How to Exempt db.Blobs from JSON in App Engine

Question

I'm using a version of the jsonGQL referenced in question 2114659 and available in the svn for eve-pos-tracker.

I have the same question as this poor app engine developer: https://groups.google.com/group/google-appengine/browse_thread/thread/f7a44b707119013c/81ba15c883ba95fe

My json serialization crashes when it hits the binary data for the png image (0x89) in my model. I don't need to serialize this data so I'd either like to skip the field or simply output something like <binary>.

this is the handler:

class FillList(webapp.RequestHandler):
    def get(self):
        fills = Fill.all()
        self.response.headers['Content-Type'] = 'application/json'
        self.response.out.write(jsonGQL.encode(fills.fetch(100)))

I tried this:

  elif isinstance(obj, db.Blob):
      return "<binary>"

but the error persists. The traceback is:

    Traceback (most recent call last):
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/webapp/_webapp25.py", line 701, in __call__
    handler.get(*groups)
  File "~/app/api.py", line 61, in get
    self.response.out.write(jsonGQL.encode(fills.fetch(100)))
  File "~/app/jsonGQL.py", line 103, in encode
    return GqlEncoder().encode(input, exclude=[])
  File "~/app/jsonGQL.py", line 87, in encode
    return simplejson.JSONEncoder.encode(self, o)
  File "~/app/simplejson/encoder.py", line 216, in encode
    chunks = list(chunks)
  File "~/app/simplejson/encoder.py", line 482, in _iterencode
    for chunk in _iterencode_list(o, _current_indent_level):
  File "~/app/simplejson/encoder.py", line 380, in _iterencode_list
    for chunk in chunks:
  File "~/app/simplejson/encoder.py", line 496, in _iterencode
    for chunk in _iterencode(o, _current_indent_level):
  File "~/app/simplejson/encoder.py", line 485, in _iterencode
    for chunk in _iterencode_dict(o, _current_indent_level):
  File "~/app/simplejson/encoder.py", line 439, in _iterencode_dict
    yield _encoder(value)
  File "~/app/simplejson/encoder.py", line 50, in py_encode_basestring_ascii
    s = s.decode('utf-8')
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/encodings/utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
UnicodeDecodeError: 'utf8' codec can't decode byte 0x89 in position 0: invalid start byte

How large an image? The trick I'd use is to convert the image into base64 before serializing. If required, http://stackoverflow.com/questions/2807251/can-i-embed-a-png-image-into-an-html-page has example of embedding base64 images directly into the HTML — Alvin K., Nov 21 '11 at 17:33
I'd rather not convert the image into anything, just skip it outright. I guess it's important to note I'm trying to serialize a whole set (edited to show) so I'm not sure how to go about dealing with the individual entries. — Regis Frey, Nov 21 '11 at 19:24

rds · Answer 1 · 2012-02-02T01:34:08.067

here is my fix

    elif isinstance(obj, db.Model):
        properties = obj.properties().items()
        output = {}
        for field, value in properties:
            data =  getattr(obj, field)
            if isinstance(data, str):
                # db.Blob inherits from str
                data = data.encode('string-escape')
            output[field] = data
        output['id'] = obj.key().id()
        return output

How to Exempt db.Blobs from JSON in App Engine

1 Answers1