Python常用装饰器汇总
本节将介绍一些比较常用的装饰器。
1、类装饰器
下面的代码演示了 staticmethod 和 classmethod 两个类装饰器的用法，分别用来装饰类成员函数，用来表示某个方法是静态方法或类方法。
class DemoClass1(object):
attr1 = 1 # 类属性
@staticmethod # 静态方法
def static_func1():
print(DemoClass1.attr1)
DemoClass1.attr1 += 1
@classmethod # 类方法
def class_func1(cls):
print(cls.attr1)
cls.attr1 += 2
DemoClass1.static_func1()
DemoClass1.class_func1()
DemoClass1.static_func1()
DemoClass1.class_func1()
运行后输出结果如下：
$ python staticclassdemo1.py
1
2
4
5
另外一个常见的装饰器是 property，该装饰器表明被装饰函数会在读取与函数同名的属性时被调用，并且得到的是被装饰函数的返回值。注意，该属性是只读的，不能被赋值。下面是一个例子。
>>> class PropertyDemo1(object): ... def __init__(self): ... self.field1 = 0 ... self.field2 = 0 ... @property ... def count(self): ... return self.field1 ... # 类定义结束 >>> obj1 = PropertyDemo1() >>> obj1.count 0 >>> obj1.count = 4 # 只读属性，不能被赋值 Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: can't set attribute如果希望可读也可写，那么需要先定义一个被 property 装饰的函数，然后定义一个被 setter 装饰的函数。下面是一个例子。
>>> class GetterSetter(object):
... def __init__(self):
... self.field1 = 0
... @property # 表明是一个属性，默认是只读的
... def count(self):
... return self.field1
... @count.setter # 添加设置写操作函数
... def count(self, value):
... if not isinstance(value, int):
... raise ValueError('count must be an integer!')
... self.field1 = value
... # 类定义结束
>>> obj1 = GetterSetter() # 创建一个对象
>>> obj1.count = 3 # 赋值
>>> obj1.count # 读出
3
2、退出时执行的函数
接下来介绍的装饰器用来装饰在程序退出时执行的函数。演示例子如下：
import time
def onexit(f): # 装饰器
import atexit
atexit.register(f) # 将被装饰函数注册到系统中
return f
@onexit
def on_exit_func1():
print("on_exit_func1() is running")
@onexit
def on_exit_func2():
print("on_exit_func2() is running")
print("Starting running") # 开始执行脚本
time.sleep(3)
print("Ending") # 脚本执行完毕
运行后的输出结果如下：
$ python onexit.py
Starting running
Ending
on_exit_func2() is running
on_exit_func1() is running
3、单例模式
这时装饰器装饰的类，表示该类最多只能有一个实例对象。演示例子如下：
>>> def singleton(cls): # 装饰器，不过传入的是一个类对象而不是函数对象
... instances = {}
... def getinstance():
... if cls not in instances:
... instances[cls] = cls() # 生成一个对象
... return instances[cls]
... return getinstance
... # 装饰器定义结束
>>> @singleton
... class DemoClass1:
... def __init__(self):
... self.field1 = 8
... self.field2 = "python"
... # 被装饰的类定义结束
>>> obj1 = DemoClass1() # 创建两个对象
>>> obj2 = DemoClass1() # 第二个对象
>>> obj1 is obj2 # 这两个对象是否是同一个对象
True # 是同一个对象
4、执行时间限制
有时需要限定某个函数的执行时间，如果超出该时间，需要强制结束运行。提供这种装饰器的 Python 包很多，这里仅介绍其中一个，即 timeout_decorator。使用该 timeout_decorator 之前，需要自行安装，方法是运行下面的命令行：
pip install timeout-decorator
下面是一个使用该装饰器的例子。
import time
import timeout_decorator # 引入装饰器
@timeout_decorator.timeout(5) # 不能超时5秒
def func_with_time_limit():
print("Start")
for i in range(1,10): # 循环10次
time.sleep(1)
print("{} seconds have passed".format(i))
if __name__ == '__main__':
func_with_time_limit()
运行该脚本，输出如下：
$ python timeoutdemo1.py
Start
1 seconds have passed
2 seconds have passed
3 seconds have passed
4 seconds have passed
Traceback (most recent call last): # 打印出来调用栈
