语言特性

    class Excutor:
        def __enter__(self):
            set things up
            return thing
        def __exit__(self, type, value, traceback):
            tear things down

    with Excutor() as thing:
         some code

class yrange:
    def __init__(self, n):
        self.i = 0
        self.n = n

    def __iter__(self):
        return self

    def next(self):
        if self.i < self.n:
            i = self.i
            self.i += 1
            return i
        else:
            raise StopIteration()

>>> def foo():
...     print "begin"
...     for i in range(3):
...         print "before yield", i
...         yield i
...         print "after yield", i
...     print "end"
...
>>> f = foo()
>>> f.next()
begin
before yield 0
0
>>> f.next()
after yield 0
before yield 1
1
>>> f.next()
after yield 1
before yield 2
2
>>> f.next()
after yield 2
end
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>>

>>> def choose_class(name):
…       if name == ‘foo‘:
…           class Foo(object):
…               pass
…           return Foo     # 返回的是类，不是类的实例
…       else:
…           class Bar(object):
…               pass
…           return Bar
…
>>> MyClass = choose_class(‘foo‘)
>>> print MyClass              # 函数返回的是类，不是类的实例
<class ‘__main__‘.Foo>
>>> print MyClass()            # 你可以通过这个类创建类实例，也就是对象
<__main__.Foo object at 0x89c6d4c>

type 接受一个字典来为类定义属性，因此
>>> class Foo(A):
…       bar = True
可以翻译为：
>>> Foo = type(‘Foo‘, (A,), {‘bar‘:True})