python使用metaclass不允许类方法同名

我这里大部分代码出自The Python Master一书。

1. 在python的class里面,允许同名的方法存在。但是后定义的方法会覆盖掉先定义的方法.
例如:

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class Dod1:
def method1(self):
return "first definition"
'''The second definition takes precedence because
it overwrites the first entry in the namespace
dictionary as the class definition is processed
'''
def method1(self):
return "second definition"
def test_():
d1 = Dod1()
print(d1.method1())
test_()

test_() 会返回second definition。

又如：　

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class T:
def test1(self, second:str):
print("I am test1, and second is ", second)
def test1(self):
print("I am test1")
def test_():
t = T()
a = range(1,5)
try:
t.test1(a)
except TypeError as e:
print(e)
t.test1()
test_()

这一段代码的运行结果是

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test1() takes 1 positional argument but 2 were given
I am test1

2. 但是可以使用metaclass 避免这个情况，即不允许class里面有同名的method.
we can write metaclass which detects and prevents this.
rather than using a regular dictionary as the namespace object used during class construction,
we need a dictionary which raises an error when we try to assign to an existing key.
代码如下:

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class OneShotDict(dict):
def __init__(self, existing=None):
super().__init__()
if existing is not None:
for k, v in existing:
self[k] = v
def __setitem__(self, key, value):
if key in self:
raise KeyError("Cannot assign existing key {!r} "
"in {!r}".format(key, type(self).__name__))
super().__setitem__(key, value)
class ProhibitDuplicateMeta(type):
@classmethod
def __prepare__(mcs, name, bases):
return OneShotDict()

但是出错信息不够清晰如下：
KeyError: "Cannot assign existing key 'method1' in 'OneShotDict'"

改进以后则如下:

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class OneShotClassNamespace(dict):
def __init__(self, name, existing=None):
super().__init__()
self._name = name
if existing is not None:
for k,v in existing:
self[k] = v
def __setitem__(self, key, value):
if key in self:
raise TypeError("Can not reassign existing class "
"attribute {!r} of {!r}".format(key, self._name))
super().__setitem__(key, value)
class ProhibitDupInClass(type):
@classmethod
def __prepare__(mcs, name, bases):
return OneShotClassNamespace(name)
if __name__ == "__main__":
'''Can not define a class with duplicate methods using this metaclass'''
class Dod2(metaclass=ProhibitDupInClass):
def method1(self):
return "first definition"
def method1(self):
return "second definition"

这时候出错信息如下:
TypeError: Can not reassign existing class attribute 'method1' of 'Dod2'

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if __name__ == "__main__":
'''Can not define a class with duplicate methods using this metaclass'''
class T2(metaclass=ProhibitDupInClass):
def test1(self, second:str):
print("I am test1, and second is ", second)
def test1(self):
print("I am test1")
#出错信息
TypeError: Can not reassign existing class attribute 'test1' of 'T2'

hmchzb19

python使用metaclass不允许类方法同名