UnitRegistry(system="imperial") fetches the wrong volume measures

[nickovs]

When fetching imperial gallons via a registry created just as reg1 = UnitRegistry() using reg1.sys.imperial.gallon it returns the correct unit. When fetching the same unit using reg2 = UnitRegistry(system="imperial") and then referencing reg2.gallon the wrong (US) unit is returned.

For example, this works:

import pint
print(pint.__version__)

plain_reg = pint.UnitRegistry()

print("Default system: ", plain_reg.default_system)
imp_pint = 1 * plain_reg.sys.imperial.pint
us_pint = 1 * plain_reg.sys.US.pint
print("Pints via plain registry:", imp_pint.to("litre").m, imp_pint.to("litre").m / us_pint.to("litre").m)

imp_gallon = 1 * plain_reg.sys.imperial.gallon
us_gallon = 1 * plain_reg.sys.US.gallon
print("Gallons via plain registry:", imp_gallon.to("litre").m, imp_gallon.to("litre").m / us_gallon.to("litre").m)

It gives the expected:

0.24.3
Default system:  mks
Pints via plain registry: 0.5682612500000002 1.2009499255048557
Gallons via plain registry: 4.546090000000001 1.2009499255048557

This does not work correctly:

import pint
print(pint.__version__)

imperial_reg = pint.UnitRegistry(system="imperial")
us_reg = pint.UnitRegistry(system="US")
print("Imperial system: ", imperial_reg.default_system)
print("US system: ", us_reg.default_system)

imp_reg_pint = 1 * imperial_reg.pint
us_reg_pint = 1 * us_reg.pint
print("Pints via system registries:", imp_reg_pint.to("litre").m, imp_reg_pint.to("litre").m / us_reg_pint.to("litre").m)

imp_reg_gallon = 1 * imperial_reg.gallon
us_reg_gallon = 1 * us_reg.gallon
print("Gallons via system registries:", imp_reg_gallon.to("litre").m, imp_reg_gallon.to("litre").m / us_reg_gallon.to("litre").m)

The result is:

0.24.3
Imperial system:  imperial
US system:  US
Pints via system registries: 0.4731764729999999 1.0
Gallons via system registries: 3.785411783999999 1.0

In the later case, even though a system was specified when creating the registry and is shown as being the default, it is not being used when fetching the volume measures. Instead it is fetching the US volume measurements.