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Iterative JSON parser with a standard Python iterator interface

Project description

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ijson

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

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": { ... }},
      // ...
    ]
  }
}

High-level interfaces

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)

For how to build a prefix see the Prefix section below.

Other times it might be useful to iterate over object members rather than objects themselves (e.g., when objects are too big). In that case one can use the kvitems functions instead:

import ijson

f = urlopen('http://.../')
european_places = ijson.kvitems(f, 'earth.europe.item')
names = (v for k, v in european_places if k == 'name')
for name in names:
    do_something_with(name)

Lower-level interfaces

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>')

Even more bare-bones is the ability to react on individual events without even calculating a prefix:

import ijson

events = ijson.basic_parse(urlopen('http://.../'))
num_names = sum(1 for event, value in events
                if event == 'map_key' and value == 'name')

asyncio support

In python 3.5+ all of the methods above have an *_async counterpart that works on file-like asynchronous objects, and that can be iterated asynchronously. In other words, something like this:

import asyncio
import ijson

async def run():
   f = await async_urlopen('http://..../')
   async for object in ijson.items_async(f, 'earth.europe.item'):
      if object['type'] == 'city':
         do_something_with(city)
asyncio.run(run())

Push interfaces

All examples above use a file-like object as the data input (both the normal case, and for asyncio support), and hence are “pull” interfaces, with the library reading data as necessary. If for whatever reason it’s not possible to use such method, you can still push data through yet a different interface: coroutines. Coroutines effectively allow users to send data to them at any point in time, with a final target coroutine-like object receiving the results.

In the following example the user is doing the reading instead of letting the library do it:

import ijson

@ijson.coroutine
def print_cities():
   while True:
      obj = (yield)
      if obj['type'] != 'city':
         continue
      print(obj)

coro = ijson.items_coro(print_cities(), 'earth.europe.item')
f = urlopen('http://.../')
chunk = f.read(buf_size)
while chunk:
   try:
      coro.send(chunk)
   except StopIteration:
         break
   chunk = f.read()

All four ijson iterators have a *_coro counterpart that work by pushing data into them. Instead of receiving a file-like object and option buffer size as arguments, they receive a single target argument, which should be a coroutine-like object (anything implementing a send method) through which results will be published.

Events

When using the lower-level ijson.parse function, three-element tuples are generated containing a prefix, an event name, and a value. Events will be one of the following:

  • start_map and end_map indicate the beginning and end of a JSON object, respectively. They carry a None as their value.
  • start_array and end_array indicate the beginning and end of a JSON array, respectively. They also carry a None as their value.
  • map_key indicates the name of a field in a JSON object. Its associated value is the name itself.
  • null, boolean, integer, double, number and string all indicate actual content, which is stored in the associated value.

Prefix

A prefix represents the context within a JSON document where an event originates at. It works as follows:

  • It starts as an empty string.
  • A <name> part is appended when the parser starts parsing the contents of a JSON object member called name, and removed once the content finishes.
  • A literal item part is appended when the parser is parsing elements of a JSON array, and removed when the array ends.
  • Parts are separated by ..

When using the ijson.items function, the prefix works as the selection for which objects should be automatically built and returned by ijson.

Backends

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

  • yajl2_c: a C extension using YAJL 2.x. This is the fastest, but might require a compiler and the YAJL development files to be present when installing this package. Binary wheel distributions exist for major platforms/architectures to spare users from having to compile the package.
  • yajl2_cffi: wrapper around YAJL 2.x using CFFI.
  • yajl2: wrapper around YAJL 2.x using ctypes, for when you can’t use CFFI for some reason.
  • yajl: deprecated YAJL 1.x + ctypes wrapper, for even older systems.
  • python: pure Python parser, good to use with PyPy

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

import ijson.backends.yajl2_cffi as ijson

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

Importing the top level library as import ijson uses the first available backend in the same order of the list above.

Acknowledgements

ijson was originally developed and actively maintained until 2016 by Ivan Sagalaev. In 2019 he handed over the maintenance of the project and the PyPI ownership.

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.

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