Making requests support stream file upload with a processbar

streamingupload.py

import requests
from io import BytesIO
import progressbar

class CancelledError(Exception):
    def __init__(self, msg):
        self.msg = msg
        Exception.__init__(self, msg)

    def __str__(self):
        return self.msg

    __repr__ = __str__


class BufferReader(BytesIO):
    def __init__(self, buf=b'',
                 callback=None,
                 cb_args=(),
                 cb_kwargs={}):
        self._callback = callback
        self._cb_args = cb_args
        self._cb_kwargs = cb_kwargs
        self._progress = 0
        self._len = len(buf)
        BytesIO.__init__(self, buf)


    def __len__(self):
        return self._len

    def read(self, n=-1):
        chunk = BytesIO.read(self, n)
        self._progress += int(len(chunk))
        self._cb_kwargs.update({
            'size'    : self._len,
            'progress': self._progress
        })
        if self._callback:
            try:
                self._callback(*self._cb_args, **self._cb_kwargs)
            except: # catches exception from the callback
                raise CancelledError('The upload was cancelled.')
        return chunk


############################################################

def upload(files, callback):
    url = 'ARBITRARY URL'
    (gendata, ctype) = requests.packages.urllib3.filepost.encode_multipart_formdata(files)
    headers = {
        "Content-Type": ctype
    }
    body = BufferReader(gendata, callback)

    response = requests.post(url, data=body, verify=False,headers=headers)
    return response

############################################################
class ProgressBar():
    def __init__(self):
        self.first_call = True
        self.finish = False
    def __call__(self, *args, **kwargs):
        if self.first_call:
            self.widgets = [progressbar.Percentage(), ' ', progressbar.Bar(marker=progressbar.RotatingMarker('>')),
                            ' ', progressbar.ETA()]
            self.pbar = progressbar.ProgressBar(widgets=self.widgets, maxval=kwargs['size']).start()
            self.first_call = False
        self.pbar.update(kwargs['progress'])
        if kwargs['size'] == kwargs['progress'] and self.finish:
            self.pbar.finish()
            self.finish = True

##############################################################

# MAIN
upload({'file':('filename','filecontent')},ProgressBar())