19

在使用 Openstack Swift 客户端库时，我遇到了 Python 生成器的问题。

手头的问题是我试图从特定的 url（大约 7MB）中检索一大串数据，将字符串分块成更小的位，然后发回生成器类，每次迭代都保存字符串的分块位。在测试套件中，这只是一个字符串，它被发送到 swift 客户端的一个猴子补丁类进行处理。

monkeypatched 类中的代码如下所示：

def monkeypatch_class(name, bases, namespace):
    '''Guido's monkeypatch metaclass.'''
    assert len(bases) == 1, "Exactly one base class required"
    base = bases[0]
    for name, value in namespace.iteritems():
        if name != "__metaclass__":
            setattr(base, name, value)
    return base

在测试套件中：

from swiftclient import client
import StringIO
import utils

class Connection(client.Connection):
    __metaclass__ = monkeypatch_class

    def get_object(self, path, obj, resp_chunk_size=None, ...):
        contents = None
        headers = {}

        # retrieve content from path and store it in 'contents'
        ...

        if resp_chunk_size is not None:
            # stream the string into chunks
            def _object_body():
                stream = StringIO.StringIO(contents)
                buf = stream.read(resp_chunk_size)
                while buf:
                    yield buf
                    buf = stream.read(resp_chunk_size)
            contents = _object_body()
        return headers, contents

返回生成器对象后，由存储类中的流函数调用：

class SwiftStorage(Storage):

    def get_content(self, path, chunk_size=None):
        path = self._init_path(path)
        try:
            _, obj = self._connection.get_object(
                self._container,
                path,
                resp_chunk_size=chunk_size)
            return obj
        except Exception:
            raise IOError("Could not get content: {}".format(path))

    def stream_read(self, path):
        try:
            return self.get_content(path, chunk_size=self.buffer_size)
        except Exception:
            raise OSError(
                "Could not read content from stream: {}".format(path))

最后，在我的测试套件中：

def test_stream(self):
    filename = self.gen_random_string()
    # test 7MB
    content = self.gen_random_string(7 * 1024 * 1024)
    self._storage.stream_write(filename, io)
    io.close()
    # test read / write
    data = ''
    for buf in self._storage.stream_read(filename):
        data += buf
    self.assertEqual(content,
                     data,
                     "stream read failed. output: {}".format(data))

输出结果如下：

======================================================================
FAIL: test_stream (test_swift_storage.TestSwiftStorage)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/bacongobbler/git/github.com/bacongobbler/docker-registry/test/test_local_storage.py", line 46, in test_stream
    "stream read failed. output: {}".format(data))
AssertionError: stream read failed. output: <generator object _object_body at 0x2a6bd20>

我尝试使用一个简单的 python 脚本来隔离它，该脚本遵循与上面代码相​​同的流程，它没有问题地通过：

def gen_num():
    def _object_body():
        for i in range(10000000):
            yield i
    return _object_body()

def get_num():
    return gen_num()

def stream_read():
    return get_num()

def main():
    num = 0
    for i in stream_read():
        num += i
    print num

if __name__ == '__main__':
    main()

非常感谢您对此问题的任何帮助:)

4

1 回答 1

13

在您的get_object方法中，您将返回值分配给_object_body()变量contents。但是，该变量也是保存您的实际数据的变量，并且在_object_body.

问题是这_object_body是一个生成器函数（它使用yield）。因此，当您调用它时，它会生成一个生成器对象，但该函数的代码在您迭代该生成器之前不会开始运行。这意味着当函数的代码真正开始运行时（for循环_test_stream），在你重新分配之后很久contents = _object_body()。

因此，您stream = StringIO(contents)创建了一个StringIO包含生成器对象的对象（因此您的错误消息），而不是数据。

这是一个说明问题的最小复制案例：

def foo():
    contents = "Hello!"

    def bar():
        print contents
        yield 1

    # Only create the generator. This line runs none of the code in bar.
    contents = bar()

    print "About to start running..."
    for i in contents:
        # Now we run the code in bar, but contents is now bound to 
        # the generator object. So this doesn't print "Hello!"
        pass
于 2013-12-16T16:50:08.290 回答