我来自Java世界，正在阅读Bruce Eckels的《Python 3 Patterns, Recipes and idiom》。
在阅读有关类的内容时，会继续说在Python中不需要声明实例变量。你只需要在构造函数中使用它们，它们就在那里了。
例如:
class Simple:
def __init__(self, s):
print("inside the simple constructor")
self.s = s
def show(self):
print(self.s)
def showMsg(self, msg):
print(msg + ':', self.show())
如果这是真的，那么任何Simple类的对象都可以改变类外变量s的值。
例如:
if __name__ == "__main__":
x = Simple("constructor argument")
x.s = "test15" # this changes the value
x.show()
x.showMsg("A message")
在Java中，我们已经学习了公共/私有/受保护变量。这些关键字是有意义的，因为有时您希望类中的变量在类之外没有人可以访问。
为什么Python中不需要这个?
在Python 3中，如果你只是想“封装”类属性，就像在Java中一样，你可以这样做:
class Simple:
def __init__(self, str):
print("inside the simple constructor")
self.__s = str
def show(self):
print(self.__s)
def showMsg(self, msg):
print(msg + ':', self.show())
要实例化此操作:
ss = Simple("lol")
ss.show()
注意:print(ss.__s)将抛出一个错误。
实际上，Python 3会混淆全局属性名。就像在Java中一样，它将此转换为“私有”属性。属性名仍然是全局的，但是以一种不可访问的方式，就像其他语言中的私有属性一样。
但是不要害怕。没关系。它也起作用了。；）
如前所述，可以通过在变量或方法前面加上下划线来表示它是私有的。如果您觉得这还不够，还可以使用属性装饰器。这里有一个例子:
class Foo:
def __init__(self, bar):
self._bar = bar
@property
def bar(self):
"""Getter for '_bar'."""
return self._bar
This way, someone or something that references bar is actually referencing the return value of the bar function rather than the variable itself, and therefore it can be accessed but not changed. However, if someone really wanted to, they could simply use _bar and assign a new value to it. There is no surefire way to prevent someone from accessing variables and methods that you wish to hide, as has been said repeatedly. However, using property is the clearest message you can send that a variable is not to be edited. property can also be used for more complex getter/setter/deleter access paths, as explained here: https://docs.python.org/3/library/functions.html#property
下面是我处理Python 3类字段的方法:
class MyClass:
def __init__(self, public_read_variable, private_variable):
self.public_read_variable_ = public_read_variable
self.__private_variable = private_variable
我只在MyClass方法中使用两个下划线访问__private_variable。
我用一个下划线对public_read_variable_进行读访问
在类之外，但从不修改变量:
my_class = MyClass("public", "private")
print(my_class.public_read_variable_) # OK
my_class.public_read_variable_ = 'another value' # NOT OK, don't do that.
