Python上下文管理器类和上下文管理器装饰器contextmanager用法实例分析

with open("test/test.txt","w") as f:
 f.write("hello")

class MyResource:
  # __enter__ 返回的对象会被with语句中as后的变量接受
  def __enter__(self):
    print('connect to resource')
    return self

  def __exit__(self, exc_type, exc_value, tb):
    print('close resource conection')

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

with Myresource() as r:
  r.query()

from contextlib import contextmanager
class MyResource:
  def query(self):
    print('query data')
@contextmanager
def make_myresource():
  print('start to connect')
  yield MyResource()
  print('end connect')
  pass

with make_myresource() as r:
   r.query()

  try:
    gift = Gift()
    gift.isbn = isbn
    ... 
    db.session.add(gift)
    db.session.commit()
  except Exception as e:
    db.session.rollback()
    raise e

from flask_sqlalchemy import SQLAlchemy as _SQLALchemy
class SQLAlchemy(_SQLALchemy):
  @contextmanager
  def auto_commit(self):
    try:
      yield
      self.session.commit()
    except Exception as e:
      db.session.rollback()
      raise e

 with db.auto_commit():
    gift = Gift()
    gift.isbn = isbndb.session.add(gift)
    db.session.add(gift)

with db.auto_commit():
  user = User()
  user.set_attrs(form.data)
  db.session.add(user)