Decorators 是強大的工具，允許我們修改函數或 Class 的行為。 Decorators 可以某種方式修改 Class。這通常涉及返回一個新 Class，該新 Class 用一些修改或增強的功能替換原始 Class。
PYTHON
def add_attribute(cls):
cls.new_attribute = "New attribute added by decorator"
return cls
@add_attribute
class MyClass():
pass
# Areate an instance of MyClass
obj = MyClass()
# Access the new attribute
print(obj.new_attribute) # Output: New attribute added by decorator
下面是一範例，它將任何調用的方法的結果乘以 10。 在這種情況下，double_results decorator 會檢查 Class 的所有方法，如果找到任一個可調用方法，它將用 double_decorator 函數包裝它，使結果值加倍。
PYTHON
def double_results(cls):
# iterate over all attributes
for attr_name, attr_value in cls.__dict__.items():
if callable(attr_value): # if the attribute is a method
setattr(cls, attr_name, double_decorator(attr_value))
return cls
def double_decorator(func):
def wrapper(*args, **kwargs):
return func(*args, **kwargs) * 10
return wrapper
@double_results
class MyClass:
def add(self, a, b):
return a + b
# create an instance of MyClass
obj = MyClass()
# call the 'add' method
print(obj.add(2, 2)) # Output: 40
Class decorators 是一個更高層次的概念，不僅需要了解 Classes 如何工作，還需要了解 decorators 如何工作。
class CallCount:
def __init__(self, f):
self.f = f
self.count = 0
def __call__(self, *args, **kwargs):
self.count += 1
print(f"This function is called {self.count} times")
return self.f(*args, **kwargs)
@CallCount
def hello(name):
print(f"Hello, {name}")
執行結果:
This function is called 1 times
Hello, Python
Decorator class 用於計時函數執行：
PYTHON
import time
class Timer:
def __init__(self, func):
self.func = func
def __call__(self, *args, **kwargs):
start_time = time.time()
result = self.func(*args, **kwargs)
end_time = time.time()
print(f"Executing {self.func.__name__} took {end_time - start_time:4f} s")
return result
@Timer
def complex_calculation(n):
return sum(i*i for i in range(n))
print(complex_calculation(10000))
執行結果:
Executing complex_calculation took 0.001001 s
333283335000
範例 – 帶參數的 Decorator Class
要創建帶有參數的 Class-based decorator，我們需要確保 Class 本身接受這些參數。
PYTHON
class Repeat:
def __init__(self, num_times):
self.num_times = num_times
def __call__(self, func):
def wrapper(*args, **kwargs):
for _ in range(self.num_times):
result = func(*args, **kwargs)
return result
return wrapper
@Repeat(num_times=3)
def greet(name):
print(f"Hello {name}")
greet("Alice")
執行結果:
Hello Alice
Hello Alice
Hello Alice
範例 – Decorator Class 實例
PYTHON
class DecoratorClass:
def __init__(self, value):
self.value = value
def __call__(self, func):
def wrapper(*args, **kwargs):
print(f"Before function call, value is: {self.value}")
result = func(*args, **kwargs)
print(f"After function call, value is: {self.value}")
return result
return wrapper
def increment(self):
self.value += 1
decorator = DecoratorClass(10)
decorator.increment()
@decorator
def test_function():
print("Inside the function")
test_function()
執行結果:
Hello Alice
Hello Alice
Hello Alice
PEP 614
PEP 代表 Python Enhancement Proposal (增強提案)，它是一個描述 Python 新功能和改進的標準。 這些 PEP 在被接受並在 Python 中實現之前，經過了仔細的審查和討論過程。
PEP 614 (Python 3.9 ver.) 消除了 decorators 語法的限制，允許任何有效的表達式充當裝飾器；允許更具表現力和更複雜的裝飾器。
