code: def to_pint(unit):
      
      global HAYSTACK_CONVERSION
      if unit == 'per_minute' or \
          unit == '/min' or \
          unit == 'per_second' or \
          unit == '/s' or \
          unit == 'per_hour' or \
          unit == '/h' or \
          unit == None:
          return ''
          # Those units are not units... they are impossible to fit anywhere in Pint
      for haystack_value, pint_value in HAYSTACK_CONVERSION:
          unit = unit.replace(haystack_value, pint_value)
      return unit
docs: Some parsing tweaks to fit pint units / handling of edge cases.