如何在Python中创建类(即静态)变量或方法?


当前回答

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)。

其他回答

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]

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。

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)

用这种方式,静态变量是在用户定义的类出现时创建的,

class Student:

    the correct way of static declaration
    i = 10

    incorrect
    self.i = 10

类变量并允许子类化

假设你不是在寻找一个真正的静态变量,而是一个类似于蟒蛇的东西,它可以为同意的成年人做同样的工作,那么就使用一个类变量。这将为您提供一个所有实例都可以访问(和更新)的变量

注意:其他许多使用类变量的答案都会破坏子类化。应避免直接按名称引用类。

from contextlib import contextmanager

class Sheldon(object):
    foo = 73

    def __init__(self, n):
        self.n = n

    def times(self):
        cls = self.__class__
        return cls.foo * self.n
        #self.foo * self.n would give the same result here but is less readable
        # it will also create a local variable which will make it easier to break your code
    
    def updatefoo(self):
        cls = self.__class__
        cls.foo *= self.n
        #self.foo *= self.n will not work here
        # assignment will try to create a instance variable foo

    @classmethod
    @contextmanager
    def reset_after_test(cls):
        originalfoo = cls.foo
        yield
        cls.foo = originalfoo
        #if you don't do this then running a full test suite will fail
        #updates to foo in one test will be kept for later tests

将为您提供与使用Sheldon.foo处理变量相同的功能,并将通过以下测试:

def test_times():
    with Sheldon.reset_after_test():
        s = Sheldon(2)
        assert s.times() == 146

def test_update():
    with Sheldon.reset_after_test():
        s = Sheldon(2)
        s.updatefoo()
        assert Sheldon.foo == 146

def test_two_instances():
    with Sheldon.reset_after_test():
        s = Sheldon(2)
        s3 = Sheldon(3)
        assert s.times() == 146
        assert s3.times() == 219
        s3.updatefoo()
        assert s.times() == 438

它还允许其他人简单地:

class Douglas(Sheldon):
    foo = 42

这也将起作用:

def test_subclassing():
    with Sheldon.reset_after_test(), Douglas.reset_after_test():
        s = Sheldon(2)
        d = Douglas(2)
        assert d.times() == 84
        assert s.times() == 146
        d.updatefoo()
        assert d.times() == 168 #Douglas.Foo was updated
        assert s.times() == 146 #Seldon.Foo is still 73

def test_subclassing_reset():
    with Sheldon.reset_after_test(), Douglas.reset_after_test():
        s = Sheldon(2)
        d = Douglas(2)
        assert d.times() == 84 #Douglas.foo was reset after the last test
        assert s.times() == 146 #and so was Sheldon.foo

有关创建课程时要注意的事项的最佳建议,请查看Raymond Hettinger的视频https://www.youtube.com/watch?v=HTLu2DFOdTg