有没有办法避免在初始化类时调用__init__
,比如从类方法中调用?
我试图用Python语言创建一个不区分大小写和标点符号的字符串类,用于有效的比较,但在不调用__init__
的情况下创建一个新实例时遇到了问题。
>>> class String:
def __init__(self, string):
self.__string = tuple(string.split())
self.__simple = tuple(self.__simple())
def __simple(self):
letter = lambda s: ''.join(filter(lambda s: 'a' <= s <= 'z', s))
return filter(bool, map(letter, map(str.lower, self.__string)))
def __eq__(self, other):
assert isinstance(other, String)
return self.__simple == other.__simple
def __getitem__(self, key):
assert isinstance(key, slice)
string = String()
string.__string = self.__string[key]
string.__simple = self.__simple[key]
return string
def __iter__(self):
return iter(self.__string)
>>> String('Hello, world!')[1:]
Traceback (most recent call last):
File "<pyshell#2>", line 1, in <module>
String('Hello, world!')[1:]
File "<pyshell#1>", line 17, in __getitem__
string = String()
TypeError: __init__() takes exactly 2 positional arguments (1 given)
>>>
我应该用什么替换string = String(); string.__string = self.__string[key]; string.__simple = self.__simple[key]
,才能用切片初始化新对象?
编辑:
由于受到以下答案的启发,初始化器已被编辑为快速检查无参数。
def __init__(self, string=None):
if string is None:
self.__string = self.__simple = ()
else:
self.__string = tuple(string.split())
self.__simple = tuple(self.__simple())
发布于 2013-02-07 04:30:18
在这个例子中,使用元类提供了一个很好的解决方案。元类的用法有限,但运行良好。
>>> class MetaInit(type):
def __call__(cls, *args, **kwargs):
if args or kwargs:
return super().__call__(*args, **kwargs)
return cls.__new__(cls)
>>> class String(metaclass=MetaInit):
def __init__(self, string):
self.__string = tuple(string.split())
self.__simple = tuple(self.__simple())
def __simple(self):
letter = lambda s: ''.join(filter(lambda s: 'a' <= s <= 'z', s))
return filter(bool, map(letter, map(str.lower, self.__string)))
def __eq__(self, other):
assert isinstance(other, String)
return self.__simple == other.__simple
def __getitem__(self, key):
assert isinstance(key, slice)
string = String()
string.__string = self.__string[key]
string.__simple = self.__simple[key]
return string
def __iter__(self):
return iter(self.__string)
>>> String('Hello, world!')[1:]
<__main__.String object at 0x02E78830>
>>> _._String__string, _._String__simple
(('world!',), ('world',))
>>>
附录:
六年后,我的观点更倾向于Alex Martelli's answer而不是我自己的方法。考虑到元类仍然在脑海中,下面的答案显示了如何在有元类和没有元类的情况下解决问题:
#! /usr/bin/env python3
METHOD = 'metaclass'
class NoInitMeta(type):
def new(cls):
return cls.__new__(cls)
class String(metaclass=NoInitMeta if METHOD == 'metaclass' else type):
def __init__(self, value):
self.__value = tuple(value.split())
self.__alpha = tuple(filter(None, (
''.join(c for c in word.casefold() if 'a' <= c <= 'z') for word in
self.__value)))
def __str__(self):
return ' '.join(self.__value)
def __eq__(self, other):
if not isinstance(other, type(self)):
return NotImplemented
return self.__alpha == other.__alpha
if METHOD == 'metaclass':
def __getitem__(self, key):
if not isinstance(key, slice):
raise NotImplementedError
instance = type(self).new()
instance.__value = self.__value[key]
instance.__alpha = self.__alpha[key]
return instance
elif METHOD == 'classmethod':
def __getitem__(self, key):
if not isinstance(key, slice):
raise NotImplementedError
instance = self.new()
instance.__value = self.__value[key]
instance.__alpha = self.__alpha[key]
return instance
@classmethod
def new(cls):
return cls.__new__(cls)
elif METHOD == 'inline':
def __getitem__(self, key):
if not isinstance(key, slice):
raise NotImplementedError
cls = type(self)
instance = cls.__new__(cls)
instance.__value = self.__value[key]
instance.__alpha = self.__alpha[key]
return instance
else:
raise ValueError('METHOD did not have an appropriate value')
def __iter__(self):
return iter(self.__value)
def main():
x = String('Hello, world!')
y = x[1:]
print(y)
if __name__ == '__main__':
main()
发布于 2010-01-31 03:46:52
如果可行,让__init__
被调用(并通过适当的参数使调用无伤大雅)是更可取的。然而,如果需要太多改动,您确实有另一种选择,只要您避免使用旧式类的灾难性选择(在新代码中使用旧式类没有很好的理由,以及 to的几个很好的理由):
class String(object):
...
bare_s = String.__new__(String)
这个习惯用法通常用在classmethod
中,它的作用是作为“替代构造函数”,所以你通常会看到它以这样的方式使用:
@classmethod
def makeit(cls):
self = cls.__new__(cls)
# etc etc, then
return self
(这样,类方法将被正确地继承,并在子类而不是基类上调用时生成子类实例)。
发布于 2013-02-06 08:22:17
标准的pickle和copy模块使用的一个技巧是创建一个空类,使用它实例化对象,然后将该实例的__class__
分配给“真正的”类。例如:
>>> class MyClass(object):
... init = False
... def __init__(self):
... print 'init called!'
... self.init = True
... def hello(self):
... print 'hello world!'
...
>>> class Empty(object):
... pass
...
>>> a = MyClass()
init called!
>>> a.hello()
hello world!
>>> print a.init
True
>>> b = Empty()
>>> b.__class__ = MyClass
>>> b.hello()
hello world!
>>> print b.init
False
但请注意,这种方法很少是必要的。绕过__init__
可能会产生一些意想不到的副作用,特别是在您不熟悉原始类的情况下,因此请确保您知道自己在做什么。
https://stackoverflow.com/questions/2168964
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