Wrong calculation when using temperature prefixes

[Christian-Mueller]

I'm doing following conversion:

from pint import UnitRegistry

ureg = UnitRegistry()
t = ureg.Quantity(20, ureg.degC)
print(t.to(ureg.millidegC))

output:
293150.0 millidegree_Celsius

It seems like prefixes for degree Celsius are not handled correctly. The result should be 20000 millidegree_Celsius.
Looks like it changes the unit to Kelvin internally.
It this a known issue? Is there a workaround for my use case?

[jules-ch]

It seems millidegree_Celsius is not identified as offset units in the convert stage hence the value does not make sense afterwards.

I'll investigate

[Christian-Mueller]

Any progress so far on this bug?

[jules-ch]

The issue has been identified but the implementation is tricky with the current implementation:

pint/pint/registry.py

Lines 1529 to 1550 in c99ed1f

	if src_offset_unit:
	value = self._units[src_offset_unit].converter.to_reference(value, inplace)
	src = src.remove([src_offset_unit])
	# Add reference unit for multiplicative section
	src = self._add_ref_of_log_unit(src_offset_unit, src)
	# clean dst units from offset units
	if dst_offset_unit:
	dst = dst.remove([dst_offset_unit])
	# Add reference unit for multiplicative section
	dst = self._add_ref_of_log_unit(dst_offset_unit, dst)
	# Convert non multiplicative units to the dst.
	value = super()._convert(value, src, dst, inplace, False)
	# Finally convert to offset units specified in destination
	if dst_offset_unit:
	value = self._units[dst_offset_unit].converter.from_reference(
	value, inplace
	)
	return value

Here the conversion for offset untis is handled like so:

• Value is converted to the reference unit of the source unit which is Kelvin for all temperature unit
• Conversion for remaining unit is done for non offset dimensions
• Value is converted to destination unit from reference unit of destination unit which is Kelvin for all temperature unit

In the case of prefixed units:
for DegC to mDegC

• Value is converted to the reference unit which is kelvin
• Conversion for remaining unit is done for non offset dimensions
• Value is converted from reference unit which is DegC for mDegC

calculation does output the wrong value, for those steps

• 20 -> 293.15 (OffsetConverter for DegC)
• 293.15 -> 293.15 (nothing happens no other units)
• 293.15 -> 293150.0 (ScaleConverter of prefixed unit for mDegC)

I'll probably end up with a fix for only those prefixed temperature units.

Thoughts @hgrecco?

I find it difficult to work with UnitDefinition, UnitsContainer there

[hgrecco]

At the very beginning of Pint (before offset units were implemented) there was a proposal to to write a special converter for ocassions in which prefix was removed, added or change. This was dropped as unnecesary in that case and would have helped here.

However, I do think there is a deeper thing to discuss. IMHO si-prefixing offset units is not a good practice. It is ok to express an interval (in which case the temperature becomes a multiplicative unit anyway). And is is kind of ok to write a value, but we should be careful when operating (converting, arithmetics, etc) with this value.

[dalito]

I also thought a bit about this: mDegC is the same as 1e-3 * degC which is a multiplication that is forbidden by default. So to be consistent pint should in my opinion raise errors for

1. any kind of conversion to an SI-prefixed offset unit and
2. creation of quantities with an SI-prefix (ureg.Quantity(20, ureg.milli_degC)).

I would vote for raising errors even with ureg.autoconvert_offset_to_baseunit = True.

[hgrecco]

Moving this to #1754