对python上下文管理器的认识

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with self.assertRaises(ValidateError):
    do_something()

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class Connnection(object):
    def __enter__(self):
        return self

    def __exit__(self, err_type, value, traceback):
        self.close()

with Connnection() as con:
    do_something()

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@contextmanager
def _assertRaises(self, exception):
    """ Context manager that clears the environment upon failure. """
    with super(BaseCase, self).assertRaises(exception) as cm:
        with self.env.clear_upon_failure():
            yield cm

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class Connection(object):
    pass

def get_con():
    con = Connection()
    yield con
    con.close()

with get_con() as con:
    do_something()

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with self.assertRaises(ValidateError): # 1.由于assertRaises是一个generator，这里获取到这个generator对象后就会进入这个对象的__enter__方法，就是执行assertRaises方法yield之前的部分
    do_something() # 4.
...

@contextmanager
def _assertRaises(self, exception):
    """ Context manager that clears the environment upon failure. """
    with super(BaseCase, self).assertRaises(exception) as cm:   #  2.执行到这里的时候可以看到super(BaseCase, self).assertRaises(exception)返回的是一个_AssertRaisesContext对象，它实现了__enter__和__exit__方法
        with self.env.clear_upon_failure(): # 3.执行到这里的时候由于clear_upon_failure方法是一个generator，所以会执行yield之前的部分(啥也没有)，然后到8, 然后完成3这一层with的"__enter__"，执行下一句5直接是yield，那么对于_assertRaises来说，其yield方法已经执行完了，那么回到最开始的with，开始执行do_something方法4
            yield cm    # 5.
...

class _AssertRaisesContext(object):
    """A context manager used to implement TestCase.assertRaises* methods."""

    def __init__(self, expected, test_case, expected_regexp=None):
        self.expected = expected
        self.failureException = test_case.failureException
        self.expected_regexp = expected_regexp

    def __enter__(self):    # 6.
        return self

    def __exit__(self, exc_type, exc_value, tb): # 7.这里是异常类型断言的核心逻辑，通过在__exit__里判断异常类型与expected是否一致来决定是返回True还是False来决定异常断言的结果
        if exc_type is None:
            try:
                exc_name = self.expected.__name__
            except AttributeError:
                exc_name = str(self.expected)
            raise self.failureException(
                "{0} not raised".format(exc_name))
        if not issubclass(exc_type, self.expected):
            # let unexpected exceptions pass through
            return False
        self.exception = exc_value # store for later retrieval
        if self.expected_regexp is None:
            return True

        expected_regexp = self.expected_regexp
        if not expected_regexp.search(str(exc_value)):
            raise self.failureException('"%s" does not match "%s"' %
                     (expected_regexp.pattern, str(exc_value)))
        return True
...

@contextmanager
def clear_upon_failure(self):
    """ Context manager that clears the environments (caches and fields to
        recompute) upon exception.
    """
    try:
        yield   #  8.
    except Exception:
        self.clear()    #9.
        raise   # 10.