A JSON Encoder in Python, that puts small lists on single lines.

CompactJSONEncoder.py

#!/usr/bin/env python3
from __future__ import annotations

import json


class CompactJSONEncoder(json.JSONEncoder):
    """A JSON Encoder that puts small containers on single lines."""

    CONTAINER_TYPES = (list, tuple, dict)
    """Container datatypes include primitives or other containers."""

    MAX_WIDTH = 70
    """Maximum width of a container that might be put on a single line."""

    MAX_ITEMS = 10
    """Maximum number of items in container that might be put on single line."""

    def __init__(self, *args, **kwargs):
        # using this class without indentation is pointless
        if kwargs.get("indent") is None:
            kwargs["indent"] = 4
        super().__init__(*args, **kwargs)
        self.indentation_level = 0

    def encode(self, o):
        """Encode JSON object *o* with respect to single line lists."""
        if isinstance(o, (list, tuple)):
            return self._encode_list(o)
        if isinstance(o, dict):
            return self._encode_object(o)
        if isinstance(o, float):  # Use scientific notation for floats
            return format(o, "g")
        return json.dumps(
            o,
            skipkeys=self.skipkeys,
            ensure_ascii=self.ensure_ascii,
            check_circular=self.check_circular,
            allow_nan=self.allow_nan,
            sort_keys=self.sort_keys,
            indent=self.indent,
            separators=(self.item_separator, self.key_separator),
            default=self.default if hasattr(self, "default") else None,
        )

    def _encode_list(self, o):
        if self._put_on_single_line(o):
            return "[" + ", ".join(self.encode(el) for el in o) + "]"
        self.indentation_level += 1
        output = [self.indent_str + self.encode(el) for el in o]
        self.indentation_level -= 1
        return "[\n" + ",\n".join(output) + "\n" + self.indent_str + "]"

    def _encode_object(self, o):
        if not o:
            return "{}"

        # ensure keys are converted to strings
        o = {str(k) if k is not None else "null": v for k, v in o.items()}

        if self.sort_keys:
            o = dict(sorted(o.items(), key=lambda x: x[0]))

        if self._put_on_single_line(o):
            return (
                "{ "
                + ", ".join(
                    f"{json.dumps(k)}: {self.encode(el)}" for k, el in o.items()
                )
                + " }"
            )

        self.indentation_level += 1
        output = [
            f"{self.indent_str}{json.dumps(k)}: {self.encode(v)}" for k, v in o.items()
        ]
        self.indentation_level -= 1

        return "{\n" + ",\n".join(output) + "\n" + self.indent_str + "}"

    def iterencode(self, o, **kwargs):
        """Required to also work with `json.dump`."""
        return self.encode(o)

    def _put_on_single_line(self, o):
        return (
            self._primitives_only(o)
            and len(o) <= self.MAX_ITEMS
            and len(str(o)) - 2 <= self.MAX_WIDTH
        )

    def _primitives_only(self, o: list | tuple | dict):
        if isinstance(o, (list, tuple)):
            return not any(isinstance(el, self.CONTAINER_TYPES) for el in o)
        elif isinstance(o, dict):
            return not any(isinstance(el, self.CONTAINER_TYPES) for el in o.values())

    @property
    def indent_str(self) -> str:
        if isinstance(self.indent, int):
            return " " * (self.indentation_level * self.indent)
        elif isinstance(self.indent, str):
            return self.indentation_level * self.indent
        else:
            raise ValueError(
                f"indent must either be of type int or str (is: {type(self.indent)})"
            )


if __name__ == "__main__":
    import sys

    if "--example" in sys.argv:
        data = {
            "compact_object": {"first": "element", "second": 2},
            "compact_list": ["first", "second"],
            "long_list": [
                "this",
                "is",
                "a",
                "rather",
                "long\nlist",
                "and should be broken up because of its width",
            ],
            "non_ascii": "汉语",
            1: 2,
        }
        json.dump(data, sys.stdout, cls=CompactJSONEncoder, ensure_ascii=False)
        exit()

    json.dump(json.load(sys.stdin), sys.stdout, cls=CompactJSONEncoder)

@mikethe1wheelnut @phschimm I have added an example of how to dump dictionaries to json using this encoder. Running the file with the --example option generates the example output:

$ python3 CompactJSONEncoder.py --example
{
    "compact_object": { "first": "element", "second": 2 },
    "compact_list": ["first", "second"],
    "long_list": [
        "this",
        "is",
        "a",
        "rather",
        "long",
        "list"
    ]
}

@keithzg Ahh, I see what the problem is: json.dump works with indent parameters given as int and str, while my encoder assumes and int to multiply my indentation level with.

