JSON Lines

Worked with the JSON Lines format the other day. It’s a CSV on steroids:

  • each entry is a separate line, as in CSV;
  • at the same time it is a full-fledged JSON.

For example:

{ "id":11, "name":"Diane", "city":"London", "department":"hr", "salary":70 }
{ "id":12, "name":"Bob", "city":"London", "department":"hr", "salary":78 }
{ "id":21, "name":"Emma", "city":"London", "department":"it", "salary":84 }
{ "id":22, "name":"Grace", "city":"Berlin", "department":"it", "salary":90}
{ "id":23, "name":"Henry", "city":"London", "department":"it", "salary":104}

Great stuff:

  • Suitable for objects of complex structure (unlike csv);
  • Easy to stream read without loading the entire file into memory (unlike json);
  • Easy to append new entries to an existing file (unlike json).

JSON can also be streamed. But look how much easier it is with JSON Lines:

import json
from typing import Iterator

def jl_reader(fname: str) -> Iterator[dict]:
    with open(fname) as file:
        for line in file:
            obj = json.loads(line.strip())
            yield obj

if __name__ == "__main__":
    reader = jl_reader("employees.jl")
    for employee in reader:
        id = employee["id"]
        name = employee["name"]
        dept = employee["department"]
        print(f"#{id} - {name} ({dept})")
#11 - Diane (hr)
#12 - Bob (hr)
#21 - Emma (it)
#22 - Grace (it)
#23 - Henry (it)


Great fit for logs and data processing pipelines.

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