic and prevents resource leaks idiomatically.

Diagram

flowchart TD
    WITH[with statement] --> ENTER[__enter__ called<br/>acquire resource]
    ENTER --> BODY[Execute body]
    BODY -->|normal exit| EXIT[__exit__ called<br/>release resource]
    BODY -->|exception| EXIT2[__exit__ called<br/>even on exception]
    EXIT & EXIT2 --> CLEAN[Resource always cleaned up]
    subgraph Examples
        FILE2[with open file as f<br/>auto close]
        LOCK2[with lock:<br/>auto release]
        DB2[with db.transaction:<br/>auto commit or rollback]
        MOCK2[with patch target:<br/>auto unpatch]
    end
style CLEAN fill:#238636,color:#fff
style EXIT2 fill:#d29922,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Not using with for file operations — files are not closed on exception without it.
• Not implementing __exit__ to handle exceptions — a context manager that re-raises is fine, but it must return False.
• Using contextlib.contextmanager without wrapping the yield in try/finally — cleanup skipped on exception.
• Nested with statements instead of a single with statement with multiple context managers.

Code Examples

# File not closed on exception:
f = open('data.txt')
data = f.read()  # Exception here leaves file open
f.close()        # Never reached

# Context manager — always closes:
with open('data.txt') as f:
    data = f.read()  # Exception here still closes the file

from contextlib import contextmanager

@contextmanager
def managed_transaction(conn):
    try:
        yield conn
        conn.commit()
    except Exception:
        conn.rollback()
        raise

with managed_transaction(db) as conn:
    conn.execute('INSERT ...')

References

• ↗ https://docs.python.org/3/reference/compound_stmts.html#the-with-statement

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