在`pint`中，设置每单位`default_format`

Question

Basically the title. In pint , is there a way to define the default string formatting per dimension or per unit , instead of 'across the board'?

Stated more precisely: I want to format a quantity's numerical value (ie, magnitude), based on its physical unit.

Here is what I tried, based on the code shown in the docs :

from pint import UnitRegistry, Unit
ureg = UnitRegistry()

# Specific format for km won't stick...
ureg.default_format = ".0f~"
ureg.km.default_format = ".2fP"
ureg.km.default_format  # '.0f~'

# ...as also seen here:
dist = 3 * ureg("km")
time = 500 * ureg("s")
print(f"{dist}, {time}")
# wanted: 3.00 kilometer, 500 s
# got:    3 km, 500 s

Especially when dealing with prices, it's practical to be able to set a 2-digit-default, with all other units having different default format.

PS: I know it's possible to set a default formatting on an individual quantity (eg dist.default_format = '.2f~' ), but that's too specific for my use case. I want all quantities with the unit 'km' to be displayed with 2 decimals.

Answer 1

I have constructed quite the hacky solution:

from pint import UnitRegistry, Quantity
ureg = UnitRegistry()

# Setting specific formats:
formatdict = {ureg.km: '.2fP'} # extend as required
Quantity.default_format = property(lambda self: formatdict.get(self.u, ".0f~"))

# Works:
dist = 3 * ureg("km")
time = 500 * ureg("s")
print(f"{dist}, {time}")  # 3.00 kilometer, 500 s

This works, but I'd be surprised if there isn't a better solution.

---

EDIT

It only works in a limited sense. ureg.default_format gets changed as well, which prohibits its use in eg a pandas.DataFrame :

ureg.default_format  # <property at 0x21148ed5ae0>