Python 3, read/write compressed json objects from/to gzip file

You have four steps of transformation here.

1. a Python data structure (nested dicts, lists, strings, numbers, booleans)
2. a Python string containing a serialized representation of that data structure (“JSON”)
3. a list of bytes containing a representation of that string (“UTF-8”)
4. a list of bytes containing a – shorter – representation of that previous byte list (“gzip”)

So let’s take these steps one by one.

import gzip
import json

data = []
for i in range(N):
    uid = "whatever%i" % i
    dv = [1, 2, 3]
    data.append({
        'what': uid,
        'where': dv
    })                                           # 1. data

json_str = json.dumps(data) + "\n"               # 2. string (i.e. JSON)
json_bytes = json_str.encode('utf-8')            # 3. bytes (i.e. UTF-8)

with gzip.open(jsonfilename, 'w') as fout:       # 4. fewer bytes (i.e. gzip)
    fout.write(json_bytes)                       

Note that adding "\n" is completely superfluous here. It does not break anything, but beyond that it has no use. I’ve added that only because you have it in your code sample.

Reading works exactly the other way around:

with gzip.open(jsonfilename, 'r') as fin:        # 4. gzip
    json_bytes = fin.read()                      # 3. bytes (i.e. UTF-8)

json_str = json_bytes.decode('utf-8')            # 2. string (i.e. JSON)
data = json.loads(json_str)                      # 1. data

print(data)

Of course the steps can be combined:

with gzip.open(jsonfilename, 'w') as fout:
    fout.write(json.dumps(data).encode('utf-8'))                       

and

with gzip.open(jsonfilename, 'r') as fin:
    data = json.loads(fin.read().decode('utf-8'))