python 上下文管理器使用方法小结

with open('/path/to/file', 'r') as f:
 f.read()

#管理锁
import threading
lock = threading.lock()
with lock:
 #执行一些操作
 pass

#数据库连接管理
def test_write():
 sql = """  #具体的sql语句
 """
 con = DBConnection()
 with con as cursor: 
  cursor.execute(sql)
  cursor.execute(sql)
  cursor.execute(sql)

class test_query:
  def __init__(self):
    pass

  def query(self):
    print('query')

  def __enter__(self):
    # 如果是数据库连接就可以返回cursor对象
    # cursor = self.cursor
    # return cursor
    print('begin query,')
    return self #由于这里没有资源对象就返回对象
    
  def __exit__(self,exc_type,exc_value,traceback):
    if traceback is None:
      print('End')
    else:
      print('Error')

    # 如果是数据库连接提交操作
    # if traceback is None:
    #  self.commit()
    # else:
    #  self.rollback()
    
if __name__ == '__main__':
  with test_query() as q:
    q.query()

  @contextmanager
    def some_generator(<arguments>):
      <setup>
      try:
        yield <value>
      finally:
        <cleanup>

import contextlib

@contextlib.contextmanager
def tag(name):
  print('<{}>'.format(name))
  yield
  print('</{}>'.format(name))


if __name__ == '__main__':
  with tag('h1') as t:
    print('hello')
    print('context')

@contextmanager
def locked(lock):
  lock.acquire()
  try:
    yield
  finally:
    lock.release()

with locked(myLock):
  #代码执行到这里时，myLock已经自动上锁
  pass
  #执行完后会，会自动释放锁

@contextmanager
def myopen(filename, mode="r"):
  f = open(filename,mode)
  try:
    yield f
  finally:
    f.close()

with myopen("test.txt") as f:
  for line in f:
    print(line)

@contextmanager
def transaction(db):
  db.begin()
  try：
    yield 
  except:
    db.rollback()
    raise
  else:
    db.commit()

with transaction(mydb):
  mydb.cursor.execute(sql)
  mydb.cursor.execute(sql)
  mydb.cursor.execute(sql)
  mydb.cursor.execute(sql)

import contextlib
from urllib.request import urlopen

if __name__ == '__main__':
  with contextlib.closing(urlopen('https://www.python.org')) as page:
    for line in page:
      print(line)
closing也是一个经过@contextmanager装饰的generator，这个generator编写起来其实非常简单：

@contextmanager
def closing(thing):
  try:
    yield thing
  finally:
    thing.close()

import contextlib
from urllib.request import urlopen

if __name__ == '__main__':
  with contextlib.closing(urlopen('https://www.python.org')) as page,\
      contextlib.closing(urlopen('https://www.python.org')) as p:
    for line in page:
      print(line)
        print(p)