如何在Python中创建类(即静态)变量或方法?
当前回答
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
其他回答
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。
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
是的,绝对可以在python中编写静态变量和方法。
静态变量:在类级别声明的变量称为静态变量,可以使用类名直接访问。
>>> class A:
...my_var = "shagun"
>>> print(A.my_var)
shagun
实例变量:类的实例相关和访问的变量是实例变量。
>>> a = A()
>>> a.my_var = "pruthi"
>>> print(A.my_var,a.my_var)
shagun pruthi
静态方法:与变量类似,可以使用类名直接访问静态方法。无需创建实例。
但请记住,静态方法不能在python中调用非静态方法。
>>> class A:
... @staticmethod
... def my_static_method():
... print("Yippey!!")
...
>>> A.my_static_method()
Yippey!!
在类定义中声明但不在方法中声明的变量是类或静态变量:
>>> class MyClass:
... i = 3
...
>>> MyClass.i
3
正如@millerdev所指出的,这会创建一个类级别i变量,但这与任何实例级别i变量都不同,因此您可以
>>> m = MyClass()
>>> m.i = 4
>>> MyClass.i, m.i
>>> (3, 4)
这与C++和Java不同,但与C#没有太大区别,在C#中,不能使用对实例的引用来访问静态成员。
看看Python教程对类和类对象的主题有什么看法。
@Steve Johnson已经回答了静态方法的问题,也在Python库参考中的“内置函数”中进行了说明。
class C:
@staticmethod
def f(arg1, arg2, ...): ...
@beidy推荐classmethods而不是staticmethod,因为该方法随后会接收类类型作为第一个参数。
您还可以向类动态添加类变量
>>> class X:
... pass
...
>>> X.bar = 0
>>> x = X()
>>> x.bar
0
>>> x.foo
Traceback (most recent call last):
File "<interactive input>", line 1, in <module>
AttributeError: X instance has no attribute 'foo'
>>> X.foo = 1
>>> x.foo
1
类实例可以更改类变量
class X:
l = []
def __init__(self):
self.l.append(1)
print X().l
print X().l
>python test.py
[1]
[1, 1]
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