python Singleton
1. Python __call__ special method practical example
instantiate a class: x = Foo() is really x = type(Foo).__call__(Foo), where __call__ is defined by the metaclass of Foo
2. use call to implement Singleton
class _Singleton(type):
""" A metaclass that creates a Singleton base class when called. """
_instances = {}
print("_instances")
def __call__(cls, *args, **kwargs):
ins = cls._instances.get(cls)
print("class is:", cls, "instance is:", ins, args, kwargs)
if not ins or (hasattr(ins, "_reset") and isinstance(ins, cls)
and ins._reset()):
cls._instances[cls] = super(
_Singleton, cls).__call__(*args, **kwargs)
return cls._instances[cls]
class Singleton(_Singleton('SingletonMeta', (object,), {})):
print"Singleton"
def __init__(self, *args, **kwargs):
print("Singleton __init__", args, kwargs)
class Logger(Singleton):
print"Logger"
def __init__(self, *args, **kwargs):
print("Logger __init__")
print(args, kwargs)
def _reset(self, *args, **kwargs):
print("Logger _reset")
print(args, kwargs)
return True
print("Type is:", type(Logger))
print(dir(type(Logger)))
l = Logger("hello")
l = Logger("world")
简述 __init__、__new__、__call__ 方法
3. Creating a singleton in Python
Method 2: A base class
class Singleton(object):
_instance = None
def __new__(class_, *args, **kwargs):
if not isinstance(class_._instance, class_):
class_._instance = object.__new__(class_, *args, **kwargs)
return class_._instance
class MyClass(Singleton, BaseClass):
pass
decorators
from functools import wraps
def decorator(argument):
def real_decorator(function):
@wraps(function)
def wrapper(*args, **kwargs):
funny_stuff()
something_with_argument(argument)
retval = function(*args, **kwargs)
more_funny_stuff()
return retval
return wrapper
return real_decorator
2. Python class method decorator with self arguments
#/usr/bin/env python3
from functools import wraps
def wrapper(method):
@wraps(method)
def _impl(self, *method_args, **method_kwargs):
method_output = method(self, *method_args, **method_kwargs)
return method_output + "!"
return _impl
class Foo:
@wrapper
def bar(self, word):
return word
f = Foo()
result = f.bar("kitty")
print(result)
traceback
1. python3 org
2. Extract traceback info from an exception object
web development
exercise
1 class T(object): 2 def __init__(self, data=None, left=None, right=None): 3 self.data = data 4 self.left = left 5 self.right = right 6 7 # 1 8 # / 9 # / 10 # 2 3 11 # / / 12 # 4 5 6 7 13 14 n4 = T(4) 15 n5 = T(5) 16 n6 = T(6) 17 n7 = T(7) 18 n2 = T(2, n4, n5) 19 n3 = T(3, n6, n7) 20 n1 = T(1, n2, n3) 21 22 23 def preOrder(n): 24 if not n: 25 return 26 print(n.data, end=" ") 27 preOrder(n.left) 28 preOrder(n.right) 29 30 print("PreOrder:") 31 preOrder(n1) 32 33 def inOrder(n): 34 if not n: 35 return 36 inOrder(n.left) 37 print(n.data, end=" ") 38 inOrder(n.right) 39 40 print(" inOrder:") 41 inOrder(n1) 42 43 def PostOrder(n): 44 if not n: 45 return 46 PostOrder(n.left) 47 PostOrder(n.right) 48 print(n.data, end=" ") 49 50 print(" postOrder:") 51 PostOrder(n1) 52 print() 53 54 print("*" * 20) 55 56 def preOrderStack(n): 57 if not n: 58 return 59 a = [n] 60 cur = n 61 while (a): 62 cur = a.pop() 63 print(cur.data, end=" ") 64 if cur.right: 65 a.append(cur.right) 66 if cur.left: 67 a.append(cur.left) 68 69 print("PreOrderStack:") 70 preOrderStack(n1) 71 print() 72 73 def inOrderStack(n): 74 if not n: 75 return 76 a = [] 77 cur = n 78 while (a or cur): 79 while(cur): 80 a.append(cur) 81 cur = cur.left 82 node = a.pop() 83 print(node.data, end=" ") 84 cur = node.right 85 86 print("InOrderStack:") 87 inOrderStack(n1) 88 print() 89 90 def postOrderStack(n): 91 if not n: 92 return 93 a = [n] 94 h = n 95 cur = None 96 while (a): 97 c = a[-1] 98 if (c.left and h !=c.left and h !=c.right): 99 a.append(c.left) 100 elif (c.right and h != c.right): 101 a.append(c.right) 102 else: 103 node = a.pop() 104 print(node.data, end=" ") 105 h = c 106 107 print("PostOrderStack:") 108 postOrderStack(n1) 109 print() 110 111 112 113 def levelOrder(n): 114 if not n: 115 return 116 a = [n] 117 while (a): 118 c = a.pop(0) 119 print(c.data, end=" ") 120 if (c.left): 121 a.append(c.left) 122 if (c.right): 123 a.append(c.right) 124 125 print("levelOrder:") 126 levelOrder(n1) 127 print()