如何在Python中创建类(即静态)变量或方法?
当前回答
静态和类方法
正如其他答案所指出的,静态和类方法可以使用内置的修饰符轻松完成:
class Test(object):
# regular instance method:
def my_method(self):
pass
# class method:
@classmethod
def my_class_method(cls):
pass
# static method:
@staticmethod
def my_static_method():
pass
通常,my_method()的第一个参数绑定到类实例对象。相反,my_class_method()的第一个参数绑定到类对象本身(例如,在本例中为Test)。对于my_static_method(),没有任何参数是绑定的,并且有任何参数都是可选的。
“静态变量”
然而,实现“静态变量”(好吧,可变静态变量,无论如何,如果这在术语上不是矛盾的话…)并不是那么直接。正如millerdev在回答中指出的,问题是Python的类属性并不是真正的“静态变量”。考虑:
class Test(object):
i = 3 # This is a class attribute
x = Test()
x.i = 12 # Attempt to change the value of the class attribute using x instance
assert x.i == Test.i # ERROR
assert Test.i == 3 # Test.i was not affected
assert x.i == 12 # x.i is a different object than Test.i
这是因为行x.i=12向x添加了一个新的实例属性i,而不是更改测试类i属性的值。
部分预期的静态变量行为,即在多个实例之间同步属性(但不与类本身同步;请参见下面的“gotcha”),可以通过将类属性转换为属性来实现:
class Test(object):
_i = 3
@property
def i(self):
return type(self)._i
@i.setter
def i(self,val):
type(self)._i = val
## ALTERNATIVE IMPLEMENTATION - FUNCTIONALLY EQUIVALENT TO ABOVE ##
## (except with separate methods for getting and setting i) ##
class Test(object):
_i = 3
def get_i(self):
return type(self)._i
def set_i(self,val):
type(self)._i = val
i = property(get_i, set_i)
现在您可以:
x1 = Test()
x2 = Test()
x1.i = 50
assert x2.i == x1.i # no error
assert x2.i == 50 # the property is synced
静态变量现在将在所有类实例之间保持同步。
(注意:除非类实例决定定义自己版本的_i!但如果有人决定这样做,他们应该得到什么,不是吗??)
注意,从技术上讲,i仍然不是一个“静态变量”;它是一种属性,是一种特殊类型的描述符。然而,属性行为现在相当于跨所有类实例同步的(可变)静态变量。
不可变的“静态变量”
对于不可变的静态变量行为,只需省略属性setter:
class Test(object):
_i = 3
@property
def i(self):
return type(self)._i
## ALTERNATIVE IMPLEMENTATION - FUNCTIONALLY EQUIVALENT TO ABOVE ##
## (except with separate methods for getting i) ##
class Test(object):
_i = 3
def get_i(self):
return type(self)._i
i = property(get_i)
现在尝试设置实例i属性将返回AttributeError:
x = Test()
assert x.i == 3 # success
x.i = 12 # ERROR
需要注意的一点
请注意,上述方法仅适用于类的实例-当使用类本身时,它们将不起作用。例如:
x = Test()
assert x.i == Test.i # ERROR
# x.i and Test.i are two different objects:
type(Test.i) # class 'property'
type(x.i) # class 'int'
assert Test.i==x.i行产生错误,因为Test和x的i属性是两个不同的对象。
许多人会觉得这令人惊讶。然而,它不应该是。如果我们回去检查我们的测试类定义(第二个版本),我们会注意到这一行:
i = property(get_i)
显然,Test的成员i必须是属性对象,这是从属性函数返回的对象类型。
如果您发现上述问题令人困惑,那么您很可能仍然从其他语言(例如Java或c++)的角度来考虑它。您应该研究属性对象、Python属性的返回顺序、描述符协议和方法解析顺序(MRO)。
我提出了一个解决上述问题的方法;然而,我强烈建议,至少在你彻底理解为什么断言Test.I=x.I会导致错误之前,不要尝试执行以下操作。
实际静态变量-测试.i==x.i
我在下面介绍(Python3)解决方案,仅供参考。我并不赞同这是一个“好的解决方案”。我怀疑是否真的有必要在Python中模拟其他语言的静态变量行为。然而,不管它是否实际有用,下面的内容应该有助于进一步了解Python的工作原理。
更新:这种尝试真的很糟糕;如果你坚持这样做(提示:请不要这样做;Python是一种非常优雅的语言,不需要强迫它表现得像另一种语言),请使用Ethan Furman答案中的代码。
使用元类模拟其他语言的静态变量行为
元类是类的类。Python中所有类的默认元类(即,我认为Python 2.3之后的“新样式”类)是类型。例如:
type(int) # class 'type'
type(str) # class 'type'
class Test(): pass
type(Test) # class 'type'
但是,您可以这样定义自己的元类:
class MyMeta(type): pass
并将其应用于您自己的类,如下所示(仅适用于Python 3):
class MyClass(metaclass = MyMeta):
pass
type(MyClass) # class MyMeta
下面是我创建的元类,它试图模拟其他语言的“静态变量”行为。它基本上通过用检查所请求的属性是否为“静态变量”的版本替换默认的getter、setter和deleter来工作。
“静态变量”的目录存储在StaticVarMeta.statics属性中。最初尝试使用替代解析顺序解析所有属性请求。我将其称为“静态解决顺序”或“SRO”。这是通过在给定类(或其父类)的“静态变量”集合中查找所请求的属性来完成的。如果该属性未出现在“SRO”中,则类将返回默认的属性get/set/delete行为(即“MRO”)。
from functools import wraps