File "timeoutdemo1.py", line 12, in <module>
func_with_time_limit()
File
"/anaconda3/lib/python3.7/site-packages/timeout_decorator/timeout_
decorator.py",
line 81, in new_function
return function(*args, **kwargs)
File "timeoutdemo1.py", line 8, in func_with_time_limit
time.sleep(1)
File
"/anaconda3/lib/python3.7/site-packages/timeout_decorator/timeout_
decorator.py",
line 72, in handler
_raise_exception(timeout_exception, exception_message)
File
"/anaconda3/lib/python3.7/site-packages/timeout_decorator/timeout_
decorator.py",
line 45, in _raise_exception
raise exception()
timeout_decorator.timeout_decorator.TimeoutError: 'Timed Out'
可以发现，在超时后它会自动抛出异常，我们可以捕捉该异常。下面是修改后的代码：
import time
import timeout_decorator
@timeout_decorator.timeout(5) # 不能超时5秒
def func_with_time_limit():
print("Start")
for i in range(1,10):
time.sleep(1)
print("{} seconds have passed".format(i))
if __name__ == '__main__':
try:
func_with_time_limit()
except:
print("Time-out")
运行后的输出如下：
$ python timeoutdemo2.py
Start
1 seconds have passed
2 seconds have passed
3 seconds have passed
4 seconds have passed
Time-out
5、执行时间标定
有时需要统计出某个函数每次执行的时间，此时既可以使用在函数执行之前和之后获取系统时间，然后通过计算得到实际运行时间，也可以使用现成的装饰器来达到这个目的。这里介绍的装饰器是 benchmark-decorator，这个装饰器也是需要读者自行安装的。安装命令如下：
pip install benchmark-decorator
下面是一个使用benchmark-decorator装饰器的例子。
import time
from benchmark import Benchmark # 引入标定模块
@Benchmark() # 标定该函数的执行时间
def do_real_work1():
time.sleep(0.1)
return 1
@Benchmark() # 标定该函数的执行时间
def do_real_work2():
time.sleep(0.2)
return 2
@Benchmark() # 标定该函数的执行时间
def func_benchmarked():
print("Start")
for i in range(1,10):
do_real_work1()
do_real_work2()
if __name__ == '__main__':
func_benchmarked ()
运行该脚本后会发现，在当前目录下多了一个文件 bench.log，标定数据都记录在该文件中了。下面是该文件的内容：
2019-07-28 06:10:05,921 - BENCHMARK - INFO - do_real_work1: 0.10025286674499512
2019-07-28 06:10:06,123 - BENCHMARK - INFO - do_real_work2: 0.20151209831237793
2019-07-28 06:10:06,227 - BENCHMARK - INFO - do_real_work1: 0.10357093811035156
2019-07-28 06:10:06,431 - BENCHMARK - INFO - do_real_work2: 0.2039330005645752
......
6、自动重新运行
如果执行过程中抛出了异常，那么可以尝试让 retry() 函数再次运行。该软件包也需要我们自行安装，安装命令如下：pip install retry_decorator
下面是一个使用该装饰器的例子：
from retry_decorator import * # 引入重试模块
@retry(Exception, tries=3, timeout_secs=0.3) # 自动重试3次，间隔0.3秒
def retry_demo(): # 如果抛出了任意类型的异常
import sys,time
print('working now')
raise Exception('Testing retry')
if __name__ == '__main__':
try:
retry_demo()
except Exception as e:
print('Received the last exception')
运行后的输出如下：
$ python retrydemo1.py
working now #第5行代码的输出
ERROR:root:Retrying in 0.32 seconds ... # 会在0.32秒后再次尝试运行
Traceback (most recent call last):
File "/anaconda3/lib/python3.7/site-packages/retry_decorator/retry_decorator.py",
line 26, in f_retry
return f(*args, **kwargs)
File "retrydemo1.py", line 7, in retry_demo
raise Exception('Testing retry')
Exception: Testing retry
working now # 第2轮运行，第5行代码的输出
ERROR:root:Retrying in 0.60 seconds ...
