https://github.com/CyC2018/CS-Notes
书籍：命令行的艺术https://github.com/jlevy/the-art-of-command-line/blob/master/README-zh.md#%E5%9F%BA%E7%A1%80
restful:http://www.ruanyifeng.com/blog/2011/09/restful.html
单例模式：
第一种方法：使用装饰器
def singleton(cls):
instances = {}
def wrapper(*args, **kwargs):
if cls not in instances:
instances[cls] = cls(*args, **kwargs)
return instances[cls]
return wrapper
@singleton
class Foo(object):
pass
foo1 = Foo()
foo2 = Foo()
print(foo1 is foo2) # True
第二种方法：使用基类 New 是真正创建实例对象的方法，所以重写基类的 new 方法，以此保证创建对象的时候只生成一个实例
class Singleton(object):
def __new__(cls, *args, **kwargs):
if not hasattr(cls, ‘_instance‘):
cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
return cls._instance
class Foo(Singleton):
pass
foo1 = Foo()
foo2 = Foo()
print(foo1 is foo2) # True
第三种方法：元类，元类是用于创建类对象的类，类对象创建实例对象时一定要调用 call 方法，因此在调用 call 时候保证始终只创建一个实例即可，type 是 python 的元类
class Singleton(type):
def __call__(cls, *args, **kwargs):
if not hasattr(cls, ‘_instance‘):
cls._instance = super(Singleton, cls).__call__(*args, **kwargs)
return cls._instance
# Python2
class Foo(object):
__metaclass__ = Singleton
# Python3
class Foo(metaclass=Singleton):
pass
foo1 = Foo()
foo2 = Foo()
print(foo1 is foo2) # True
原文：https://www.cnblogs.com/luoyangyang/p/11267057.html