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jsonfile - incrementally write files in JSON format.

Project description

jsonfile - incrementally generate JSON

pip install jsonfile - PyPI link

Incrementally means that, in most places, you can either send in a complete Python object, or build it a piece at a time. For example, let’s build the JSON structure [3, {"cool": true, "awesome": [1,2,3,4,5]}]:

>>> import jsonfile
>>> jp = jsonfile.JsonProto()
>>> jp.start_list()
>>> jp.list_item(3)
>>> jp.start_dict()
>>> jp.dict_item('cool', True)
>>> jp.dict_key('awesome')
>>> jp.dict_value([1,2,3,4,5])
':[1, 2, 3, 4, 5]'
>>> jp.finish_all()

That used the JsonProto object, a sans-io version that just returns the text that you should write to your output. If you are writing to a file in a normal synchronous context, you could also use the JsonWriter object, it will be slightly more convenient:

>>> import io
>>> dest = io.StringIO()
>>> import jsonfile
>>> jw = jsonfile.JsonWriter(dest)
>>> jw.start_list()
>>> jw.list_item(3)
>>> jw.list_item({'things': 'stuff'})
>>> jw.start_dict()
>>> jw.dict_item('cool', True)
>>> jw.dict_key('awesome')
>>> jw.dict_value([1,2,3,4,5])
>>> jw.finish_all()
>>> dest.getvalue()
'[3,{"things": "stuff"},{"cool":true,"awesome":[1, 2, 3, 4, 5]}]'

Because the underlying JsonProto doesn’t do IO, it should be easy to make a version of JsonWriter that works well in an async context.

Many kinds of errors are caught: having more than one object at the root level of a file, using a list item in a dictionary, etc.


  • Allow specifying your own JSONEncoder to use on complete objects.
  • Do indentation.
  • Implement an AsyncJsonWriter
  • Better tests for preventing illegal JSON from being generated (e.g. lists as keys).
  • Python 2 support?

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