如何在Python中创建类(即静态)变量或方法?


当前回答

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!!

其他回答

为了避免任何潜在的混淆,我想对比静态变量和不可变对象。

一些基本对象类型,如整数、浮点数、字符串和元组,在Python中是不可变的。这意味着由给定名称引用的对象如果属于上述对象类型之一,则不能更改。可以将名称重新分配给不同的对象,但不能更改对象本身。

通过禁止变量名指向除当前指向的对象之外的任何对象,使变量成为静态变量更进一步。(注意:这是一个通用的软件概念,并不特定于Python;请参阅其他人的帖子,了解有关在Python中实现静态的信息)。

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!!

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。

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

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