I'd like to read multiple JSON objects from a file/stream in Python, one at a time. Unfortunately json.load() just .read()s until end-of-file; there doesn't seem to be any way to use it to read a single object or to lazily iterate over the objects.
Is there any way to do this? Using the standard library would be ideal, but if there's a third-party library I'd use that instead.
At the moment I'm putting each object on a separate line and using json.loads(f.readline()), but I would really prefer not to need to do this.
import my_json as json
import sys
for o in json.iterload(sys.stdin):
print("Working on a", type(o))
{"foo": ["bar", "baz"]} 1 2 [] 4 5 6
$ python3.2 example.py < in.txt
Working on a dict
Working on a int
Working on a int
Working on a list
Working on a int
Working on a int
Working on a int
Here's a much, much simpler solution. The secret is to try, fail, and use the information in the exception to parse correctly. The only limitation is the file must be seekable.
def stream_read_json(fn):
import json
start_pos = 0
with open(fn, 'r') as f:
while True:
try:
obj = json.load(f)
yield obj
return
except json.JSONDecodeError as e:
f.seek(start_pos)
json_str = f.read(e.pos)
obj = json.loads(json_str)
start_pos += e.pos
yield obj
Edit: just noticed that this will only work for Python >=3.5. For earlier, failures return a ValueError, and you have to parse out the position from the string, e.g.
def stream_read_json(fn):
import json
import re
start_pos = 0
with open(fn, 'r') as f:
while True:
try:
obj = json.load(f)
yield obj
return
except ValueError as e:
f.seek(start_pos)
end_pos = int(re.match('Extra data: line \d+ column \d+ .*\(char (\d+).*\)',
e.args[0]).groups()[0])
json_str = f.read(end_pos)
obj = json.loads(json_str)
start_pos += end_pos
yield obj