python - 将 "NoneType object has no attribute"用于自定义上下文管理器时获取 "with ... as"

Question

我用 Python 编写了一个简单的上下文管理器来处理单元测试(并尝试学习上下文管理器):

class TestContext(object):
    test_count=1
    def __init__(self):
        self.test_number = TestContext.test_count
        TestContext.test_count += 1

    def __enter__(self):
        pass

    def __exit__(self, exc_type, exc_value, exc_traceback):
        if exc_value == None:
            print 'Test %d passed' %self.test_number
        else:
            print 'Test %d failed: %s' %(self.test_number, exc_value)
        return True

如果我按如下方式编写测试，一切正常。

test = TestContext()
with test:
   print 'running test %d....' %test.test_number
   raise Exception('this test failed')

但是，如果我尝试使用 with...as，我不会得到对 TestContext() 对象的引用。运行这个:

with TestContext() as t:
    print t.test_number

引发异常'NoneType'对象没有属性'test_number'。

我哪里错了？

Answer 1

假设您需要访问在 with 语句中创建的上下文管理器，__enter__ needs to return self .如果您不需要访问它，__enter__ 可以返回您想要的任何内容。

The with statement will bind this method’s return value to the target(s) specified in the as clause of the statement, if any.

这会起作用。

class TestContext(object):
    test_count=1
    def __init__(self):
        self.test_number = TestContext.test_count
        TestContext.test_count += 1

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, exc_traceback):
        if exc_value == None:
            print 'Test %d passed' % self.test_number
        else:
            print 'Test %d failed: %s' % (self.test_number, exc_value)
        return True