python JSON array newlines

Question

I have a large dictionary that has some large array data in it:

d = {'something': {'else': 'x'}, 'longnumbers': [1,2,3,4,54,6,67,7,7,8,8,8,6,4,3,3,5,6,7,4,3,5,6,54]}

The real dictionary has many more keys and a nested structure. When I use json.dump without indent, I get a compact, single-line output which is not readable. When I set indent, it puts newlines after every separator, including the arrays.

The numerical arrays are long and end up like this:

  "longnumbers": [
    1, 
    2, 
    3, 
    4, 
    54, 
    6, 
    67, 
    7, 
    7, 
    8, 
    8, 
    8, 
    6, 
    4, 
    3, 
    3, 
    5, 
    6, 
    7, 
    4, 
    3, 
    5, 
    6, 
    54
  ],

Is there any way to get pretty-printed JSON with an indent level, but without placing newlines after array elements? For the example above, I'd like something like this:

{
  "longnumbers": [1, 2, 3, 4, 54, 6, 67, 7, 7, 8, 8, 8, 6, 4, 3, 3, 5, 6, 7, 4, 3, 5, 6, 54],
  "something": {
    "else": "x"
  }
}

@NiklasB. that's the conclusion I came to when I had a similar issue. Fortunately, the json library is implemented in Python, and not especially hard to read, which provides a good base for things. — Karl Knechtel
– Karl Knechtel, Commented Apr 10, 2012 at 23:18
I think a good way would be to just wrap json.dumps and only override the behaviour for handling dicts, passing through all the other element types. — Niklas B.
– Niklas B., Commented Apr 10, 2012 at 23:20

Simon Zhang · Accepted Answer · 2016-02-01 13:19:37Z

15

I ended up just writing my own JSON serializer:

import numpy

INDENT = 3
SPACE = " "
NEWLINE = "\n"

def to_json(o, level=0):
    ret = ""
    if isinstance(o, dict):
        ret += "{" + NEWLINE
        comma = ""
        for k,v in o.iteritems():
            ret += comma
            comma = ",\n"
            ret += SPACE * INDENT * (level+1)
            ret += '"' + str(k) + '":' + SPACE
            ret += to_json(v, level + 1)

        ret += NEWLINE + SPACE * INDENT * level + "}"
    elif isinstance(o, basestring):
        ret += '"' + o + '"'
    elif isinstance(o, list):
        ret += "[" + ",".join([to_json(e, level+1) for e in o]) + "]"
    elif isinstance(o, bool):
        ret += "true" if o else "false"
    elif isinstance(o, int):
        ret += str(o)
    elif isinstance(o, float):
        ret += '%.7g' % o
    elif isinstance(o, numpy.ndarray) and numpy.issubdtype(o.dtype, numpy.integer):
        ret += "[" + ','.join(map(str, o.flatten().tolist())) + "]"
    elif isinstance(o, numpy.ndarray) and numpy.issubdtype(o.dtype, numpy.inexact):
        ret += "[" + ','.join(map(lambda x: '%.7g' % x, o.flatten().tolist())) + "]"
    elif o is None:
        ret += 'null'
    else:
        raise TypeError("Unknown type '%s' for json serialization" % str(type(o)))
    return ret

edited Feb 1, 2016 at 13:19

Simon Zhang

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answered May 2, 2012 at 19:05

jterrace

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2 Comments

jterrace Over a year ago

This function is free to use under a BSD license.

soulmachine Over a year ago

This answer saved my day!

David Kim · Accepted Answer · 2020-07-29 21:26:14Z

@jterrace's answer was written for Python 2, which has since deprecated for Python 3 with changes to types. So, with all due credit to his answer, I tweaked it a tad bit for my personal use & compatibility with Python 3, including support for tuples as lists:

import numpy

INDENT = 3
SPACE = " "
NEWLINE = "\n"

# Changed basestring to str, and dict uses items() instead of iteritems().
def to_json(o, level=0):
  ret = ""
  if isinstance(o, dict):
    ret += "{" + NEWLINE
    comma = ""
    for k, v in o.items():
      ret += comma
      comma = ",\n"
      ret += SPACE * INDENT * (level + 1)
      ret += '"' + str(k) + '":' + SPACE
      ret += to_json(v, level + 1)

    ret += NEWLINE + SPACE * INDENT * level + "}"
  elif isinstance(o, str):
    ret += '"' + o + '"'
  elif isinstance(o, list):
    ret += "[" + ",".join([to_json(e, level + 1) for e in o]) + "]"
  # Tuples are interpreted as lists
  elif isinstance(o, tuple):
    ret += "[" + ",".join(to_json(e, level + 1) for e in o) + "]"
  elif isinstance(o, bool):
    ret += "true" if o else "false"
  elif isinstance(o, int):
    ret += str(o)
  elif isinstance(o, float):
    ret += '%.7g' % o
  elif isinstance(o, numpy.ndarray) and numpy.issubdtype(o.dtype, numpy.integer):
    ret += "[" + ','.join(map(str, o.flatten().tolist())) + "]"
  elif isinstance(o, numpy.ndarray) and numpy.issubdtype(o.dtype, numpy.inexact):
    ret += "[" + ','.join(map(lambda x: '%.7g' % x, o.flatten().tolist())) + "]"
  elif o is None:
    ret += 'null'
  else:
    raise TypeError("Unknown type '%s' for json serialization" % str(type(o)))
  return ret

spinkus · Accepted Answer · 2018-08-19 03:08:50Z

Ugh, should really be an option for specifying different indents for the two different JSON container types by now. An alternative approach if you want to stay compatible with the core Python JSON lib is to override the function (_make_iterencode() currently) in that lib that is responsible for handling indent.

Had a crack at reimplementation of _make_iterencode(). Only had to change a few lines to make the indent option, optionally take a tuple (hash-indent, array-indent). But unfortunately have to replace an entire _make_iterencode() which turns out to be pretty big and poorly decomposed. Anyway, following works for 3.4-3.6:

import sys
import json

dat = {"b": [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], "a": 1, "c": "x"}
indent = 2
print(json.dumps(dat, indent=indent))

if sys.version_info.major == 3 and 4 <= sys.version_info.minor <= 6:
  import _make_iterencode
  json.encoder._make_iterencode = _make_iterencode._make_iterencode
  indent = (2, None)

print(json.dumps(dat, indent=indent))

Gives:

{
  "c": "x",
  "a": 1,
  "b": [
    1,
    2,
    3,
    4,
    5,
    6,
    7,
    8,
    9,
    10
  ]
}
{
  "c": "x",
  "a": 1,
  "b": [1,2,3,4,5,6,7,8,9,10]
}

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