with 文を深く理解する

導入

withの基本的な使い方

with open('/tmp/cm_test.txt','w') as f:
    f.write('hello')
print('file closed:', f.closed)

なぜwithが存在するのか

# Without with:
# f = open('f.txt')
# try: ...
# finally: f.close()
# With with:
# with open('f.txt') as f: ...
print('with is cleaner')

複数のコンテキストマネージャー

import tempfile, os
a = tempfile.mktemp(); b = tempfile.mktemp()
with open(a,'w') as fa, open(b,'w') as fb:
    fa.write('A'); fb.write('B')
os.unlink(a); os.unlink(b)
print('both closed')

withは例外を抑制しない

try:
    with open('/nonexistent.txt') as f:
        pass
except FileNotFoundError as e:
    print('caught:', e)

コンテキストマネージャーとしてのLock

import threading
lock = threading.Lock()
with lock:
    print('locked section')
print('lock released')

データベース接続のパターン

# Typical DB pattern:
# with db.transaction() as conn:
#     conn.execute(sql)
# commits on success, rolls back on exception
print('db pattern demo')

ネストしたwith文

import tempfile
tmp = tempfile.mktemp()
with open(tmp,'w') as outer:
    with open(tmp,'r+') as inner:
        outer.write('x')
import os; os.unlink(tmp)
print('nested exit order')

contextlib.suppress

from contextlib import suppress
with suppress(FileNotFoundError):
    open('/does/not/exist.txt')
print('no error raised')

contextlib.redirect_stdout

from contextlib import redirect_stdout
import io
buf = io.StringIO()
with redirect_stdout(buf):
    print('captured')
print('got:', buf.getvalue())

ExitStack

from contextlib import ExitStack
import tempfile, os
with ExitStack() as stack:
    files = [stack.enter_context(open(tempfile.mktemp(),'w')) for _ in range(3)]
    for f in files: f.write('x')
print('all closed')

クイックチェック

復習

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