Using the requests library:

import base64
import requests


def get_as_base64(url):

    return base64.b64encode(requests.get(url).content)

I think it's just

import base64
import requests

response = requests.get(url)
uri = ("data:" + 
       response.headers['Content-Type'] + ";" +
       "base64," + base64.b64encode(response.content))

Assuming content-type is set.

I'm not sure I understand your question. I assume you are doing something along the lines of:

import base64

with open("yourfile.ext", "rb") as image_file:
    encoded_string = base64.b64encode(image_file.read())

You have to open the file first of course, and read its contents - you cannot simply pass the path to the encode function.

Edit: Ok, here is an update after you have edited your original question.

First of all, remember to use raw strings (prefix the string with 'r') when using path delimiters on Windows, to prevent accidentally hitting an escape character. Second, PIL's Image.open either accepts a filename, or a file-like (that is, the object has to provide read, seek and tell methods).

That being said, you can use cStringIO to create such an object from a memory buffer:

import cStringIO
import PIL.Image

# assume data contains your decoded image
file_like = cStringIO.StringIO(data)

img = PIL.Image.open(file_like)
img.show()