pyqgis - Letting user choose from 2 options for QGIS Processing script input parameter?

I want to make a drop down box that shows 2 or 3 options and let the user choose 1 from them?

I would like to let the user to choose from "millimeter", "centimeter" and "meter". These Options are not from the field of input.

It seems like none of them here is valid:

Answer

Germán Carrillo's answer is the most convenient. A possible alternative (if you're stuck with QGIS 2.8) could be to:

• 
  

  Use a string which allows the user to enter the units they want to use:

  
  

  ##units=string mm
  
  if units == 'mm':
      # Do something
  elif units == 'cm':
  
      # Do something
  elif units == 'm':
      # Do something
  

  
  

  

  
  

---

• 
  
  

  Or use three boolean checkboxes:

  
  

  ##Millimeter=boolean
  ##Centimeter=boolean
  ##Meter=boolean
  
  if Millimeter == True:
      # Do something
  elif Centimeter == True:
      # Do something
  elif Meter == True:
  
      # Do something