I have reworked it so indent can be provided both as int and str. Providing indent="\t" works as expected now.

elif isinstance(o, str):  # escape newlines
    o = o.replace("\n", "\\n")
    return f'"{o}"'

Not only newlines need to be escaped, I think return json.dumps(o) is better.
Be ware of ensure_ascii argument or the CJK words will be print as unicode escape.

@PenutChen I'm not sure why I didn't do that in the first place. 😅 Thanks for the hint!

@jannismain, what is the license of this code? (just making sure I can reuse it)

Note: sort_keys=True when calling dumps(..., sort_keys=True, indent=2, cls=CompactJSONEncoder) doesn't seem to work. Indent does work though.

@jannismain, what is the license of this code? (just making sure I can reuse it)

Thanks for asking! Feel free to reuse it in any way you want. Consider its license as MIT.

Note: sort_keys=True when calling dumps(..., sort_keys=True, indent=2, cls=CompactJSONEncoder) doesn't seem to work. Indent does work though.

I‘ll have a look at sorting when I get a chance. If somebody knows how to preserve the sorting feature with this way of custom encoding in the meantime, let me know 🙂

@jannismain maybe done by this way in _encode_object?

items = [(k, el) for k, el in o.items()]
if self.sort_keys:
    items = sorted(items)

# (... for k, el in items)

For me, inserting the following branch in L31 does the trick:

            if self.sort_keys:
                o = dict(sorted(o.items()))

@jannismain
Excuse me, can you tell me how to use this? I have a small script that writes data from a binary file to json. How should I use it with your compact version?

My script:

def readInt(file, size):
    return int.from_bytes(file.read(size), "little")

with open("club.dat", 'rb') as datFile:
    datFile.read(8)
    size = readInt(datFile, 4)
    clubs = []
    for i in range(size):
        print(f"#{i + 1}/{size} - {hex(datFile.tell())}")
        club = {}
        club['uid'] = readInt(datFile, 4)
        club['player'] = [(readInt(datFile, 4)) for i in range(readInt(datFile, 2))]
        club['mainClubId'] = readInt(datFile, 4)
        club['isNational'] = readInt(datFile, 2)
        clubs.append(club)

with open("clubs.json", 'wt') as jsonFile:
    json.dump(clubs, jsonFile, ensure_ascii=False, indent=2)

@Royal724 Line 118 is what you are looking for:
json.dump(data, stream, cls=CompactJSONEncoder)

@@jannismain
Thank you for your response. I downloaded your module and crossed it to the same folder where my script is, then made the changes I wrote below, but it didn't work. My json file stays as it is, no compact format. I'm sorry, I'm making a mistake somewhere, but I can't figure out where, as I don't have much experience.
And created a new folder pycache in my folder and in it is the file CompactJSONEncoder.cpython-311.pyc

from CompactJSONEncoder import CompactJSONEncoder

json.dump(clubs, jsonFile, cls=CompactJSONEncoder)

@Royal724 maybe your json file have too long width or too many items, try to set MAX_ITEMS or MAX_WIDTH to a larger number.

@PenutChen
Yes, that was the point, thank you.

@jannismain
Excellent work! Thank you so much!

There's a small bug: If the keys of a dict are integers of floats, they don't get converted to strings. Perhaps adding inner str() might be a solution…

There's a small bug: If the keys of a dict are integers of floats, they don't get converted to strings. Perhaps adding inner str() might be a solution…

You are right, I didn’t even think to treat the keys in any way.. will push a revision soon to address this! 👍

@olin256 I'm now converting keys to string to ensure the output produced is valid JSON.

@oesteban @PenutChen sort_keys=True is now supported 👍

Little suggestion to also correctly process Numpy types:

    def encode(self, o):
        """Encode JSON object *o* with respect to single line lists."""
        if isinstance(o, (list, tuple)):
            return self._encode_list(o)
        elif isinstance(o, dict):
            return self._encode_object(o)
        if isinstance(o, float):  # Use scientific notation for floats
            return format(o, "g") 
        elif isinstance(o, np.integer):  # process numpy integers
            return self.encode(int(o))
        elif isinstance(o, np.floating): # process numpy floats
            return self.encode(float(o))
        elif isinstance(o, np.ndarray): # flatten numpy arrays as lists
            return self._encode_list(o.tolist())

        return json.dumps(
            o,
            skipkeys=self.skipkeys,
            ensure_ascii=self.ensure_ascii,
            check_circular=self.check_circular,
            allow_nan=self.allow_nan,
            sort_keys=self.sort_keys,
            indent=self.indent,
            separators=(self.item_separator, self.key_separator),
            default=self.default if hasattr(self, "default") else None,
        )

Also, personally, I would remove the format(o, "g") part, as it sometimes results in converting floats into exponential notation.