python 初次认识with 和 contextlib.closing_closing+python

我打开两个数据库的时候，都是 

with xxx as conn1:
    with yyy as conn2:
        code

with xxx as conn1, yyy as conn2:
    code

from contextlib import closing
from urllib2 import urlopen
 
with closing(urlopen('http://www.python.org';)) as page:
    for line in page:
        print(line)

class Door(object):
    def open(self):
        print 'Door is opened'
 
    def close(self):
        print 'Door is closed'
 
with Door() as d:
    d.open()

# 报错：
Traceback (most recent call last):
  File "1.py", line 38, in <module>
    with Door() as d:
AttributeError: __exit__

class Door(object):
    def open(self):
        print 'Door is opened'
 
    def close(self):
        print 'Door is closed'
 
with contextlib.closing(Door()) as door:
    door.open()

Door is opened
Door is closed

class closing(object):
    """Context to automatically close something at the end of a block.
    Code like this:
        with closing(<module>.open(<arguments>)) as f:
            <block>
    is equivalent to this:
        f = <module>.open(<arguments>)
        try:
            <block>
        finally:
            f.close()
    """
    def __init__(self, thing):
        self.thing = thing
    def __enter__(self):
        return self.thing
    def __exit__(self, *exc_info):
        self.thing.close()

from contextlib import contextmanager
 
@contextmanager
def tag(name):
    print("<%s>" % name)
    yield
    print("</%s>" % name)
 
with tag("h1"):
    print 'hello world!'

<h1>
hello world!
</h1>

import time
def wrapper(func):
    def new_func(*args, **kwargs):
        t1 = time.time()
        ret = func(*args, **kwargs)
        t2 = time.time()
        print 'cost time=', (t2-t1)
        return ret
    return new_func
 
@wrapper
def hello(a,b):
    time.sleep(1)
    print 'a + b = ', a+b
 
hello(100,200)

a + b =  300
cost time= 1.00243401527

from contextlib import contextmanager
 
@contextmanager
def cost_time():
    t1 = time.time()
    yield
    t2 = time.time()
    print 'cost time=',t2-t1
 
with cost_time():
    time.sleep(1)
    a = 100
    b = 200
    print 'a + b = ', a + b

a + b =  300
cost time= 1.00032901764

class GeneratorContextManager(object):
    """Helper for @contextmanager decorator."""
 
    def __init__(self, gen):
        self.gen = gen
 
    def __enter__(self):
        try:
            return self.gen.next()
        except StopIteration:
            raise RuntimeError("generator didn't yield")
 
    def __exit__(self, type, value, traceback):
        if type is None:
            try:
                self.gen.next()
            except StopIteration:
                return
            else:
                raise RuntimeError("generator didn't stop")
        else:
            if value is None:
                # Need to force instantiation so we can reliably
                # tell if we get the same exception back
                value = type()
            try:
                self.gen.throw(type, value, traceback)
                raise RuntimeError("generator didn't stop after throw()")
            except StopIteration, exc:
                return exc is not value
            except:
                if sys.exc_info()[1] is not value:
                    raise
 
 
def contextmanager(func):
    @wraps(func)
    def helper(*args, **kwds):
        return GeneratorContextManager(func(*args, **kwds))
    return helper