如何在Python中创建类(即静态)变量或方法?
当前回答
我发现最好的方法是使用另一个类。您可以创建一个对象,然后将其用于其他对象。
class staticFlag:
def __init__(self):
self.__success = False
def isSuccess(self):
return self.__success
def succeed(self):
self.__success = True
class tryIt:
def __init__(self, staticFlag):
self.isSuccess = staticFlag.isSuccess
self.succeed = staticFlag.succeed
tryArr = []
flag = staticFlag()
for i in range(10):
tryArr.append(tryIt(flag))
if i == 5:
tryArr[i].succeed()
print tryArr[i].isSuccess()
通过上面的示例,我创建了一个名为staticFlag的类。
此类应显示静态变量__success(私有静态变量)。
tryIt类表示我们需要使用的常规类。
现在我为一个标志(staticFlag)创建了一个对象。此标志将作为对所有常规对象的引用发送。
所有这些对象都被添加到列表tryArr中。
此脚本结果:
False
False
False
False
False
True
True
True
True
True
其他回答
关于Python的属性查找,一个非常有趣的点是它可以用来创建“虚拟变量”:
class A(object):
label="Amazing"
def __init__(self,d):
self.data=d
def say(self):
print("%s %s!"%(self.label,self.data))
class B(A):
label="Bold" # overrides A.label
A(5).say() # Amazing 5!
B(3).say() # Bold 3!
通常情况下,在创建它们之后,不会有任何分配给它们。请注意,查找使用self,因为尽管标签在不与特定实例关联的意义上是静态的,但值仍然取决于(的类)实例。
@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
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]
python中的静态方法称为classmethods。查看以下代码
class MyClass:
def myInstanceMethod(self):
print 'output from an instance method'
@classmethod
def myStaticMethod(cls):
print 'output from a static method'
>>> MyClass.myInstanceMethod()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unbound method myInstanceMethod() must be called [...]
>>> MyClass.myStaticMethod()
output from a static method
注意,当我们调用方法myInstanceMethod时,会得到一个错误。这是因为它要求在此类的实例上调用该方法。方法myStaticMethod使用decorator@classmethod设置为类方法。
为了好玩,我们可以通过传入类的实例来调用类上的myInstanceMethod,如下所示:
>>> MyClass.myInstanceMethod(MyClass())
output from an instance method
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