Context Managers

Python · Reference cheat sheet

📋 Overview

Context managers guarantee setup/teardown via with. They close files, release locks, and restore state even when exceptions occur. Implement with __enter__/__exit__ or @contextlib.contextmanager.

🔧 Core concepts

API	Role
`with cm as x:`	Enter, bind result, exit on leave
Multiple	`with a, b:`
Parenthesized	Multi-line `with` (3.10+)
`__enter__`	Setup; return bound value
`__exit__`	Cleanup; return truthy to suppress exc
`contextlib.contextmanager`	Generator-based CM
`ExitStack`	Dynamic stack of contexts
`AsyncExitStack` / `async with`	Async variants

__exit__(exc_type, exc, tb) — return True to suppress the exception (rare).

💡 Examples

Built-in file and lock:

from pathlib import Path
from threading import Lock

lock = Lock()
path = Path("out.txt")

with lock, path.open("w", encoding="utf-8") as f:
    f.write("hello\n")

Generator context manager:

from contextlib import contextmanager
from collections.abc import Iterator

@contextmanager
def temp_attr(obj: object, name: str, value: object) -> Iterator[None]:
    old = getattr(obj, name)
    setattr(obj, name, value)
    try:
        yield
    finally:
        setattr(obj, name, old)

class C:
    mode = "prod"

with temp_attr(C, "mode", "test"):
    assert C.mode == "test"

Class-based:

class suppress_os:
    def __enter__(self) -> "suppress_os":
        return self

    def __exit__(self, exc_type, exc, tb) -> bool:
        return exc_type is not None and issubclass(exc_type, OSError)

ExitStack:

from contextlib import ExitStack
from pathlib import Path

paths = [Path("a.txt"), Path("b.txt")]
with ExitStack() as stack:
    files = [stack.enter_context(p.open("w", encoding="utf-8")) for p in paths]
    for f in files:
        f.write("x\n")

⚠️ Pitfalls

Returning True from __exit__ swallows exceptions — usually wrong.
Yielding inside @contextmanager after errors needs careful try/finally.
Nested with order: enter left-to-right, exit right-to-left.
Don't open resources without with unless you manage close yourself.
Async code needs async with and async context managers.

Context Managers

Context Managers

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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