What is the correct way to generate a json from file in GoogleAppEngine?

Question

I'm quite new to python and GAE, can anyone please provide some help/sample code for doing the following simple task? I managed to read a simple file and output it as a webpage but I need some slightly more complicated logic. Here is the pseudo code:

  open file;
  for each line in file {
    store first line as album title;
    for each song read {
      store first line as song title;
      store second line as song URL;
    }
  }
  Output the read in data as a json;

The file format will be something like this

Album title1
song1 title
song1 url
song2 title
song2 url

Album title2
song1 title
song1 url
song2 title
song2 url
..

Answer 1

Here's a generator-based solution with a few nice features:

• Tolerates multiple blank lines between albums in text file
• Tolerates leading/trailing blank lines in text file
• Uses only an album's worth of memory at a time
• Demonstrates a lot of neato things you can do with Python :)

albums.txt

Album title1
song1 title
song1 url
song2 title
song2 url

Album title2
song1 title
song1 url
song2 title
song2 url

Code

from django.utils import simplejson

def gen_groups(lines):
   """ Returns contiguous groups of lines in a file """

   group = []

   for line in lines:
      line = line.strip()
      if not line and group:
         yield group
         group = []
      elif line:
         group.append(line)


def gen_albums(groups):
   """ Given groups of lines in an album file, returns albums  """

   for group in groups:
      title    = group.pop(0)
      songinfo = zip(*[iter(group)]*2)
      songs    = [dict(title=title,url=url) for title,url in songinfo]
      album    = dict(title=title, songs=songs)

      yield album


input = open('albums.txt')
groups = gen_groups(input)
albums = gen_albums(groups)

print simplejson.dumps(list(albums))

Output

[{"songs": [{"url": "song1 url", "title": "song1 title"}, {"url": "song2 url", "title": "song2 title"}], "title": "song2
title"},
{"songs": [{"url": "song1 url", "title": "song1 title"}, {"url": "song2 url", "title": "song2 title"}], "title": "song2
title"}]

Album information could then be accessed in Javascript like so:

var url = albums[1].songs[0].url;

Lastly, here's a note about that tricky zip line.

Answer 2

from django.utils import simplejson

def albums(f):
  "" yields lists of strings which are the
     stripped lines for an album (blocks of
     nonblank lines separated by blocks of
     blank ones.
  """
  while True:
    # skip leading blank lines if any
    for line in f:
      if not line: return
      line = line.strip()
      if line: break
    result = [line]
    # read up to next blank line or EOF
    for line in f:
      if not line:
        yield result
        return
      line = line.strip()
      if not line: break
      result.append(line)
    yield result

def songs(album):
  """ yields lists of 2 lines, one list per song.
  """
  for i in xrange(1, len(album), 2):
    yield (album[i:i+2] + ['??'])[:2]

result = dict()
f = open('thefile.txt')
for albumlines in albums(f):
  current = result[albumlines[0]] = []
  for songlines in songs(albumlines):
    current.append( {
      'songtitle': songlines[0],
      'songurl': songlines[1]
    } )

response.out.write(simplejson.dumps(result))