Traceback (most recent call last):
File "/anaconda3/lib/python3.7/site-packages/retry_decorator/retry_
decorator.py",
line 26, in f_retry
return f(*args, **kwargs)
File "retrydemo1.py", line 7, in retry_demo
raise Exception('Testing retry')
Exception: Testing retry
working now # 第3轮运行
Received the last exception
7、状态机
对于一个状态机来说，需要定义状态和状态迁移函数，在某个事件发生时，会调用对应的状态迁移函数，如果迁移函数执行失败，其会调用用户定义的失败处理函数。下面是 pysta temachine 包的源代码：
import functools
import inspect
class InvalidStateTransition(Exception):
pass
class StateInfo(object):
@staticmethod
def get_states(cls):
if not inspect.isclass(cls):
raise TypeError('"{0}" is no class object!'.format(cls))
if not hasattr(cls, '___pystatemachine_cls_states'):
states = tuple(state for _, state in
inspect.getmembers(cls, lambda member: isinstance(member,
State)))
setattr(cls, '___pystatemachine_cls_states', states)
return getattr(cls, '___pystatemachine_cls_states')
@staticmethod
def get_initial_state(cls):
if not inspect.isclass(cls):
raise TypeError('"{0}" is no class object!'.format(cls))
if not hasattr(cls, '___pystatemachine_cls_initial_state'):
states = StateInfo.get_states(cls)
initial_states = [state for state in states if state.is_
initial]
assert initial_states
assert len(initial_states) == 1
initial_state = initial_states[0]
setattr(cls, '___pystatemachine_cls_initial_state', initial_
state)
return getattr(cls, '___pystatemachine_cls_initial_state')
@staticmethod
def get_current_state(obj):
if not hasattr(obj, '___pystatemachine_obj_current_state'):
initial_state = StateInfo.get_initial_state(obj.__class__)
setattr(obj, '___pystatemachine_obj_current_state', initial_
state)
return getattr(obj, '___pystatemachine_obj_current_state')
@staticmethod
def set_current_state(obj, state):
assert isinstance(state, State), 'invalid state type!'
setattr(obj, '___pystatemachine_obj_current_state', state)
class State(object): # 定义状态类
def __init__(self, name, initial=False):
super(State, self).__init__()
self.is_initial = True if initial else False
self.name = name.upper()
def __str__(self):
return '<{0}.State[{1}] object at 0x{2:X}>'.format(__name__,
self.name, id(self))
def event(from_states=None, to_state=None): # 装饰器
""" a decorator for transitioning from certain states to a target
state.
