A rough JSON parser in Python

jsonparser.py

import re
from typing import Any, Tuple, Union

"""A very rough JSON parser.
Implementing the standard outlined in https://www.json.org/json-en.html
Number parsing is handled by Python.

Usage:
>>> data, _ = parse_json('{"key": ["value", -1e20, true, false, null]}')
>>> assert data == {"key": ["value", -1e20, True, False, None]}
"""

whitespace_re = re.compile("^[ \t\n\r]+")
float_re = re.compile(r"-?\d+(?:\.\d+)?(?:[Ee][+-]?\d+)?")
single_char_escape = {"\\\\": "\\", "\\/": "/", '\\"': '"', "\\b": "\b", "\\f": "\f", "\\n": "\n", "\\r": "\r", "\\t": "\t"}
plain_str_content_re = re.compile(r"([^\\\"]|\n\r\t)+")

def leading_whitespaces(data: str) -> int:
    match = whitespace_re.match(data)
    if not match:
        return 0
    return len(match[0])

def parse_string(data: str) -> Tuple[str, int]:
    assert data[0] == '"'
    result = ""
    ptr = 1
    while ptr < len(data):
        if data[ptr] == "\\":
            # Escape sequence
            if data[ptr + 1] == "u":
                # Unicode escape
                result += chr(int(data[ptr + 2:ptr + 6], 16))
                ptr += 6
            else:
                # Single character escape
                for i in single_char_escape:
                    if data[ptr:ptr+2] == i:
                        result += single_char_escape[i]
                        ptr += 2
                        break
        elif data[ptr] == '"':
            # End of string
            ptr += 1
            break
        else:
            # Plain string
            match = plain_str_content_re.match(data[ptr:])
            result += match[0]
            ptr += len(match[0])
    return result, ptr


def pares_number(s: str) -> Tuple[Union[float, int], int]:
    match = float_re.match(s)[0]
    if "e" not in match and "E" not in match and "." not in match:
        return int(match), len(match)
    return float(match), len(match)


def parse_object(data: str) -> Tuple[dict, int]:
    assert data[0] == "{"
    result = {}
    ptr = 1
    while ptr < len(data):
        ptr += leading_whitespaces(data[ptr:])

        # Empty object
        if data[ptr] == "}":
            ptr += 1
            break
        
        key, proc_len = parse_string(data[ptr:])
        ptr += proc_len
        ptr += leading_whitespaces(data[ptr:])
        assert data[ptr] == ":"
        ptr += 1
        value, proc_len = parse_json(data[ptr:])
        ptr += proc_len
        ptr += leading_whitespaces(data[ptr:])
        result[key] = value
        if data[ptr] == "}":
            ptr += 1
            break
        assert data[ptr] == ","
        ptr += 1
    return result, ptr


def parse_array(data: str) -> Tuple[list, int]:
    assert data[0] == "["
    result = []
    ptr = 1
    while ptr < len(data):
        ptr += leading_whitespaces(data[ptr:])
        # Empty array
        if data[ptr] == "]":
            ptr += 1
            break
        value, proc_len = parse_json(data[ptr:])
        ptr += proc_len
        result.append(value)
        ptr += leading_whitespaces(data[ptr:])
        if data[ptr] == "]":
            ptr += 1
            break
        assert data[ptr] == ","
        ptr += 1
    return result, ptr


def parse_json(data: str) -> Tuple[Any, int]:
    """Returns: parsed value and the number of characters consumed."""
    ptr = leading_whitespaces(data)
    if ptr == len(data):
        return None, ptr
    if data[ptr] == "{":
        val, proc_chr = parse_object(data[ptr:])
        return val, ptr + proc_chr
    elif data[ptr] == "[":
        val, proc_chr = parse_array(data[ptr:])
        return val, ptr + proc_chr
    elif data[ptr] == '"':
        val, proc_chr = parse_string(data[ptr:])
        return val, ptr + proc_chr
    elif data[ptr:].startswith("true"):
        return True, ptr + 4
    elif data[ptr:].startswith("false"):
        return False, ptr + 5
    elif data[ptr:].startswith("null"):
        return None, ptr + 4
    elif data[ptr] in "-+0123456789":
        val, proc_chr = pares_number(data[ptr:])
        return val, ptr + proc_chr
    else:
        raise ValueError(f"Unexpected character: {data[ptr:]}")