Python 如何通过上下文管理器强制对象实例化？

Question

我想通过类上下文管理器强制对象实例化。所以不能直接实例化。

我实现了这个解决方案，但从技术上讲，用户仍然可以实例化对象。

class HessioFile:
    """
    Represents a pyhessio file instance
    """
    def __init__(self, filename=None, from_context_manager=False):
        if not from_context_manager:
            raise HessioError('HessioFile can be only use with context manager')

和上下文管理器:

@contextmanager
def open(filename):
    """
    ...
    """
    hessfile = HessioFile(filename, from_context_manager=True)

有更好的解决方案吗？

Answer 1

如果您认为您的客户将遵循基本的 Python 编码原则，那么您可以保证如果您不在上下文中，您的类中的任何方法都不会被调用。

您的客户端不应该显式调用 __enter__，因此如果调用了 __enter__，您就知道您的客户端使用了 with 语句并且是因此在上下文中(__exit__ 将被调用)。

你只需要有一个 bool 变量来帮助你记住你是在上下文中还是在上下文之外。

class Obj:
    def __init__(self):
        self._inside_context = False

    def __enter__(self):
        self._inside_context = True
        print("Entering context.")
        return self

    def __exit__(self, *exc):
        print("Exiting context.")
        self._inside_context = False

    def some_stuff(self, name):
        if not self._inside_context:
            raise Exception("This method should be called from inside context.")
        print("Doing some stuff with", name)

    def some_other_stuff(self, name):
        if not self._inside_context:
            raise Exception("This method should be called from inside context.")
        print("Doing some other stuff with", name)


with Obj() as inst_a:
    inst_a.some_stuff("A")
    inst_a.some_other_stuff("A")

inst_b = Obj()
with inst_b:
    inst_b.some_stuff("B")
    inst_b.some_other_stuff("B")

inst_c = Obj()
try:
    inst_c.some_stuff("c")
except Exception:
    print("Instance C couldn't do stuff.")
try:
    inst_c.some_other_stuff("c")
except Exception:
    print("Instance C couldn't do some other stuff.")

这将打印:

Entering context.
Doing some stuff with A
Doing some other stuff with A
Exiting context.
Entering context.
Doing some stuff with B
Doing some other stuff with B
Exiting context.
Instance C couldn't do stuff.
Instance C couldn't do some other stuff.

由于您可能有很多方法想要“保护”不被外部上下文调用，因此您可以编写一个装饰器来避免重复相同的代码来测试您的 bool 值:

def raise_if_outside_context(method):
    def decorator(self, *args, **kwargs):
        if not self._inside_context:
            raise Exception("This method should be called from inside context.")
        return method(self, *args, **kwargs)
    return decorator

然后将您的方法更改为:

@raise_if_outside_context
def some_other_stuff(self, name):
    print("Doing some other stuff with", name)