笔者的上一篇python文章阅读量不错，看来python爱好者很多，所以再写一篇，以飨读者。
先接着上一篇讲一个问题，下面这段code有没有问题？
def countcalls(func):
count = 0
def wrapper(*args, **kwargs):
count += 1
print('num of calls: {}'.format(count))
return func(*args, **kwargs)
return wrapper @countcalls
def foo(x):
print (x+1) foo(1)
运行时会发现：
UnboundLocalError: local variable 'count' referenced before assignment
原因是count在wrapper下面除非global，不然是不可见的，那么就没有初始化了。但是这是不能加global的，因为它不是global，如果移出函数外，那么结果又不对了。怎么解决呢？
python3的解决方案：
nonlocal count
count += 1
python2的解决方案：
def countcalls(func):
def wrapper(*args, **kwargs):
wrapper.count += 1
print('num of calls: {}'.format(wrapper.count))
return func(*args, **kwargs)
wrapper.count = 0
return wrapper
基于class的decorator
class PrintLog:
def __init__(self, func):
self.func = func
def __call__(self, *args, **kwargs):
print ('CALLING: {}'.format(self.func.__name__))
return self.func(*args, **kwargs)
主要是__call__决定的，任何object只要定义了__call__方法就能当函数用。下面对比一下：
class PrintLog:
def __init__(self, func):
self.func = func
def __call__(self, *args, **kwargs):
print ('CALLING: {}'.format(self.func.__name__))
return self.func(*args, **kwargs) def printlog(func):
def wrapper(*args, **kwargs):
print("CALLING: " + func.__name__)
return func(*args, **kwargs)
return wrapper @printlog
def f(n):
return n+2 @PrintLog
def f_class(n):
return n+2
CALLING: f
5
CALLING: f_class
5
完全等效。__call__就相当于wrapper function
magic methods
相当于是python中的syntax sugar，让+，-，*，／啥的拿过来就用，举例如下：
class Angle:
def __init__(self, value):
self.value = value % 360
def __add__(self, other_angle):
return Angle(self.value + other_angle.value) a = Angle(45)
b = Angle(90)
c = a + b
print (c.value)
135
常用方法如下：
数学运算
__add__ a + b
__sub__ a - b
__mul__ a * b
__truediv__ a/b(浮点除)
__mod__ a % b
__pow__ a ** b
位运算
__lshift__ a << b
__rshift__ a >> b
__and__ a & b
__xor__ a ^ b
__or__ a | b
比较运算
__eq__ a == b
__ne__ a != b
__lt__ a < b
__le__ a <= b
__gt__ a > b
__ge__ a >= b
举个例子：
class Money(object):
def __init__(self, dollar, cent):
self.dollars = dollar
self.cents = cent
def __str__(self):
return "$" + str(self.dollars) + "." + "{:02}".format(self.cents)
def __repr__(self):
return "Money(" + str(self.dollars) + ", " + str(self.cents) + ")"
def __add__(self, other):
cents = (self.cents + other.cents) % 100
dollars = self.dollars + other.dollars + (self.cents + other.cents)/100
return Money(dollars, cents)
def __sub__(self, other):
if self.cents < other.cents:
cents = 100 + self.cents - other.cents
dollars = self.dollars - other.dollars - 1
else:
cents = self.cents - other.cents
dollars = self.dollars - other.dollars
return Money(dollars, cents)
def __mul__(self, other):
cents = (self.cents * other) % 100
dollars = self.dollars * other + (self.cents * other) / 100
return Money(dollars, cents)
def __eq__(self, other):
return self.dollars == other.dollars and self.cents == other.cents
def __ge__(self, other):
return self.dollars >= other.dollars and self.cents >= other.cents
def __lt__(self, other):
return self.dollars < other.dollars and self.cents < other.dollars
__str__和__repr__也是会时常用到的方法，它们都会放回string。__str__被用在print()时，同时str()也会调用它。而__repr__则是告诉你如何重现这个object，python命令行交互中输入object会调用__repr__，同时repr会调用它。
python collection类型
__getitem__相当于[index]
据两个例子大家就明白了，第一个例子是关于list，第二个例子是针对dict
class UniqueList:
def __init__(self, items):
self.items = []
for item in items:
self.append(item) def append(self, item):
if item not in self.items:
self.items.append(item) def __getitem__(self, index):
return self.items[index] ul = UniqueList([2,2,2,3,3,3,4,4,4]) print ul[0]
print ul[1]
print ul[2]
2
3
4
当然在具体实现的过程中对于index 要有input check，有问题需要raise IndexException，negative index要考虑语义的支持。
class MultiDict:
def __init__(self):
self.data = {} def insert(self, key, value):
if key not in self.data:
self.data[key] = []
self.data[key].append(value) def get_values(self, key):
return self.data[key] def get(self, key):
return self.get_values(key)[-1] def __getitem__(self, key):
return self.get_values(key)[-1] md = MultiDict()
md.insert("a", 1)
md.insert("a", 2)
print md['a']#2
Iterator
回到list那个例子，你觉得下面这个语法能够工作吗？
for i in ul:
print i
答：可以的，通过__getitem__我们不小心实现了iterator，事实上只要__getitem__能够接受0,1,2,... 并且在访问越界的时候raise IndexException，就相当于实现了iterator
Iterator的另一种实现是:
def __iter__(self):
return self def __next__(self):
...
raise StopIteration
return ...
可见__getitem__显得更简洁。