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import subprocess
import sys
import re
import datetime

child = subprocess.Popen("ffmpeg -i test.mp3 test2.mp3", shell=True, stderr=subprocess.PIPE, stdout=subprocess.PIPE)
complete = False
myline = ''
while True:
    out = child.stderr.read(1)
    if out == '' and child.poll() != None:
        break
    #create myline string
    myline += out
    if out in ('\r','\n'):
        #print(myline)
        # Duration / start
        m = re.search("Duration: ([0-9:.]+), start: ([0-9.]+)", myline)
        if m:
            # duration in hh:mm:ss format
            m_duration = m.group(1)
            # start (in seconds?)
            m_start = m.group(2)
            print("Duration: {0}, start: {1}".format(m_duration, m_start))
        n =  re.search("time=([0-9.]+)", myline)
        if n:
            n_tpassed = float(n.group(1))
            n_dt = datetime.timedelta(seconds=n_tpassed)
            print("time processed: {0} out of {1}".format(n_dt, m_duration))
            # Calculate time left:
            tformat = '%H:%M:%S.%f'
            tdelta = datetime.datetime.strptime(m_duration, tformat) - datetime.timedelta(seconds=n_tpassed)
            # Sometimes ffmpeg shows more time processed than the end time. It reverts the date to 1899 and shows a ValueError.
            if tdelta.year != 1899:
                # The datetime module calculates according to date and time. date is set as 1900-01-01 and we don't need that.
                print("Time left to process: {0}".format(tdelta.strftime("%H:%M:%S")))
        #reset myline
        myline = ''