英文:
How to wrap __getitem__ method correctly to keep square brackets functionality
问题
我有一个类B，它包装了类A的`__getitem__`方法：
def my_decorator(method):
def wrapper(*args, **kwargs):
print('Decorator')
method(*args, **kwargs)
return wrapper
class A:
def __getitem__(self, idx):
print(idx)
class B:
def __init__(self):
self._a = A()
self._a.__getitem__ = my_decorator(self._a.__getitem__)
def get_a(self, idx):
self._a[idx]
b = B()
b.get_a(0)
我期望的输出应该是：
Decorator
0
但是实际上我得到的输出是：
0
如果我将`get_a`方法改为如下所示，我可以得到期望的行为：
def get_a(self, idx):
self._a.__getitem__(idx)
似乎包装`__getitem__`方法并不影响使用方括号的访问方式。有没有办法使原始代码在使用方括号访问时也能正常工作？我不想修改`get_a`的实现方式。
英文:
I have a class B that wraps the the __getitem__ method of class A:
def my_decorator(method):
def wrapper(*args, **kwargs):
print('Decorator')
method(*args, **kwargs)
return wrapper
class A:
def __getitem__(self, idx):
print(idx)
class B:
def __init__(self):
self._a = A()
self._a.__getitem__ = my_decorator(self._a.__getitem__)
def get_a(self, idx):
self._a[idx]
b = B()
b.get_a(0)
I would expect the following output:
Decorator
0
But instead I just get:
0
If I change get_a method as follows, I get the expected behaviour:
def get_a(self, idx):
self._a.__getitem__(idx)
It seems that wrapping the __getitem__ method doesn't affect the access with square brackets. Is there any way to make the original code work with square brackets access? I don't want to modify the implementation of get_a.
答案1
得分: 4
self._a[idx] 相当于 type(self._a).__getitem__(idx)，而不是 self._a.__getitem__(idx)。你定义的实例属性 __getitem__ 在 [] 语法中会被忽略。
你需要定义一个继承自 A 的子类，重写 __getitem__ 方法。
class A:
def __getitem__(self, idx):
print(idx)
class SubA(A):
def __getitem__(self, idx):
print('Decorator')
return super().__getitem__(idx)
class B:
def __init__(self):
self._a = SubA()
def get_a(self, idx):
self._a[idx]
b = B()
b.get_a(0)
（请注意：__getitem__ 应该真正返回一个值，这样的话 get_a 需要显式地返回 self._a[idx] 的值。）
英文:
self._a[idx] is equivalent to type(self._a).__getitem__(idx), not self._a.__getitem__(idx). The instance attribute __getitem__ you define is ignored by the [] syntax.
You need to define a subclass of A that overrides __getitem__.
class A:
def __getitem__(self, idx):
print(idx)
class SubA(A):
def __getitem__(self, idx):
print('"Decorator"')
return super().__getitem__(idx)
class B:
def __init__(self):
self._a = SubA()
def get_a(self, idx):
self._a[idx]
b = B()
b.get_a(0)
(Just to note: __getitem__ should really return a value, in which case get_a needs to explicitly return the value of self._a[idx].)
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