为什么 del 在 with 块的末尾被调用？

Question

在with语句中创建的变量范围在with块之外（参考：Variable defined with with-statement available outside of with-block?）。但是当我运行下面的代码：

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 Foo.__del__ 在打印 foo 之前被调用（在 # line 1 上面）：

__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，为什么会在这里调用 Foo.__del__？

另外，如果在第二个with块中调用连接的__del__方法，我不明白为什么my_curs1.connection之后仍然打开（参见# line 2以上）。

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。但是一旦完成，该对象就不再需要了，Python 可以随意丢弃它。

换句话说，这个：

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

与此相同：

_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，就会发生您期望的行为。例如...

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

版画

__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__ 会打开它的游标。

为什么 del 在 with 块的末尾被调用？

Why is del called at the end of a with block?

python

mysql-python

with-statement

为什么 __del__ 在 with 块的末尾被调用？

Why is __del__ called at the end of a with block?

python

mysql-python

with-statement

为什么 del 在 with 块的末尾被调用？

Why is del called at the end of a with block?