Python srt to csv converter

#This script converts .srt subtitle files to .csv files with timestamp,
#subtitle number, and utterance stored as separate variables
#I only sort of know Python, so sorry in advance.

import csv
import os
import string

filePath = "./"
dirList = os.listdir(filePath)

for filename in dirList:
    timestamps = ["timestamp"] #the complete timestamp of each subtitle
    timeStarts = ["start time"] #when each subtitle appears on screen
    timeEnds = ["end time"] #when each subtitle is removed from the screen
    subtitleIndices = ["index"]
    utterances = ["utterance"]
    extension = str(os.path.splitext(filename)[1])
    root = str(os.path.splitext(filename)[0])
    if extension != ".srt": #skip files other than subtitle files
        continue
    currentFile = open(filename)
    prevLineWasUtt = False #was the most recent line looked at an utterance?
    for line in currentFile: #populate the lists that will be put in the csv
        line = line.strip() #get rid of whitespace characters on end of line
        if line.find("-") == 0: #prevents Excel from viewing entry as formula
            line = "\'" + line
        if line.isdigit():
            subtitleIndices.append(line)
        elif "-->" in line:
            timestamps.append(line)
            timeStarts.append(line[:8])
            timeEnds.append(line[17:25])
        elif len(line) > 1:
            if prevLineWasUtt:
                utterances[len(utterances)-1] = utterances[len(utterances)-1] + " " + line
            else:
                utterances.append(line)
            prevLineWasUtt = True
            continue
        prevLineWasUtt = False
    #put all the lists in a csv file with the same name as the subtitle file
    csvWriter = csv.writer(file(root + ".csv",'wb'), dialect = 'excel')
    csvWriter.writerows([subtitleIndices, timestamps, timeStarts, timeEnds, utterances])