为什么 __del__ 在 with 块的末尾被调用？

Question

在with 语句中创建的变量的范围在with 块之外（参考：Variable defined with with-statement available outside of with-block?）。但是当我运行以下代码时：

class Foo:
    def __init__(self):
        print "__int__() called."

    def __del__(self):
        print "__del__() called."

    def __enter__(self):
        print "__enter__() called."
        return "returned_test_str"

    def __exit__(self, exc, value, tb):
        print "__exit__() called."

    def close(self):
        print "close() called."

    def test(self):
        print "test() called."

if __name__ == "__main__":
    with Foo() as foo:
        print "with block begin???"
        print "with block end???"

    print "foo:", foo  # line 1

    print "-------- Testing MySQLdb -----------------------"
    with MySQLdb.Connect(host="xxxx", port=0, user="xxx", passwd="xxx", db="test") as my_curs2:
        print "(1)my_curs2:", my_curs2
        print "(1)my_curs2.connection:", my_curs2.connection
    print "(2)my_curs2.connection:", my_curs2.connection
    print "(2)my_curs2.connection.open:", my_curs2.connection.open  # line 2

输出显示在打印 foo 之前调用了 Foo.__del__（在上面的 # line 1）：

__int__() called.
__enter__() called.
with block begin???
with block end???
__exit__() called.
__del__() called.
foo: returned_test_str
-------- Testing MySQLdb -----------------------
(1)my_curs2: <MySQLdb.cursors.Cursor object at 0x7f16dc95b290>
(1)my_curs2.connection: <_mysql.connection open to 'xxx' at 2609870>
(2)my_curs2.connection: <_mysql.connection open to 'xxx' at 2609870>
(2)my_curs2.connection.open: 1

我的问题是，如果with 语句没有创建新的执行范围，为什么在这里调用Foo.__del__？

另外，如果连接的__del__ 方法在第二个with 块中被调用，我不明白为什么my_curs1.connection 之后仍然打开（参见上面的# line 2）。

Answer 1

请务必注意 foo 不是 Foo 类型的对象。您确实创建了一个Foo 并需要保留它，因为它可能包含调用__exit__ 所需的状态信息。但一旦完成，对象就不再需要，Python 可以随意丢弃它。

换一种说法，就是这样：

with Foo() as foo:
    print ('Hello World!')

和这个是一样的：

_bar = Foo()
foo = _bar.__enter__()
print ('Hello World!')
_bar.__exit__()
del _bar # This will call __del__ because _bar is the only reference

如果foo 是对with 块的foo 的引用，您所期望的行为将会发生。比如……

class Foo:
    def __init__(self):
        print ("__int__() called.")

    def __del__(self):
        print ("__del__() called.")

    def __enter__(self):
        print ("__enter__() called.")
        return self # foo now stores the Foo() object

    def __str__(self):
        return 'returned_test_str'

    def __exit__(self, exc, value, tb):
        print ("__exit__() called.")

    def close(self):
        print ("close() called.")

    def test(self):
        print ("test() called.")

if __name__ == "__main__":
    with Foo() as foo:
        print ("with block begin???")
        print ("with block end???")

    print ("foo:", foo)  # line 1

打印

__int__() called.
__enter__() called.
with block begin???
with block end???
__exit__() called.
foo: returned_test_str
__del__() called.

---

我不知道为什么 Connection.__exit__ 会保持光标打开。