Python script that downloads xkcd comics to basePath, prepends the comic number to the filename, and puts the alt text in 0-alt.txt. I recommend setting basePath to an absolute path.

xkcd.py

#!/usr/bin/python
#Download xkcd comics by their index numbers (passed as arguments)
import sys
import re
import json
from urllib.request import *

basePath = '/home/caleb/Pictures/Comics/xkcd/'

#Read alt text file into memory
altFile = open(basePath+'0-alt.txt', encoding='utf-8')
altText = [line.strip() for line in altFile]
altFile.close()

sys.argv.pop(0) #remove argv[0] (name of script)
for comicNum in sys.argv:
    #Skip duplicates
    skip = False
    for l in altText:
        if (str(comicNum)+':' in l):
            skip = True
    if skip == True:
        continue
    #Download alt text
    url = urlopen('http://xkcd.com/'+str(comicNum)+'/info.0.json')
    data = json.loads(url.read().decode('utf-8'))
    altText.append(str(comicNum)+': '+data['alt'])
    #Download image
    m = re.search('(?<=\/)[\w]+\.(jpg|png)', data['img'])
    url = urlretrieve(data['img'], basePath+str(comicNum)+'-'+m.group())

#Write alt text file to disk
altFile = open(basePath+'0-alt.txt', 'w', encoding='utf-8')
for l in altText:
    if l == '': continue
    altFile.write(l+'\n\n')
altFile.close()