Python 3, read/write compressed json objects from/

Question

For Python3, I followed @Martijn Pieters's code with this:

import gzip
import json

# writing
with gzip.GzipFile(jsonfilename, 'w') as fout:
    for i in range(N):
        uid = "whatever%i" % i
        dv = [1, 2, 3]
        data = json.dumps({
            'what': uid,
            'where': dv})

        fout.write(data + '\n')

but this results in an error:

Traceback (most recent call last):
    ...
  File "C:\Users\Think\my_json.py", line 118, in write_json
    fout.write(data + '\n')
  File "C:\Users\Think\Anaconda3\lib\gzip.py", line 258, in write
    data = memoryview(data)
TypeError: memoryview: a bytes-like object is required, not 'str'

Any thoughts about what is going on?

Answer 1

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 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.GzipFile(jsonfilename, 'w') as fout:   # 4. 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.

Reading works exactly the other way around:

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

and

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