Python: Parsing a JSON HTTP chunking stream
I’ve been playing around with meetup.com’s API again and this time wanted to consume the chunked HTTP RSVP stream and filter RSVPs for events I’m interested in.
I use Python for most of my hacking these days and if HTTP requests are required the requests library is my first port of call.
I started out with the following script
import requests import json def stream_meetup_initial(): uri = "http://stream.meetup.com/2/rsvps" response = requests.get(uri, stream = True) for chunk in response.iter_content(chunk_size = None): yield chunk for raw_rsvp in stream_meetup_initial(): print raw_rsvp try: rsvp = json.loads(raw_rsvp) except ValueError as e: print e continue
This mostly worked but I also noticed the following error from time to time:
No JSON object could be decoded
Although less frequent, I also saw errors suggesting I was trying to parse an incomplete JSON object. I tweaked the function to keep a local buffer and only yield that if the chunk ended in a new line character:
def stream_meetup_newline(): uri = "http://stream.meetup.com/2/rsvps" response = requests.get(uri, stream = True) buffer = "" for chunk in response.iter_content(chunk_size = 1): if chunk.endswith("\n"): buffer += chunk yield buffer buffer = "" else: buffer += chunk
This mostly works although I’m sure I’ve seen some occasions where two JSON objects were being yielded and then the call to 'json.loads' failed. I haven’t been able to reproduce that though.
A second read through the requests documentation made me realise I hadn’t read it very carefully the first time since we can make our lives much easier by using 'iter_lines' rather than 'iter_content':
r = requests.get('http://stream.meetup.com/2/rsvps', stream=True) for raw_rsvp in r.iter_lines(): if raw_rsvp: rsvp = json.loads(raw_rsvp) print rsvp
We can then process 'rsvp', filtering out the ones we’re interested in.