class StaticVarsMeta(type):
'''A metaclass for creating classes that emulate the "static variable" behavior
of other languages. I do not advise actually using this for anything!!!
Behavior is intended to be similar to classes that use __slots__. However, "normal"
attributes and __statics___ can coexist (unlike with __slots__).
Example usage:
class MyBaseClass(metaclass = StaticVarsMeta):
__statics__ = {'a','b','c'}
i = 0 # regular attribute
a = 1 # static var defined (optional)
class MyParentClass(MyBaseClass):
__statics__ = {'d','e','f'}
j = 2 # regular attribute
d, e, f = 3, 4, 5 # Static vars
a, b, c = 6, 7, 8 # Static vars (inherited from MyBaseClass, defined/re-defined here)
class MyChildClass(MyParentClass):
__statics__ = {'a','b','c'}
j = 2 # regular attribute (redefines j from MyParentClass)
d, e, f = 9, 10, 11 # Static vars (inherited from MyParentClass, redefined here)
a, b, c = 12, 13, 14 # Static vars (overriding previous definition in MyParentClass here)'''
statics = {}
def __new__(mcls, name, bases, namespace):
# Get the class object
cls = super().__new__(mcls, name, bases, namespace)
# Establish the "statics resolution order"
cls.__sro__ = tuple(c for c in cls.__mro__ if isinstance(c,mcls))
# Replace class getter, setter, and deleter for instance attributes
cls.__getattribute__ = StaticVarsMeta.__inst_getattribute__(cls, cls.__getattribute__)
cls.__setattr__ = StaticVarsMeta.__inst_setattr__(cls, cls.__setattr__)
cls.__delattr__ = StaticVarsMeta.__inst_delattr__(cls, cls.__delattr__)
# Store the list of static variables for the class object
# This list is permanent and cannot be changed, similar to __slots__
try:
mcls.statics[cls] = getattr(cls,'__statics__')
except AttributeError:
mcls.statics[cls] = namespace['__statics__'] = set() # No static vars provided
# Check and make sure the statics var names are strings
if any(not isinstance(static,str) for static in mcls.statics[cls]):
typ = dict(zip((not isinstance(static,str) for static in mcls.statics[cls]), map(type,mcls.statics[cls])))[True].__name__
raise TypeError('__statics__ items must be strings, not {0}'.format(typ))
# Move any previously existing, not overridden statics to the static var parent class(es)
if len(cls.__sro__) > 1:
for attr,value in namespace.items():
if attr not in StaticVarsMeta.statics[cls] and attr != ['__statics__']:
for c in cls.__sro__[1:]:
if attr in StaticVarsMeta.statics[c]:
setattr(c,attr,value)
delattr(cls,attr)
return cls
def __inst_getattribute__(self, orig_getattribute):
'''Replaces the class __getattribute__'''
@wraps(orig_getattribute)
def wrapper(self, attr):
if StaticVarsMeta.is_static(type(self),attr):
return StaticVarsMeta.__getstatic__(type(self),attr)
else:
return orig_getattribute(self, attr)
return wrapper
def __inst_setattr__(self, orig_setattribute):
'''Replaces the class __setattr__'''
@wraps(orig_setattribute)
def wrapper(self, attr, value):
if StaticVarsMeta.is_static(type(self),attr):
StaticVarsMeta.__setstatic__(type(self),attr, value)
else:
orig_setattribute(self, attr, value)
return wrapper
def __inst_delattr__(self, orig_delattribute):
'''Replaces the class __delattr__'''
@wraps(orig_delattribute)
def wrapper(self, attr):
if StaticVarsMeta.is_static(type(self),attr):
