Metric failed, Python 3

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from re import split

__name__ = '__metric__'

class DataModel(object):
  pass

class UnitError(Exception):
  pass

class SpecialDict(dict):
  pass

class MetricSystem(DataModel):
  def __init__(self, metric_type, base_unit, unit_dict = {}):
    self.metric_type = metric_type
    self.base_unit = base_unit
    self.unit_dict = unit_dict
  def __getitem__(self, unit):
    return self.unit_dict[self.strip_power(unit)[0]]
  def is_same_type(self, unit):
    return self.strip_power(unit)[0] in self.unit_dict
  def convert(self, exp, unit, convert_to = None):
    if not self.is_same_type(convert_to):
      raise UnitError('Undefined or inconvertible type')
    convert_to = convert_to or self.base_unit
    return '(({:s})*{}){}'.format(exp, self._convert_to(self._convert_from(unit), convert_to), convert_to)
  def _convert_from(self, unit):
    return '{}^{}'.format(self[unit], self.strip_power(unit)[1])
  def _convert_to(self, exp, convert_to):
    return '({})/{}^{}'.format(exp, self[convert_to], self.strip_power(convert_to)[1])
  def strip_power(self, unit):
    return split(r'^|\*\*', unit) if '^' in unit or '**' in unit else [unit, 1]
  def eval(self, final_exp):
    pass
  def __str__(self):
    return '<Metric System for {:s} (base unit: {!r})>'.format(self.metric_type, self.base_unit)

length = MetricSystem('length', 'm', { 'feet': 0.3048, 'cm': 0.01, 'pm': 1e-12 })
print(length, length.convert('48', 'cm', 'pm'))

[ + ] Show input
Absolute running time: 0.12 sec, cpu time: 0.1 sec, memory peak: 6 Mb, absolute service time: 0,13 sec