如何在不丢失逗号的情况下将列表写入JSON文件

Question

I'm trying to develop code in Python that formats a list in a JSON file without removing the commas or brackets. The list is supposed to have each set of data on a new line. I'm able to handle the brackets on my own, but I'm having issues with the commas.

I've already tried include a indent statement in .dump, but it's not the correct format.

    #Attempt 1

    for data in data:
        outfile.write('\t')
        json.dump(data, outfile)
        outfile.write('\n')

    #Attempt 2
    for obj in data:
        outfile.write('\t' + json.dumps(obj) + '\n')

Expected output

[
    [1, 12],
    [2, 7],
    [3, 6]
]

Actual output

[
    [1, 12]
    [2, 7]
    [3, 6]
]

Answer 1

Why are you iterating at all? You should just dump the whole list in one go:

outfile.write(json.dumps(data))

Answer 2

You could modify your Attempt 2 to add the comma to each outputted item except the last:

for ndx, obj in enumerate(data, 1):
    outfile.write(
        '\t'
        + json.dumps(obj)
        + (',' if ndx != len(data) else '')
        + '\n'
    )

Answer 3

I would use something like:

# Python 3+
import json

objects = [[1, 2], [3, 4]]
# the magic happens next line:
dump = "[\n" + ",\n".join([ "\t" + json.dumps(obj) for obj in objects ]) + "\n]"
print(dump)

with open("out", "w") as outfile:
  outfile.write(dump)

json.dumps(obj) outputs JSON representation of the object as string. A tab character is appended to each object representation and they are joined using ,\\n .

Output:

[
    [1, 2],
    [3, 4]
]