Load an element with python from large json file

问题

So, here is my json file. I want to load the data list from it, one by one, and only it. And then, for exemple plot it...

This is an exemple, because I am dealing with large data set, with wich I could not load all the file (that would create a memory error).

{
  "earth": {
    "europe": [
      {"name": "Paris", "type": "city"},
      {"name": "Thames", "type": "river"}, 
      {"par": 2, "data": [1,7,4,7,5,7,7,6]}, 
      {"par": 2, "data": [1,0,4,1,5,1,1,1]}, 
      {"par": 2, "data": [1,0,0,0,5,0,0,0]}
        ],
    "america": [
      {"name": "Texas", "type": "state"}
    ]
  }
}

Here is what I tried:

import ijson
filename = "testfile.json"

f = open(filename)
mylist = ijson.items(f, 'earth.europe[2].data.item')
print mylist

It returns me nothing, even when I try to convert it into a list:

[]

回答1:

You need to specify a valid prefix; ijson prefixes are either keys in a dictionary or the word item for list entries. You can't select a specific list item (so [2] doesn't work).

If you wanted all the data keys dictionaries in the europe list, then the prefix is:

earth.europe.item.data
# ^ ------------------- outermost key must be 'earth'
#       ^ ------------- next key must be 'europe'
#              ^ ------ any value in the array
#                   ^   the value for the 'data' key

This produces each such list:

>>> l = ijson.items(f, 'earth.europe.item.data')
>>> for data in l:
...     print data
...
[1, 7, 4, 7, 5, 7, 7, 6]
[1, 0, 4, 1, 5, 1, 1, 1]
[1, 0, 0, 0, 5, 0, 0, 0]

You can't put wildcards in that, so you can't get earth.*.item.data for example.

If you need to do more complex prefixing matching, you'd have to use the ijson.parse() function and handle the events this produces. You can reuse the ijson.ObjectBuilder() class to turn events you are interested in into Python objects:

parser = ijson.parse(f)
for prefix, event, value in parser:
    if event != 'start_array':
        continue
    if prefix.startswith('earth.') and prefix.endswith('.item.data'):
        continent = prefix.split('.', 2)[1]
        builder = ijson.ObjectBuilder()
        builder.event(event, value)
        for nprefix, event, value in parser:
            if (nprefix, event) == (prefix, 'end_array'):
                break
            builder.event(event, value)
        data = builder.value
        print continent, data

This will print every array that's in a list under a 'data' key (so lives under a prefix that ends with '.item.data'), with the 'earth' key. It also extracts the continent key.

回答2:

Given the structure of your json I would do this:

import json

filename = "test.json"

with open(filename) as data_file:
    data = json.load(data_file)
print data['earth']['europe'][2]['data']
print type(data['earth']['europe'][2]['data'])

回答3:

So, I will explain how I finally solved this problem. The first answer will work. But you have to know that loading elements one per one with ijson will be very long... and by the end, you do not have the loaded file.

So, the important information is that windows limit your memory per process to 2 or 4 GB, depending on wich windows you use (32 or 64). If you use pythonxy, that will be 2 GB (it only exists in 32). Anyway, in both way, that's very very low!

I solved this problem by installing a virtual Linux in my windows, and it works. Here are the main step to do so:

1. Install Virtual Box
2. Install Ubuntu (for exemple)
3. Install python for scientist on your computer, like SciPy
4. Create a share file between the 2 "computers" (you will find tutorial on google)
5. Execute your code on your ubuntu "computer": it sould work ;)

NB: Do not forget to allow sufficient RAM and memory to you virtual computer.

This works for me. I don't have anymore this "memory error" problem.

来源：https://stackoverflow.com/questions/40330820/load-an-element-with-python-from-large-json-file