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

CompactJSONEncoder.py

import json
from typing import Union


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 = 80
    """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.update({"indent": 4})
        super().__init__(*args, **kwargs)
        self.indentation_level = 0
        self.kwargs = kwargs

    def encode(self, o):
        """Encode JSON object *o* with respect to single line lists."""
        if isinstance(o, (list, tuple)):
            if self._put_on_single_line(o):
                return "[" + ", ".join(self.encode(el) for el in o) + "]"
            else:
                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 + "]"
        elif isinstance(o, dict):
            if o:
                if self._put_on_single_line(o):
                    return (
                        "{ "
                        + ", ".join(
                            f"{self.encode(k)}: {self.encode(el)}"
                            for k, el in o.items()
                        )
                        + " }"
                    )
                else:
                    self.indentation_level += 1
                    output = [
                        self.indent_str + f"{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 + "}"
            else:
                return "{}"
        elif isinstance(
            o, float
        ):  # Use scientific notation for floats, where appropiate
            return format(o, "g")
        elif isinstance(o, str):  # escape string
            return json.dumps(o, **self.kwargs)
        else:
            return json.dumps(o)

    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: Union[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

    data = json.loads(sys.stdin.read())
    print(json.dumps(data, cls=CompactJSONEncoder, ensure_ascii=False))