How to use the quantities.unitquantity.UnitMass function in quantities

If both system and individual defaults are given, the system
    defaults will be applied first, followed by the individual ones.
    """
    if system is not None:
        system = system.lower()
        try:
            assert system in ('si', 'cgs')
        except AssertionError:
            raise ValueError('system must be "SI" or "cgs", got "%s"' % system)
        if system == 'si':
            UnitCurrent.set_default_unit('A')
            UnitLength.set_default_unit('m')
            UnitMass.set_default_unit('kg')
        elif system == 'cgs':
            UnitLength.set_default_unit('cm')
            UnitMass.set_default_unit('g')
        UnitLuminousIntensity.set_default_unit('cd')
        UnitSubstance.set_default_unit('mol')
        UnitTemperature.set_default_unit('degK')
        UnitTime.set_default_unit('s')

    UnitCurrency.set_default_unit(currency)
    UnitCurrent.set_default_unit(current)
    UnitInformation.set_default_unit(information)
    UnitLength.set_default_unit(length)
    UnitLuminousIntensity.set_default_unit(luminous_intensity)
    UnitMass.set_default_unit(mass)
    UnitSubstance.set_default_unit(substance)
    UnitTemperature.set_default_unit(temperature)
    UnitTime.set_default_unit(time)

dr = dram = UnitMass(
    'dram',
    oz/16,
    symbol='dr',
    aliases=['drams'],
    doc='avoirdupois dram'
)
drachm = apdram = UnitMass(
    'drachm',
    60*gr,
    u_symbol='  ',
    aliases=['drachms', 'apdram', 'apdrams'],
    doc='also known as the apothecary dram'
)

bag = UnitMass(
    'bag',
    94*lb,
    aliases=['bags']
)

ton = short_ton = UnitMass(
    'short_ton',
    2000*lb,
    aliases=['short_tons']
)
long_ton = UnitMass(
    'long_ton', 2240*lb,
    aliases=['long_tons']
) # both long and short tons are referred to as "ton" so we wont use it

############################################################

]
)
u = amu = atomic_mass_unit = dalton = Da = UnitMass(
    'atomic_mass_unit',
    1.660538782e-27*kg,
    symbol='u',
    aliases=['amu', 'Da', 'dalton'],
    doc='relative uncertainty = 5e-8'
)
scruple = UnitMass(
    'scruple',
    20*gr,
    u_symbol='℈',
    aliases=['scruples']
)
dr = dram = UnitMass(
    'dram',
    oz/16,
    symbol='dr',
    aliases=['drams'],
    doc='avoirdupois dram'
)
drachm = apdram = UnitMass(
    'drachm',
    60*gr,
    u_symbol='  ',
    aliases=['drachms', 'apdram', 'apdrams'],
    doc='also known as the apothecary dram'
)

bag = UnitMass(
    'bag',

symbol='kg',
    aliases=['kilograms']
)
g = gram = UnitMass(
    'gram',
    kg/1000,
    symbol='g',
    aliases=['grams']
)
mg = milligram = UnitMass(
    'milligram',
    gram/1000,
    symbol='mg',
    aliases=['milligrams']
)
oz = ounce = avoirdupois_ounce = UnitMass(
    'ounce',
    28.349523125*g,
    symbol='oz',
    aliases=['ounces','avoirdupois_ounce', 'avoirdupois_ounces'],
    doc='exact'
)
lb = pound = avoirdupois_pound = UnitMass(
    'pound',
    0.45359237*kg,
    symbol='lb',
    aliases=['pounds', 'avoirdupois_pound', 'avoirdupois_pounds'],
    doc='exact'
)
st = stone = UnitMass(
    'stone',
    14*lb,

)
drachm = apdram = UnitMass(
    'drachm',
    60*gr,
    u_symbol='  ',
    aliases=['drachms', 'apdram', 'apdrams'],
    doc='also known as the apothecary dram'
)

bag = UnitMass(
    'bag',
    94*lb,
    aliases=['bags']
)

ton = short_ton = UnitMass(
    'short_ton',
    2000*lb,
    aliases=['short_tons']
)
long_ton = UnitMass(
    'long_ton', 2240*lb,
    aliases=['long_tons']
) # both long and short tons are referred to as "ton" so we wont use it

