python上下文类中的类变量

Question

I am writing a series of nested contexts, and need to keep trace of their relative relationship. But I ran in the following behavior that I did not expect:

class NestedContext():
    _current = None
    def __enter__(self):
        self.parent = self._current
        self._current = self

    def __exit__(self, _type, _value, _tb):
         self._current = self.parent

with NestedContext():
    assert NestedContext._current is not None # fails
    with NestedContext():
        pass

The assert fail because the class variable _current is None, which is unexpected, since I thought __enter__ just set it to self. This seems to be related some deeper behavior of Python context that I don't understand. Can anyone explain? Thanks.

Answer 1

You are testing the class attribute . You set self._current , an instance attribute .

If you wanted to set the class attribute, you need to qualify it as such:

class NestedContext(object):
    _current = None

    def __enter__(self):
        self.parent = self._current
        NestedContext._current = self

    def __exit__(self, _type, _value, _tb):
         NestedContext._current = self.parent

When reading an attribute, Python searches through the layers of instance, class, and base classes. But when setting an attribute, no such layering takes place. So self._current will fall back to NestedContext if there is no such attribute on self , but self._current = some_value sets that attribute on self , not on the class.

I also used object as the parent class for NestedContext ; unless there are compelling reasons not to (eg you are working with legacy code that relies on the old behaviours), you always want to get new style classes .

Demo (with addition of a _name attribute and a __repr__ method):

>>> class NestedContext(object):
...     _current = None
...     def __init__(self, name):
...         self._name = name
...     def __repr__(self):
...         return '<NestedContext({._name!r})>'.format(self)
...     def __enter__(self):
...         self.parent = self._current
...         NestedContext._current = self
...     def __exit__(self, _type, _value, _tb):
...          NestedContext._current = self.parent
... 
>>> with NestedContext('outer'):
...     print NestedContext._current
...     with NestedContext('inner'):
...         print NestedContext._current
...         print NestedContext._current.parent
... 
<NestedContext('outer')>
<NestedContext('inner')>
<NestedContext('outer')>