Demo rounding modes in Python (https://docs.python.org/3/library/decimal.html#rounding-modes)

demo_rounding.py

#! /usr/bin/env python3
# Copyright (c) 2025 Sebastian Pipping <[email protected]>
# Licensed under the MIT license

import decimal
import math
import sys
from decimal import Decimal

import pylab
from matplotlib import pyplot

PERCENT_FORMAT = "PERCENT_FORMATTING"
HUMAN_ROUNDING = decimal.ROUND_HALF_UP
NO_DECIMAL_PLACES = Decimal("0")


def create_label(rounding_mode: str) -> str:
    if rounding_mode == decimal.ROUND_HALF_EVEN:
        return f"Python decimal default (i.e. decimal.{rounding_mode})"
    elif rounding_mode == HUMAN_ROUNDING:
        return f"Human default (i.e. decimal.{rounding_mode})"
    elif rounding_mode == PERCENT_FORMAT:
        return f"Percent formatting (i.e. decimal.ROUND_HALF_EVEN)"
    return f"decimal.{rounding_mode}"


def values_for_rounding_mode(rounding_mode: str, x_values: list[float]) -> list[float]:
    if rounding_mode == PERCENT_FORMAT:
        return [Decimal("%.0f" % x) for x in x_values]

    with decimal.localcontext(rounding=rounding_mode):
        return [x.quantize(NO_DECIMAL_PLACES) for x in x_values]


if __name__ == '__main__':
    if sys.version_info < (3, 11):
        sys.exit("ERROR: Python >=3.11 is needed.")

    ROUNDING_MODES = [getattr(decimal, k) for k in dir(decimal) if k.startswith('ROUND_')]
    X = [Decimal(i / 10) for i in range(-20, 20 + 1)]
    Y_HUMAN = values_for_rounding_mode(decimal.ROUND_HALF_UP, X)

    for rounding_mode in ROUNDING_MODES + [PERCENT_FORMAT]:
        Y = values_for_rounding_mode(rounding_mode, X)
        pyplot.clf()
        pyplot.grid(visible=True, which='major', axis='both')

        pyplot.scatter(X, X, label="id(x)", marker='o')
        if rounding_mode != HUMAN_ROUNDING:  # to not plot the the same thing twice
            pyplot.scatter(X, Y_HUMAN, label=create_label(decimal.ROUND_HALF_UP), marker='o')
        pyplot.scatter(X, Y, label=create_label(rounding_mode), marker='.')

        pyplot.legend()

        filename = f"{rounding_mode.lower()}.svg"
        print(f"Writing file {filename!r}...")
        pyplot.savefig(filename)

    print("Done.")

Expected output

percent_formatting.svg

round_05up.svg

round_ceiling.svg

round_down.svg

round_floor.svg

round_half_down.svg

round_half_even.svg

round_half_up.svg

round_up.svg