docs: def eta_letters(seconds, shortest=False, leading_zero=False):
      """Converts seconds remaining into human readable strings (e.g. '1s' or '5h 22m 2s').
  
      Positional arguments:
      seconds -- integer/float indicating seconds remaining.
  
      Keyword arguments:
      shortest -- show the shortest possible string length by only showing the biggest unit.
      leading_zero -- always show a leading zero for the minutes and seconds.
  
      Returns:
      Human readable string.
      """
      if not seconds:
          return '00s' if leading_zero else '0s'
  
      # Convert seconds to other units.
      final_weeks, final_days, final_hours, final_minutes, final_seconds = 0, 0, 0, 0, seconds
      if final_seconds >= 604800:
          final_weeks = int(final_seconds / 604800.0)
          final_seconds -= final_weeks * 604800
      if final_seconds >= 86400:
          final_days = int(final_seconds / 86400.0)
          final_seconds -= final_days * 86400
      if final_seconds >= 3600:
          final_hours = int(final_seconds / 3600.0)
          final_seconds -= final_hours * 3600
      if final_seconds >= 60:
          final_minutes = int(final_seconds / 60.0)
          final_seconds -= final_minutes * 60
      final_seconds = int(ceil(final_seconds))
  
      # Handle shortest:
      if shortest:
          if final_weeks:
              formatted = str(final_weeks) + 'w'
          elif final_days:
              formatted = str(final_days) + 'd'
          elif final_hours:
              formatted = str(final_hours) + 'h'
          elif final_minutes:
              formatted = '{0:0{1}d}m'.format(final_minutes, 2 if leading_zero else 1)
          else:
              formatted = '{0:0{1}d}s'.format(final_seconds, 2 if leading_zero else 1)
          return formatted
  
      # Determine which string template to use.
      if final_weeks:
          template = '{0:d}w {1:d}d {2:d}h {3:02d}m {4:02d}s' if leading_zero else '{0}w {1}d {2}h {3}m {4}s'
      elif final_days:
          template = '{1:d}d {2:d}h {3:02d}m {4:02d}s' if leading_zero else '{1}d {2}h {3}m {4}s'
      elif final_hours:
          template = '{2:d}h {3:02d}m {4:02d}s' if leading_zero else '{2}h {3}m {4}s'
      elif final_minutes:
          template = '{3:02d}m {4:02d}s' if leading_zero else '{3}m {4}s'
      else:
          template = '{4:02d}s' if leading_zero else '{4}s'
  
      return template.format(final_weeks, final_days, final_hours, final_minutes, final_seconds)