pint-cf 0.2.0

CF-compliant unit registry and formatter for Pint

pint-cf

This package extends Pint with a CF-compliant unit registry and formatter.

Features

• A CF-compliant Pint UnitRegistry, understanding UDUNITS-2 unit strings.
• A cf format for Unit/Quantity, to write results back out as CF strings.
• CFContext, to correctly read CF's units_metadata temperature attribute.
• CF units that UDUNITS-2 itself doesn't define (level, psu, decibel, sverdrup's Sv symbol, ...), included by default.

Known limitations:

• No support for time coordinates (e.g. days since 2001-01-01) or climate calendars (e.g. 360_day). Use cftime for those instead.
• decibel/bel are treated as plain dimensionless ratio units, matching cfunits/cf-units - CF's own reference level for a dB value depends on the variable's standard_name, but neither of those reference packages resolves it either, so pint-cf follows the same convention rather than the letter of the spec.

Installation

Install via pip:

pip install pint-cf

Usage

Create a CF-ready unit registry with cf_unitregistry(). This also registers the cf format.

from pint_cf import cf_unitregistry

ureg = cf_unitregistry()

q = ureg('10 meters per second^2').to('km s-2')

print(f"{q:cf}")   # 0.01 kilometer-second^-2
print(f"{q:~cf}")  # 0.01 km/s2

Temperature units_metadata

CF distinguishes an on-scale temperature (e.g. today's temperature) from a temperature difference (e.g. an anomaly) with a units_metadata attribute - the plain units string alone can't tell them apart.

When reading a variable that has this attribute, wrap the value in CFContext:

from pint_cf import cf_unitregistry, CFContext

ureg = cf_unitregistry()

with CFContext(units_metadata="temperature: difference"):
    q = ureg.Quantity(1, "degree_C")

print(q.units)  # delta_degree_Celsius

Without CFContext (or with units_metadata absent), nothing changes - an on-scale temperature is read as usual.

When writing a result back out, cf_attributes_for does the reverse: it derives the units_metadata attribute from an already-computed Unit or Quantity, ready to merge into a NetCDF variable's attributes:

from pint_cf import cf_attributes_for

print(cf_attributes_for(q))                   # {"units_metadata": "temperature: difference"}
print(cf_attributes_for(ureg.Unit("meter")))  # {} - not a temperature unit

ds[name].attrs.update(cf_attributes_for(q.units))

This round-trips exactly for Celsius and Fahrenheit. Kelvin and Rankine have no zero-offset, so on-scale and difference look identical for them - cf_attributes_for reports "temperature: unknown" rather than guessing.

CF units not in UDUNITS-2

CF defines a few units that UDUNITS-2 doesn't (details), sourced from cfunits. cf_unitregistry() includes them by default:

Unit	Behavior
level, sigma_level, layer	Dimensionless; kept only for legacy COARDS files, so using one raises a DeprecationWarning.
practical_salinity_unit / psu	1e-3, not 1 (CF's own FAQ is out of date here).
decibel / dB, bel	Plain dimensionless ratio units (see "Known limitations" above).
sverdrup / Sv	Sv now means sverdrup (ocean volume transport), not sievert - sievert itself still works by its full name.

Pass cf_extensions=False to cf_unitregistry() for a registry that matches plain UDUNITS-2 instead, without any of these additions.