Контекстные менеджеры и оператор with

with open('notes.txt', 'w', encoding='utf-8') as f:
    f.write('first line\n')
    print('inside:', f.closed)
print('after:', f.closed)

try:
    with open('notes.txt', encoding='utf-8') as f:
        text = f.read()
        raise ValueError('something went wrong')
except ValueError as e:
    print('error:', e)
print('closed even after the error:', f.closed)

class Transaction:
    def __init__(self, account):
        self.account = account

    def __enter__(self):
        self.backup = dict(self.account)
        return self.account

    def __exit__(self, exc_type, exc, tb):
        if exc_type is not None:
            self.account.clear()
            self.account.update(self.backup)
        return False

wallet = {'Aysel': 100, 'Murad': 50}
for amount in (30, 500):
    try:
        with Transaction(wallet) as w:
            w['Aysel'] -= amount
            w['Murad'] += amount
            if w['Aysel'] < 0:
                raise ValueError('not enough money')
    except ValueError as e:
        print('cancelled:', e)
    print(wallet)

from contextlib import contextmanager

@contextmanager
def tag(name):
    print(f'<{name}>')
    try:
        yield name
    finally:
        print(f'</{name}>')

with tag('ul'):
    for item in ['tea', 'plov']:
        with tag('li') as t:
            print(f'  {item} (inside {t})')

import time
from contextlib import contextmanager

@contextmanager
def timer(label):
    start = time.perf_counter()
    try:
        yield
    finally:
        elapsed = time.perf_counter() - start
        print(f'{label}: {elapsed * 1000:.1f} ms')

with timer('sum of squares'):
    total = sum(n * n for n in range(1_000_000))
print(total)

from contextlib import suppress, redirect_stdout
import io
import os

with suppress(FileNotFoundError):
    os.remove('no-such-file.txt')
print('no crash')

buffer = io.StringIO()
with redirect_stdout(buffer):
    print('this goes into the buffer')
print('captured:', buffer.getvalue().strip())

from contextlib import ExitStack

names = ['a.txt', 'b.txt', 'c.txt']
for i, name in enumerate(names):
    with open(name, 'w', encoding='utf-8') as f:
        f.write(f'file {i}\n')

with ExitStack() as stack:
    files = [stack.enter_context(open(n, encoding='utf-8')) for n in names]
    print([f.readline().strip() for f in files])
print(all(f.closed for f in files))

from decimal import Decimal, localcontext

print(Decimal(1) / Decimal(7))
with localcontext() as ctx:
    ctx.prec = 5
    print(Decimal(1) / Decimal(7))
print(Decimal(1) / Decimal(7))

from contextlib import contextmanager

@contextmanager
def section(title):
    # print '== title ==' on entry and '-- end of title --' on exit, even after an error
    yield

with section('Report'):
    print('all good')

try:
    with section('Import'):
        raise ValueError('bad file')
except ValueError as e:
    print('error:', e)

class Ignore:
    def __init__(self, *exceptions):
        self.exceptions = exceptions

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc, tb):
        # return True only if the exception is one of self.exceptions
        return False

with Ignore(ZeroDivisionError):
    print(1 / 0)
print('still running')

with Ignore(KeyError, IndexError):
    [][5]
print('done')