retail.price-per-unit

def unit_price(price: Money, quantity: str, unit: str, per: UnitPriceBasis) -> Money

UnitPriceBasis = Literal["kg", "100g", "10g", "L", "100mL", "10mL", "75cL", "m", "m2", "m3", "item"]

from fune.retail.price_per_unit import unit_price  # retail.price-per-unit@^1

import re

from .money_amount import Money, money  ← from money.amount ^1.0.0 · built alongside by fune
from .units_convert import convert_units  ← from units.convert ^1.0.0 · built alongside by fune
from .retail_price_per_unit_types import UnitPriceBasis

# Each basis is a units.convert symbol and how many of it the price is for.
_BASES = {
    "kg": ("kg", 1),
    "100g": ("g", 100),
    "10g": ("g", 10),
    "L": ("L", 1),
    "100mL": ("mL", 100),
    "10mL": ("mL", 10),
    "75cL": ("mL", 750),
    "m": ("m", 1),
    "m2": ("m2", 1),
    "m3": ("m3", 1),
    "item": ("item", 1),
}

_DECIMAL = re.compile(r"[0-9]+(\.[0-9]+)?")


def unit_price(price: Money, quantity: str, unit: str, per: UnitPriceBasis) -> Money:
    """The price of one ``per`` of the product, rounded half up to the minor unit.

    The quantity is converted exactly (units.convert, 12 decimal places), then
    price x multiplier / quantity is done in integers, so the one rounding step
    is the last one.
    """
    if price.minor < 0:
        raise ValueError("price must not be negative, received %d" % price.minor)
    if per not in _BASES:
        raise ValueError('unknown unit price basis "%s"' % (per,))
    target, multiplier = _BASES[per]

    if target == "item" or unit == "item":
        if target != "item":
            raise ValueError('a quantity in items can only be priced per "item"')
        if unit != "item":
            raise ValueError('per "item" needs the quantity in "item"')
        if not isinstance(quantity, str) or not _DECIMAL.fullmatch(quantity):
            raise ValueError('quantity must be more than 0, received "%s"' % (quantity,))
        converted = quantity
    else:
        converted = convert_units(quantity, unit, target, 12)
    if converted.startswith("-") or not re.search(r"[1-9]", converted):
        raise ValueError('quantity must be more than 0, received "%s"' % (quantity,))

    # converted = digits / 10^places, so price / converted = price x 10^places / digits.
    whole, _, fraction = converted.partition(".")
    digits = int(whole + fraction)
    numerator = price.minor * multiplier * 10 ** len(fraction)
    quotient, remainder = divmod(numerator, digits)
    if remainder * 2 >= digits:
        quotient += 1
    return money(quotient, price.currency)

Install

fune build

With that line in your source, in a Python project (language python in fune.project), fune build resolves it and its 2 dependencies, pins them in fune.lock, downloads only the Python package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:

fune add retail.price-per-unit

The manifest, vectors and README with only the Python implementation. Install it without the registry with fune add ./retail.price-per-unit-1.0.1-python.fune, or fetch it from a terminal with fune pull retail.price-per-unit@1.0.1:python.

The whole function, every language, is one file too: retail.price-per-unit-1.0.1.fune, 18,495 bytes, sha256 bb9baca1fabbefbfb0d311fb8f46242ae294c3a78f05a541398b361ce3e780e5. It installs into a project of any language.

# fune: before retail.price-per-unit

# fune: after retail.price-per-unit

# fune: replace money.amount in retail.price-per-unit
# fune: replace units.convert in retail.price-per-unit

# fune: step retail.price-per-unit after <n|label>

More from the author

The Price Marking Order 2004 (SI 2004/102) requires a unit price alongside the selling price for most goods sold loose or in pre-set quantities, and defines it as the final price, VAT included, per kilogram, litre, metre, square metre or cubic metre, or per item for goods sold by number.

- **Until 5 April 2026**, Schedule 1 of the Order let listed products use other units: per 10 g or 10 ml (herbs, spices, make-up), per 100 g or 100 ml (biscuits, coffee, soft drinks, sauces and many more), per 75 cl for wine, per 50 kg for coal. - **From 6 April 2026**, the Price Marking (Amendment) Order 2024 (SI 2024/1055, commencement moved from 1 October 2025 by SI 2025/592) omits Schedule 1. The unit price is per item for goods sold by number, per kilogram for goods marked only by weight, per litre for goods marked only by volume, and otherwise per kilogram, litre, metre, square metre or cubic metre. The deposit (for example a deposit return scheme charge) is excluded.

This function does not decide which unit a product needs; that depends on the product and on the date, and the caller's product data knows both. It takes the unit as `per` and does the arithmetic exactly. `100g`, `10g`, `100mL`, `10mL` and `75cL` stay available for labels printed under the old Schedule 1, for shelf labels outside Great Britain, and for voluntary extra prices.

## Arithmetic

The pack quantity is converted to the unit of `per` with `units.convert`, which is exact rational arithmetic, to 12 decimal places (so 8 oz is exactly 226.796185 g). The price is then divided by that quantity in exact integer arithmetic and rounded once, half up, to the minor unit: 6 rolls for £1.00 is 16.67p, shown as 17p. There is no floating point anywhere, so £4.00 per pound is 882p per kg in every language.

## Edge cases

- `unit` `"item"` goes with `per` `"item"` only, and the other way round. - A quantity of zero, or one so small it converts to 0 at 12 decimal places, is an error; so is a negative price. A free item's unit price is 0. - Mass and volume do not convert into each other: pricing 500 g per litre is an error from `units.convert`. Where the law lets a product be sold by weight or volume, pass whichever the pack is marked with.

## Sources

- The Price Marking Order 2004, SI 2004/102, article 1 (definition of "unit price") and Schedule 1, as made: https://www.legislation.gov.uk/uksi/2004/102/article/1/made and https://www.legislation.gov.uk/uksi/2004/102/schedule/1/made - The Price Marking (Amendment) Order 2024, SI 2024/1055 (new definition of "unit price", Schedule 1 omitted): https://www.legislation.gov.uk/uksi/2024/1055/made - The Price Marking (Amendment) Order 2025, SI 2025/592, article 2(2) (commencement moved to 6 April 2026): https://www.legislation.gov.uk/uksi/2025/592/body/made

1.0.1 fixes Python accepting a trailing newline in quantity (priced per item); adds tests.

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