Python用多处理程序构建字典

Question

I am quite beginner with the multiprocessing module. In my code I am trying to build a dictionary with images from a given path. I wrote the following code:

from multiprocessing import Pool
from PIL import Image, ImageTk
import glob

def process(path):
    print path
    im=ImageTk.PhotoImage(Image.open(path).resize((600, 600), Image.ANTIALIAS))
    name = (path.split('/')[1]).split('.')[0]
    return (name, im)

p = Pool(4)
input = glob.glob('./*.jpg')
image_list = dict(p.map(process, input))

If the code works correctly I would expect something like the following:

{'-22': PIL.ImageTk.PhotoImage object at 0x7f6b66507150,

'-23': PIL.ImageTk.PhotoImage object at 0x7f6b66507190, ... and so on}

... but instead I get the following error:

`multiprocessing.pool.MaybeEncodingError: Error sending result:`
`'[('-51', PIL.ImageTk.PhotoImage object at 0x7f6b664f6990),`
`('-47', PIL.ImageTk.PhotoImage object at 0x7f6b664f6fd0),`
`('-54', PIL.ImageTk.PhotoImage object at 0x7f6b66507050),`
`('-13', PIL.ImageTk.PhotoImage object at 0x7f6b665070d0),`
`('-45', PIL.ImageTk.PhotoImage object at 0x7f6b66507110),`
`('-49', PIL.ImageTk.PhotoImage object at 0x7f6b66507150),`
`('-48', PIL.ImageTk.PhotoImage object at 0x7f6b66507190),`
`('-26', PIL.ImageTk.PhotoImage object at 0x7f6b665071d0),`
`('-10', PIL.ImageTk.PhotoImage object at 0x7f6b66507210)]'.`
`Reason: 'UnpickleableError(tkapp object at 0x7f6b67c88e30,)'`

How can I solve this?

Answer 1

Multiprocessing is NOT threading. It is a completely separate process with its own interpreter. This has some advantages- you can't accidentally created shared mutable state, which is great! It has some disadvantages though- this is one of them.

All data structures that are passed to or from a multiprocessing process have to be serialized/deserialized. So the return value of that function, behind the scenes, has to be pickled- which as you can see, it cannot be.

With your current design use Threading instead of Multiprocessing.