must be used on bound methods of a class instance, only. """
from_states_tuple = (from_states, ) if isinstance(from_states,
State) \
else tuple(from_states or [])
if not len(from_states_tuple) >= 1:
raise ValueError()
if not all(isinstance(state, State) for state in from_states_tuple):
raise TypeError()
if not isinstance(to_state, State):
raise TypeError()
def wrapper(wrapped):
@functools.wraps(wrapped)
def transition(instance, *a, **kw):
if instance.current_state not in from_states_tuple:
raise InvalidStateTransition()
try:
result = wrapped(instance, *a, **kw)
except Exception as error:
error_handlers = getattr(instance,
'___pystatemachine_transition_failure_handlers', [])
for error_handler in error_handlers:
error_handler(instance, wrapped, instance.current_state,
to_state, error)
if not error_handlers:
raise error
else:
StateInfo.set_current_state(instance, to_state)
return result
return transition
return wrapper
def transition_failure_handler(calling_sequence=0):
def wrapper(wrapped):
setattr(wrapped, '___pystatemachine_is_transition_failure_
handler', True)
setattr(wrapped,
'___pystatemachine_transition_failure_handler_calling_
sequence',
int(calling_sequence))
return wrapped
return wrapper
def acts_as_state_machine(cls):
"""
a decorator which sets two properties on a class:
* the 'current_state' property: a read-only property,
returning the state machine's current state, as 'State' object
* the 'states' property: a tuple of all valid state machine states,
as 'State' objects
class objects may use current_state and states freely
:param cls:
:return:
"""
assert not hasattr(cls, 'current_state')
assert not hasattr(cls, 'states'), '{0} already has a "states"
attribute!'.format(cls)
def get_states(obj):
return StateInfo.get_states(obj.__class__)
def is_transition_failure_handler(obj):
return all([
any([
inspect.ismethod(obj), # Python 2
inspect.isfunction(obj), # python3
]),
getattr(obj, '___pystatemachine_is_transition_failure_
handler', False),
])
transition_failure_handlers = sorted(
[value for name, value in
inspect.getmembers(cls, is_transition_failure_handler)],
key=lambda m:
getattr(m,
'___pystatemachine_transition_failure_handler_calling_
sequence', 0),
)
setattr(cls,
'___pystatemachine_transition_failure_handlers',
transition_failure_handlers)
cls.current_state = property(fget=StateInfo.get_current_state)
cls.states = property(fget=get_states)
return cls
下面是使用该状态机装饰器的例子，该例子描述了闸机的状态。闸机只有两个状态，一个是锁死，一个是可以推动的非锁死状态。事件也有两个，一个是让闸机锁死，一个是让闸机解锁。状态转换函数应该有4个，但有两个可以和别的共享，所以实际上只有两个状态转换函数，一个是解锁处理函数，一个是锁定处理函数。由于任意状态转换的过程中可能发生异常情况，所以需要定义异常，该函数会在转换函数抛出异常时被调用。下面是一个例子：
@acts_as_state_machine
class Turnstile(object): # 这个类对应一个状态机
locked = State('locked', initial=True) # 定义两个状态
unlocked = State('unlocked')
@event(from_states=(locked, unlocked), to_state=unlocked)
def coin(self): # 解锁的处理函数
assert random.random() > .5, 'failing for demonstration
purposes, only ..'
print('*blingbling* .. unlocked!')
@event(from_states=(locked, unlocked), to_state=locked)
def push(self): # 锁定处理函数
print('*push* .. locked!')
@transition_failure_handler(calling_sequence=2)
def turnstile_malfunction(self, method, from_state, to_state,
error):
print(" *** turnstile_malfunction() is Running")
@transition_failure_handler(calling_sequence=1)
def before_turnstile_malfunction(self, method, from_state, to_state,
error):
print(" *** before_turnstile_malfunction() is Running")
import random
turnstile = Turnstile() # 创建状态机
for _ in range(10): # 产生10个事件
handler = random.choice([turnstile.coin, turnstile.push])
handler() # 触发事件
print("======================="). # 分隔符
运行后的输出如下：
$ python statemachine.py
*blingbling* .. unlocked!
=======================
*push* .. locked!
=======================
*** before_turnstile_malfunction() is Running
*** turnstile_malfunction() is Running
=======================
*** before_turnstile_malfunction() is Running
*** turnstile_malfunction() is Running
=======================
*** before_turnstile_malfunction() is Running
*** turnstile_malfunction() is Running
=======================
*** before_turnstile_malfunction() is Running
*** turnstile_malfunction() is Running
=======================
*push* .. locked!
=======================
*push* .. locked!
=======================
*push* .. locked!
=======================
*push* .. locked!
=======================
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