SI Unit

Canonicalizes one SI unit expression per call — a symbol, a name, or a product/quotient compound — to its canonical symbol.

In plain language: give it "Kilogram", "megahertz", or "m/s²" and it hands back "kg", "MHz", or "m/s2" if the SI Brochure or ISO 80000 says that expression is a valid unit. Quantities like "25°C" are not its domain (the number is not an SI unit).

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What it recognizes — and what it does not

Recognizes	Does not recognize
Symbols (kg, Pa, kΩ, m/s²)	Quantities / magnitudes (25°C, 5 kg) — MISSING; identity-only, no amounts
Names (Kilogram, megahertz) — case-insensitive, hyphen-tolerant	Name-compounds (metre per second) — words recognized separately → AMBIGUOUS
Prefixed symbols and names (MHz, kPa)	Symbol-prefix spacing (k g) — INVALID, never accepted
Product/quotient compounds (m/s², kg·m/s², kPa)	Multi-solidus without flag (kg/m/s) — INVALID unless allow_multi_solidus=True
Spoken word-prefix forms (kilo gram) — only when allow_split_word_prefixes=True	kilo gram without that flag — INVALID

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Canonical output

Single format — the canonical symbol form.

output_format	Renders
(only) symbol / None / "default"	canonical symbol (kg, MHz, m/s2)

from paxman.capabilities import SIUnit
import paxman

paxman.register_all_shipped()
paxman.canonicalize("Kilogram", SIUnit.create_contract()).canonicalized_value  # "kg"
paxman.canonicalize("megahertz", SIUnit.create_contract()).canonicalized_value  # "MHz"
paxman.canonicalize("m/s²", SIUnit.create_contract()).canonicalized_value  # "m/s2"

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Contract

contract = SIUnit.create_contract(
    allow_split_word_prefixes=False,  # bool, default False — "kilo gram" → "kg" when True
    allow_multi_solidus=False,  # bool, default False — preserve "kg/m/s" when True
    output_format=None,  # "symbol" (only format)
    # plus every common field: excluded_rules / pinned_rules / year / extra_grammars
)

• With allow_split_word_prefixes=False (default), kilo gram is recognized as INVALID — it is not a valid SI identity. Symbol-prefix spacing like k g is always INVALID; there is no flag for it.
• With allow_multi_solidus=False (default) per ISO 80000-1 §6.6.2, more than one top-level solidus is INVALID. Set True to preserve the legacy accept-multi-solidus behavior.

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Statuses

Input	Contract	Status	Value / why
Kilogram	defaults	SUCCESS	→ kg
m/s²	defaults	SUCCESS	compound → m/s2
metre per second	defaults	AMBIGUOUS	words recognized separately, competing groupings
25°C	any	MISSING	quantity, not an identity-only unit
kilo gram	defaults	INVALID	needs allow_split_word_prefixes
kilo gram	allow_split_word_prefixes=True	SUCCESS	→ kg
k g	any	INVALID	symbol-prefix spacing always rejected
kg/m/s	defaults	INVALID	multi-solidus without flag
kg/m/s	allow_multi_solidus=True	SUCCESS	preserved

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Notebook snippet

import paxman
from paxman.capabilities import SIUnit
from paxman.core.domain import Resolution

paxman.register_all_shipped()
c = SIUnit.create_contract()
c_spoken = SIUnit.create_contract(allow_split_word_prefixes=True)
c_multi = SIUnit.create_contract(allow_multi_solidus=True)

for text in [
    "Kilogram",
    "m/s²",
    "metre per second",
    "kilo gram",
    "k g",
    "kg/m/s",
    "25°C",
]:
    for label, cc in [("default", c), ("spoken", c_spoken), ("multi", c_multi)]:
        r = paxman.canonicalize(text, cc)
        val = r.canonicalized_value if r.status == Resolution.SUCCESS else "—"
        print(f"{text!r:18} [{label:8}] → {r.status.value:10} {val!r}")

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Provenance

• BIPM SI Brochure (9th ed.) — base units, derived units, non-SI units accepted for use, prefixes, names.
• ISO 80000-1 — compound expressions.

See also: Execution Result, Candidates & Ambiguity, Segmentation.