如何从回调函数访问(和编辑)变量？

Question

我使用Boto访问Amazon S3.对于文件上传,我可以分配一个回调函数.问题是我无法从该回调函数访问所需的变量,直到我将它们设为全局变量.另一方面,如果我将它们设置为全局,那么它们也是所有其他Celery任务的全局(直到我重新启动Celery),因为文件上载是从Celery任务执行的.

这是一个上传JSON文件的函数,其中包含有关视频转换进度的信息.

def upload_json():
    global current_frame
    global path_to_progress_file
    global bucket
    json_file = Key(bucket)
    json_file.key = path_to_progress_file
    json_file.set_contents_from_string('{"progress": "%s"}' % current_frame,
    cb=json_upload_callback, num_cb=2, policy="public-read")

这里有2个回调函数,用于上传视频转换过程中ffmpeg生成的帧和带有进度信息的JSON文件.

# Callback functions that are called by get_contents_to_filename.
# The first argument is representing the number of bytes that have
# been successfully transmitted from S3 and the second is representing
# the total number of bytes that need to be transmitted.
def frame_upload_callback(transmitted, to_transmit):
    if transmitted == to_transmit:
        upload_json()
def json_upload_callback(transmitted, to_transmit):
    global uploading_frame
    if transmitted == to_transmit:
        print "Frame uploading finished"
        uploading_frame = False

从理论上讲,我可以将uploaded_frame变量传递给upload_json函数,但它不会到达由Boto执行的json_upload_callback.

事实上,我可以写这样的东西.

In [1]: def make_function(message):
   ...:     def function():
   ...:         print message
   ...:     return function
   ...: 

In [2]: hello_function = make_function("hello")

In [3]: hello_function
Out[3]: <function function at 0x19f4c08>

In [4]: hello_function()
hello

但是,这不允许您编辑函数中的值,只是让您读取值.

def myfunc():
    stuff = 17
    def lfun(arg):
        print "got arg", arg, "and stuff is", stuff
    return lfun

my_function = myfunc()
my_function("hello")

这有效.

def myfunc():
    stuff = 17
    def lfun(arg):
        print "got arg", arg, "and stuff is", stuff
        stuff += 1
    return lfun

my_function = myfunc()
my_function("hello")

这给出了一个UnboundLocalError:在赋值之前引用的局部变量'stuff'.

谢谢.

Answer 1

在Python 2.x中,闭包是只读的.但是,你可以使用一个可变值的闭包...即

def myfunc():
    stuff = [17] # <<---- this is a mutable object
    def lfun(arg):
        print "got arg", arg, "and stuff[0] is", stuff[0]
        stuff[0] += 1
    return lfun

my_function = myfunc()
my_function("hello")
my_function("hello")

如果您使用的是Python 3.x,nonlocal则可以使用关键字指定闭包中读/写中使用的变量不是本地的,但应从封闭范围捕获:

def myfunc():
    stuff = 17
    def lfun(arg):
        nonlocal stuff
        print "got arg", arg, "and stuff is", stuff
        stuff += 1
    return lfun

my_function = myfunc()
my_function("hello")
my_function("hello")