類實例內的上下文管理器

Question

我想創建一個特定於另一個類的實例的上下文管理器類。 我可以通過調用創建類的方法來做到這一點，但是我不確定這是最好，最好的方法：

class MyClass(object):
    def __init__(self):
        self.level = 0
        self.Nest = self.create_context_manager()
    def inclev(self):
        self.level += 1
    def declev(self):
        self.level -= 1

    def create_context_manager(self):
        self2 = self
        class Nest(object):
            def __init__(self):
                pass
            def __enter__(self):
                self2.inclev()
            def __exit__(self, exc_type, exc_value, traceback):
                self2.declev()
        return Nest

# Manually increase/decrease level
my_instance = MyClass()
print(my_instance.level)
my_instance.inclev()
print(my_instance.level)
my_instance.inclev()
print(my_instance.level)
my_instance.declev()
print(my_instance.level)
my_instance.declev()
print(my_instance.level)

# Use instance-specific context manager
other_instance = MyClass()
print(other_instance.level)
with other_instance.Nest():
    print(other_instance.level)
    with other_instance.Nest():
        print(other_instance.level)
    print(other_instance.level)
print(other_instance.level)

Answer 1

為什么需要嵌套類？ 只需讓主要對象直接實現上下文管理協議即可：

class MyClass(object):
    def __init__(self):
        self.level = 0
    def inclev(self):
        self.level += 1
    def declev(self):
        self.level -= 1

    __enter__ = inclev  # For __enter__, just alias inclev, no need for wrapper
    def __exit__(self, exc_type, exc_value, traceback):
        self.declev()

然后將其與：

with other_instance:
    print(other_instance.level)
    with other_instance:
        print(other_instance.level)
    print(other_instance.level)

如果你真的需要上下文管理協議作為構建東西叫Nest ，你仍然可以簡化一點與內建contextlib ：

from contextlib import contextmanager

class MyClass(object):
    def __init__(self):
        self.level = 0
    def inclev(self):
        self.level += 1
    def declev(self):
        self.level -= 1

    @contextmanager
    def Nest(self):
        self.inclev()
        try:
            yield
        finally:
            self.declev()