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

Question

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 1

If you are not limited to QGIS v2.8 (your screenshot points to v2.8 documentation), you can install a newer version. For example, in QGIS v2.14 you have a selection parameter type available (see the docs):

You can use it in this way in the header of the script, separating options with a semicolon:

##Units=selection Millimeter;Centimeter;Meter

And later in the script you can get the chosen value in this way:

if Units == 0:
    # User chose millimeter
elif Units == 1:
    # User chose centimeter
elif Units == 2:
    # User chose meter

For reference, here you have an example script.

Answer 2

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
  

  

Answer 3

The selection option does work on QGIS 2.18.2 (Las Palmas). Please note that the selection parameter will get an integer value,starting from 0, (and not a string value) correspond to the order of the string as you write it in your selection (just like in Germán Carrillo answer).