如何在Python中创建类(即静态)变量或方法?
当前回答
您还可以使用元类强制类为静态类。
class StaticClassError(Exception):
pass
class StaticClass:
__metaclass__ = abc.ABCMeta
def __new__(cls, *args, **kw):
raise StaticClassError("%s is a static class and cannot be initiated."
% cls)
class MyClass(StaticClass):
a = 1
b = 3
@staticmethod
def add(x, y):
return x+y
然后,每当您意外尝试初始化MyClass时,都会收到StaticClassError。
其他回答
可以使用静态类变量,但可能不值得这样做。
这里有一个用Python 3编写的概念证明——如果任何确切的细节都是错误的,那么可以对代码进行调整,以匹配静态变量所指的任何内容:
class Static:
def __init__(self, value, doc=None):
self.deleted = False
self.value = value
self.__doc__ = doc
def __get__(self, inst, cls=None):
if self.deleted:
raise AttributeError('Attribute not set')
return self.value
def __set__(self, inst, value):
self.deleted = False
self.value = value
def __delete__(self, inst):
self.deleted = True
class StaticType(type):
def __delattr__(cls, name):
obj = cls.__dict__.get(name)
if isinstance(obj, Static):
obj.__delete__(name)
else:
super(StaticType, cls).__delattr__(name)
def __getattribute__(cls, *args):
obj = super(StaticType, cls).__getattribute__(*args)
if isinstance(obj, Static):
obj = obj.__get__(cls, cls.__class__)
return obj
def __setattr__(cls, name, val):
# check if object already exists
obj = cls.__dict__.get(name)
if isinstance(obj, Static):
obj.__set__(name, val)
else:
super(StaticType, cls).__setattr__(name, val)
使用中:
class MyStatic(metaclass=StaticType):
"""
Testing static vars
"""
a = Static(9)
b = Static(12)
c = 3
class YourStatic(MyStatic):
d = Static('woo hoo')
e = Static('doo wop')
以及一些测试:
ms1 = MyStatic()
ms2 = MyStatic()
ms3 = MyStatic()
assert ms1.a == ms2.a == ms3.a == MyStatic.a
assert ms1.b == ms2.b == ms3.b == MyStatic.b
assert ms1.c == ms2.c == ms3.c == MyStatic.c
ms1.a = 77
assert ms1.a == ms2.a == ms3.a == MyStatic.a
ms2.b = 99
assert ms1.b == ms2.b == ms3.b == MyStatic.b
MyStatic.a = 101
assert ms1.a == ms2.a == ms3.a == MyStatic.a
MyStatic.b = 139
assert ms1.b == ms2.b == ms3.b == MyStatic.b
del MyStatic.b
for inst in (ms1, ms2, ms3):
try:
getattr(inst, 'b')
except AttributeError:
pass
else:
print('AttributeError not raised on %r' % attr)
ms1.c = 13
ms2.c = 17
ms3.c = 19
assert ms1.c == 13
assert ms2.c == 17
assert ms3.c == 19
MyStatic.c = 43
assert ms1.c == 13
assert ms2.c == 17
assert ms3.c == 19
ys1 = YourStatic()
ys2 = YourStatic()
ys3 = YourStatic()
MyStatic.b = 'burgler'
assert ys1.a == ys2.a == ys3.a == YourStatic.a == MyStatic.a
assert ys1.b == ys2.b == ys3.b == YourStatic.b == MyStatic.b
assert ys1.d == ys2.d == ys3.d == YourStatic.d
assert ys1.e == ys2.e == ys3.e == YourStatic.e
ys1.a = 'blah'
assert ys1.a == ys2.a == ys3.a == YourStatic.a == MyStatic.a
ys2.b = 'kelp'
assert ys1.b == ys2.b == ys3.b == YourStatic.b == MyStatic.b
ys1.d = 'fee'
assert ys1.d == ys2.d == ys3.d == YourStatic.d
ys2.e = 'fie'
assert ys1.e == ys2.e == ys3.e == YourStatic.e
MyStatic.a = 'aargh'
assert ys1.a == ys2.a == ys3.a == YourStatic.a == MyStatic.a
您还可以使用元类强制类为静态类。
class StaticClassError(Exception):
pass
class StaticClass:
__metaclass__ = abc.ABCMeta
def __new__(cls, *args, **kw):
raise StaticClassError("%s is a static class and cannot be initiated."
% cls)
class MyClass(StaticClass):
a = 1
b = 3
@staticmethod
def add(x, y):
return x+y
然后,每当您意外尝试初始化MyClass时,都会收到StaticClassError。
@Blair Conrad表示,在类定义中声明的静态变量,而不是在方法中声明的是类或“静态”变量:
>>> class Test(object):
... i = 3
...
>>> Test.i
3
这里有几家餐厅。从以上示例继续:
>>> t = Test()
>>> t.i # "static" variable accessed via instance
3
>>> t.i = 5 # but if we assign to the instance ...
>>> Test.i # we have not changed the "static" variable
3
>>> t.i # we have overwritten Test.i on t by creating a new attribute t.i
5
>>> Test.i = 6 # to change the "static" variable we do it by assigning to the class
>>> t.i
5
>>> Test.i
6
>>> u = Test()
>>> u.i
6 # changes to t do not affect new instances of Test
# Namespaces are one honking great idea -- let's do more of those!
>>> Test.__dict__
{'i': 6, ...}
>>> t.__dict__
{'i': 5}
>>> u.__dict__
{}
请注意,当直接在t上设置属性i时,实例变量t.i如何与“static”类变量不同步。这是因为我在t命名空间中重新绑定,这与Test命名空间不同。如果要更改“静态”变量的值,必须在其最初定义的范围(或对象)内更改它。我把“static”放在引号里,因为Python实际上没有C++和Java那样的静态变量。
尽管Python教程没有具体说明静态变量或方法,但它提供了一些关于类和类对象的相关信息。
@Steve Johnson还回答了静态方法的问题,也在Python库参考中的“内置函数”中进行了记录。
class Test(object):
@staticmethod
def f(arg1, arg2, ...):
...
@beid还提到了classmethod,它类似于staticmethod。类方法的第一个参数是类对象。例子:
class Test(object):
i = 3 # class (or static) variable
@classmethod
def g(cls, arg):
# here we can use 'cls' instead of the class name (Test)
if arg > cls.i:
cls.i = arg # would be the same as Test.i = arg1
您可以使用列表或字典来获取实例之间的“静态行为”。
class Fud:
class_vars = {'origin_open':False}
def __init__(self, origin = True):
self.origin = origin
self.opened = True
if origin:
self.class_vars['origin_open'] = True
def make_another_fud(self):
''' Generating another Fud() from the origin instance '''
return Fud(False)
def close(self):
self.opened = False
if self.origin:
self.class_vars['origin_open'] = False
fud1 = Fud()
fud2 = fud1.make_another_fud()
print (f"is this the original fud: {fud2.origin}")
print (f"is the original fud open: {fud2.class_vars['origin_open']}")
# is this the original fud: False
# is the original fud open: True
fud1.close()
print (f"is the original fud open: {fud2.class_vars['origin_open']}")
# is the original fud open: False
与@staticmethod不同,但类变量是类的静态方法,并与所有实例共享。
现在您可以像这样访问它
instance = MyClass()
print(instance.i)
or
print(MyClass.i)
必须为这些变量赋值
我在努力
class MyClass:
i: str
并在一个方法调用中赋值,在这种情况下,它将不起作用,并将抛出错误
i is not attribute of MyClass
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