############################################################
##                 Mass per unit length                   ##
############################################################

denier = UnitQuantity(
    'denier',
    g/(9000*m),

aliases=['grams']
)
mg = milligram = UnitMass(
    'milligram',
    gram/1000,
    symbol='mg',
    aliases=['milligrams']
)
oz = ounce = avoirdupois_ounce = UnitMass(
    'ounce',
    28.349523125*g,
    symbol='oz',
    aliases=['ounces','avoirdupois_ounce', 'avoirdupois_ounces'],
    doc='exact'
)
lb = pound = avoirdupois_pound = UnitMass(
    'pound',
    0.45359237*kg,
    symbol='lb',
    aliases=['pounds', 'avoirdupois_pound', 'avoirdupois_pounds'],
    doc='exact'
)
st = stone = UnitMass(
    'stone',
    14*lb,
    symbol='st',
    doc='As defined in the UK, 1 stone = 14 avoirdupois pounds'
)

carat = UnitMass(
    'carat',
    200*mg,

symbol='st',
    doc='As defined in the UK, 1 stone = 14 avoirdupois pounds'
)

carat = UnitMass(
    'carat',
    200*mg,
    aliases=['carats']
)
gr = grain = UnitMass(
    'grain',
    64.79891*mg,
    symbol='gr',
    aliases=['grains']
)
long_hundredweight = UnitMass(
    'long_hundredweight',
    112*lb,
    aliases=['long_hundredweights']
)
short_hundredweight = UnitMass(
    'short_hundredweight',
    100*lb,
    aliases=['short_hundredweights']
) # cwt is used for both short and long hundredweight, so we wont use it
t = metric_ton = tonne = UnitMass(
    'tonne',
    1000*kg,
    symbol='t',
    aliases=['tonnes']
)
dwt = pennyweight = UnitMass(

aliases=[
        'apounce', 'apounces', 'apothecary_ounce', 'apothecary_ounces',
        'troy_ounces'
    ]
)
troy_pound = appound = apothecary_pound = UnitMass(
    'troy_pound',
    12*toz,
    symbol='tlb',
    u_symbol='℔',
    aliases=[
        'troy_pounds', 'appound', 'appounds', 'apothecary_pound',
        'apothecary_pounds'
    ]
)
u = amu = atomic_mass_unit = dalton = Da = UnitMass(
    'atomic_mass_unit',
    1.660538782e-27*kg,
    symbol='u',
    aliases=['amu', 'Da', 'dalton'],
    doc='relative uncertainty = 5e-8'
)
scruple = UnitMass(
    'scruple',
    20*gr,
    u_symbol='℈',
    aliases=['scruples']
)
dr = dram = UnitMass(
    'dram',
    oz/16,
    symbol='dr',

doc='relative uncertainty = 5e-8'
)
scruple = UnitMass(
    'scruple',
    20*gr,
    u_symbol='℈',
    aliases=['scruples']
)
dr = dram = UnitMass(
    'dram',
    oz/16,
    symbol='dr',
    aliases=['drams'],
    doc='avoirdupois dram'
)
drachm = apdram = UnitMass(
    'drachm',
    60*gr,
    u_symbol='  ',
    aliases=['drachms', 'apdram', 'apdrams'],
    doc='also known as the apothecary dram'
)

bag = UnitMass(
    'bag',
    94*lb,
    aliases=['bags']
)

ton = short_ton = UnitMass(
    'short_ton',
    2000*lb,

aliases=['pounds', 'avoirdupois_pound', 'avoirdupois_pounds'],
    doc='exact'
)
st = stone = UnitMass(
    'stone',
    14*lb,
    symbol='st',
    doc='As defined in the UK, 1 stone = 14 avoirdupois pounds'
)

carat = UnitMass(
    'carat',
    200*mg,
    aliases=['carats']
)
gr = grain = UnitMass(
    'grain',
    64.79891*mg,
    symbol='gr',
    aliases=['grains']
)
long_hundredweight = UnitMass(
    'long_hundredweight',
    112*lb,
    aliases=['long_hundredweights']
)
short_hundredweight = UnitMass(
    'short_hundredweight',
    100*lb,
    aliases=['short_hundredweights']
) # cwt is used for both short and long hundredweight, so we wont use it
t = metric_ton = tonne = UnitMass(