python - 在 Python 中，统计类中变量的数量或防止添加新的类变量

Question

在python中，有没有一种方法可以防止在定义对象后添加新的类变量？

例如:

class foo:
    def __init__(self):
        self.a = 1
        self.b = 2
        self.c = 3

bar = foo()
try:
    bar.d = 4
except Exception, e:
    print "I want this to always print"

或者，有没有办法计算对象中变量的数量？

class foo:
    def __init__(self):
        self.a = 1
        self.b = 2
        self.c = 3
    def count(self):
        ...

bar = foo()
if bar.count() == 3:
    print "I want this to always print"

我想到的唯一方法是使用字典或列表:

class foo:
    def __int__(self):
        self.dict = {'foo':1, 'bar':2} 
        self.len  = 2
    def chk():
        return self.len == len(self.list)

但是这样做对于python来说感觉比较麻烦。 (obj.dict['foo'])。如果可能的话，我更喜欢 obj.foo。

我想要这个，这样我就不会在想要更改现有变量时不小心声明一个变量。

f = foo()
f.somename = 3
...
f.simename = 4 #this is a typo

if f.somename == 3:
    solve_everything()

Answer 1

我建议使用 __setattr__ 来避免 __slots__ 的奇怪之处。

你在使用 __setattr__ 时一定要小心，因为它负责设置 所有 实例属性，包括你在 __init__ 中设置的那些>。因此它必须有某种方式知道什么时候允许设置属性，什么时候拒绝它。在此解决方案中，我指定了一个特殊属性来控制是否允许使用新属性:

class A(object):
    def __init__(self):
        self.a = 1
        self.b = 2
        self.c = 3
        self.freeze = True

    def __setattr__(self, attr, value):
        if getattr(self, "freeze", False) and not hasattr(self, attr):
            raise AttributeError("You shall not set attributes!")
        super(A, self).__setattr__(attr, value)

测试:

a = A()
try:
    a.d = 89
except AttributeError:
    print "It works!"
else:
    print "It doesn't work."
a.c = 42
print a.a
print a.c
a.freeze = False
a.d = 28
a.freeze = True
print a.d

结果:

It works!14228

Also see gnibblers answer that wraps this concept neatly up in a class decorator, so it doesn't clutter up the class definition and can be reused in several classes without duplicating code.

---

EDIT:

Coming back to this answer a year later, I realize a context manager might solve this problem even better. Here's a modified version of gnibbler's class decorator:

from contextlib import contextmanager

@contextmanager
def declare_attributes(self):
    self._allow_declarations = True
    try:
        yield
    finally:
        self._allow_declarations = False

def restrict_attributes(cls):
    cls.declare_attributes = declare_attributes
    def _setattr(self, attr, value):
        disallow_declarations = not getattr(self, "_allow_declarations", False)
        if disallow_declarations and attr != "_allow_declarations":
            if not hasattr(self, attr):
                raise AttributeError("You shall not set attributes!")
        super(cls, self).__setattr__(attr, value)
    cls.__setattr__ = _setattr

    return cls

下面是如何使用它:

@restrict_attributes
class A(object):
    def __init__(self):
        with self.declare_attributes():
            self.a = 1
            self.b = 2
            self.c = 3

所以每当你想设置新的属性时，只要像上面那样使用with语句即可。也可以从实异常(exception)部完成:

a = A()
try:
    a.d = 89
except AttributeError:
    print "It works!"
else:
    print "It doesn't work."
a.c = 42
print a.a
print a.c
with a.declare_attributes():
    a.d = 28
print a.d