我尝试在python
shell中对图像进行编码和解码。我第一次在PIL中打开解码的base64字符串时，没有错误，如果我重复执行Image.open（）命令，我将得到
IOError：无法识别图像文件。

>>> with open("/home/home/Desktop/gatherify/1.jpg", "rb") as image_file:
...     encoded_string = base64.b64encode(image_file.read())
>>> image_string = cStringIO.StringIO(base64.b64decode(encoded_string))
>>> img = Image.open(image_string)
>>> img
<PIL.JpegImagePlugin.JpegImageFile image mode=RGB size=576x353 at 0xA21F20C>
>>> img.show() <-- Image opened without any errors. 
>>> img = Image.open(image_string)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/lib/python2.6/dist-packages/PIL/Image.py", line 1980, in open
    raise IOError("cannot identify image file")
IOError: cannot identify image file

为什么会这样呢？