`pint` 中派生维度中数量的标准格式

Question

TLDR: I'd like to have pint quantities, that are in a certain (derived) dimension, to be converted into a pre-set unit by default.

---

Details:

I deal with 5 dimensions, as specified below. Note that [power] (usually in MW) and [price] (usually in Eur/MWh) are the derived dimensions.

# units.txt

# Base dimensions and their units
hour = [time] = h = hr
megawatthour = [energy] = MWh
euro = [currency] = Eur = €

# Derived dimensions and their units
[power] = [energy] / [time]
megawatt = megawatthour / hour = MW
[price] = [currency] / [energy]
euro_per_MWh = euro / megawatthour  = Eur/MWh

My question: is it possible to specify that calculated quantities with dimension [power] are by default to be converted to MW?

---

Here's an example:

import pint

ureg = pint.UnitRegistry("units.txt",)
ureg.default_format = ".0f~P"

energy = 100 * ureg.MWh
time = 4 * ureg.h
revenue = 4000 * ureg.euro

price = revenue / energy
power = energy / time

print(energy, time, revenue, price, power) 
# 100 MWh 4 h 4000 Eur 40 Eur/MWh 25 MWh/h

Here, the power is expressed in MWh/h , because of the way it is calculated, and I can 'convert' it to MW by calling power.to('MW') . Is it possible to automatically do this every time a quantity in this dimension is calculated?

Note that doing a blanket .to_base_units() on all quantities reverts it back to MWh/h .

Remarks:

• I don't want to use [power] as a base dimension. It moves the problem to the price anyway.
• I'm aware of prefixes; I just left them out here to keep the example as short as possible.

Answer 1

The issue is that MWh is defined as a base unit in the units.txt file instead of using MW as base unit and constructing MWh from MW and hour. Once the unit construction is sorted, .to_base_units() will yield the desired result.

# units-2.txt

# Base dimensions and their units
hour = [time] = h = hr
megawatt = [power] = MW 
euro = [currency] = Eur = €

# Derived dimensions and their units
[energy] = [power] * [time]
megawatthour = megawatt * hour = MWh
[price] = [currency] / [energy]
euro_per_MWh = euro / megawatthour  = Eur/MWh

>>> def test_defs(file):
...     ureg = pint.UnitRegistry(file,)
...     ureg.default_format = ".0f~P"
...     
...     energy = 100 * ureg.MWh
...     time = 4 * ureg.h
...     revenue = 4000 * ureg.euro
...     
...     price = revenue / energy
...     power = energy / time
...     
...     print(energy, time, revenue, price, power) 
...     print(power.to('MW'))
...     print(power.to_base_units())
... 
>>> test_defs("units.txt")
100 MWh 4 h 4000 Eur 40 Eur/MWh 25 MWh/h
25 MW
25 MWh/h
>>> test_defs("units-2.txt")
100 MWh 4 h 4000 Eur 40 Eur/MWh 25 MWh/h
25 MW
25 MW
>>>