StaticVarsMeta.__delstatic__(type(self),attr)
else:
orig_delattribute(self, attr)
return wrapper
def __getstatic__(cls,attr):
'''Static variable getter'''
for c in cls.__sro__:
if attr in StaticVarsMeta.statics[c]:
try:
return getattr(c,attr)
except AttributeError:
pass
raise AttributeError(cls.__name__ + " object has no attribute '{0}'".format(attr))
def __setstatic__(cls,attr,value):
'''Static variable setter'''
for c in cls.__sro__:
if attr in StaticVarsMeta.statics[c]:
setattr(c,attr,value)
break
def __delstatic__(cls,attr):
'''Static variable deleter'''
for c in cls.__sro__:
if attr in StaticVarsMeta.statics[c]:
try:
delattr(c,attr)
break
except AttributeError:
pass
raise AttributeError(cls.__name__ + " object has no attribute '{0}'".format(attr))
def __delattr__(cls,attr):
'''Prevent __sro__ attribute from deletion'''
if attr == '__sro__':
raise AttributeError('readonly attribute')
super().__delattr__(attr)
def is_static(cls,attr):
'''Returns True if an attribute is a static variable of any class in the __sro__'''
if any(attr in StaticVarsMeta.statics[c] for c in cls.__sro__):
return True
return False
其他回答
关于这个答案,对于常量静态变量,可以使用描述符。下面是一个示例:
class ConstantAttribute(object):
'''You can initialize my value but not change it.'''
def __init__(self, value):
self.value = value
def __get__(self, obj, type=None):
return self.value
def __set__(self, obj, val):
pass
class Demo(object):
x = ConstantAttribute(10)
class SubDemo(Demo):
x = 10
demo = Demo()
subdemo = SubDemo()
# should not change
demo.x = 100
# should change
subdemo.x = 100
print "small demo", demo.x
print "small subdemo", subdemo.x
print "big demo", Demo.x
print "big subdemo", SubDemo.x
导致。。。
small demo 10
small subdemo 100
big demo 10
big subdemo 10
如果您不喜欢忽略设置值(上面的传递),您总是可以引发异常。如果您正在寻找C++、Java风格的静态类变量:
class StaticAttribute(object):
def __init__(self, value):
self.value = value
def __get__(self, obj, type=None):
return self.value
def __set__(self, obj, val):
self.value = val
请查看此答案和HOWTO官方文件,以了解有关描述符的更多信息。
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
例如,如果您试图共享一个静态变量,在其他实例之间增加它,则类似以下脚本的操作很正常:
# -*- coding: utf-8 -*-
class Worker:
id = 1
def __init__(self):
self.name = ''
self.document = ''
self.id = Worker.id
Worker.id += 1
def __str__(self):
return u"{}.- {} {}".format(self.id, self.name, self.document).encode('utf8')
class Workers:
def __init__(self):
self.list = []
def add(self, name, doc):
worker = Worker()
worker.name = name
worker.document = doc
self.list.append(worker)
if __name__ == "__main__":
workers = Workers()
for item in (('Fiona', '0009898'), ('Maria', '66328191'), ("Sandra", '2342184'), ('Elvira', '425872')):
workers.add(item[0], item[1])
for worker in workers.list:
print(worker)
print("next id: %i" % Worker.id)
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
当然是的,Python本身没有任何静态数据成员,但我们可以这样做
class A:
counter =0
def callme (self):
A.counter +=1
def getcount (self):
return self.counter
>>> x=A()
>>> y=A()
>>> print(x.getcount())
>>> print(y.getcount())
>>> x.callme()
>>> print(x.getcount())
>>> print(y.getcount())
输出
0
0
1
1
解释
here object (x) alone increment the counter variable
from 0 to 1 by not object y. But result it as "static counter"
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