ijson 1.0

Iterative JSON parser with a standard Python iterator interface

Ijson is an iterative JSON parser with a standard Python iterator interface.

Usage

All usage example will be using a JSON document describing geographical objects:

{
  "earth": {
    "europe": [
      {"name": "Paris", "type": "city", "info": { ... }},
      {"name": "Thames", "type": "river", "info": { ... }},
      // ...
    ],
    "america": [
      {"name": "Texas", "type": "state", "info": { ... }},
      // ...
    ]
  }
}

Most common usage is having ijson yield native Python objects out of a JSON stream located under a prefix. Here’s how to process all European cities:

import ijson

f = urlopen('http://.../')
objects = ijson.items(f, 'earth.europe.item')
cities = (o for o in objects if o['type'] == 'city')
for city in cities:
    do_something_with(city)

Sometimes when dealing with a particularly large JSON payload it may worth to not even construct individual Python objects and react on individual events immediately producing some result:

import ijson

parser = ijson.parse(urlopen('http://.../'))
stream.write('<geo>')
for prefix, event, value in parser:
    if (prefix, event) == ('earth', 'map_key'):
        stream.write('<%s>' % value)
        continent = value
    elif prefix.endswith('.name'):
        stream.write('<object name="%s"/>' % value)
    elif (prefix, event) == ('earth.%s' % continent, 'end_map'):
        stream.write('</%s>' % continent)
stream.write('</geo>')

Backends

Ijson provides several implementations of the actual parsing in the form of backends located in ijson/backends:

• yajl2: wrapper around YAJL version 2.x

• yajl: wrapper around YAJL version 1.x

• python: pure Python parser (good to use under PyPy)

You can import a specific backend and use it in the same way as the top level library:

import ijson.backends.python as ijson

for item in ijson.items(...):
    # ...

Importing the top level library as import ijson tries to import all backends in order, so it either finds an appropriate version of YAJL or falls back to the Python backend if none is found.

Acknowledgements

Python parser in ijson is relatively simple thanks to Douglas Crockford who invented a strict, easy to parse syntax.

The YAJL library by Lloyd Hilaiel is the most popular and efficient way to parse JSON in an iterative fashion.

Ijson was inspired by yajl-py wrapper by Hatem Nassrat. Though ijson borrows almost nothing from the actual yajl-py code it was used as an example of integration with yajl using ctypes.