Python上下文管理器全实例详解

f = None
try:
 print("try")
 f = open("__init__.py", "r")
 print(f.read())
except Exception as e:
 print("exception")
finally:
 if f:
  print("finally")
  f.close()

class OpenHandle:

 def __init__(self, filename, mode):
  self.filename = filename
  self.mode = mode

 def __enter__(self):
  self.f = open(self.filename, self.mode)
  return self.f

 def __exit__(self, exc_type, exc_val, exc_tb):
  if exc_type:
   print("exception")
  else:
   print("normal")
  self.f.close()

with OpenHandle("book.txt", "r") as f:
 print(f.read())

from contextlib import contextmanager

@contextmanager
def managed_resource(*args, **kwds):
 # Code to acquire resource, e.g.:
 resource = acquire_resource(*args, **kwds)
 try:
  yield resource
 finally:
  # Code to release resource, e.g.:
  release_resource(resource)

>>> with managed_resource(timeout=3600) as resource:
...  # Resource is released at the end of this block,
...  # even if code in the block raises an exception

class DbTransaction:

 def __init__(self, session_maker):
  self.session_maker = session_maker

 def __enter__(self):
  self.session = self.session_maker()
  return self.session

 def __exit__(self, exc_type, exc_val, exc_tb):
  if exc_type:
   self.session.rollback()
  else:
   self.session.commit()
  self.session.close()
  return False if exc_type else True