Python with 用法

with open('em.txt', 'w') as f:
    f.write('ememem')

等价于

tmp = open('em.txt', 'w')
f = tmp.__enter__()
#tmp == f  True
f.write('ememem')
f.__exit__()
print(f)
# <_io.TextIOWrapper name='em.txt' mode='w' encoding='cp65001'>

• 打开文件

  with open('em.txt', 'w') as f:
      f.write('ememem')
  

• 线程lock

  lock = threading.Lock()
  # with里面获取lock, 退出with后释放lock
  with lock:
      # Critical section of code
      ...
  

• 数据库游标

  db_connection = DatabaseConnection()
  with db_connection as cursor:
      # with里面获取cursor, 退出with后释放cursor
      cursor.execute('insert into ...')
      cursor.execute('delete from ...')
      # ... more operations ...
  

• 同时获取lock和数据库游标

  lock = threading.Lock()
  cur = db.cursor()
  with cur as cu, lock as locked:
      ...
  

• 请求后关闭

  import urllib, sys
  from contextlib import closing
  
  with closing(urllib.urlopen('http://www.yahoo.com')) as f:
      for line in f:
          sys.stdout.write(line)