Concatenate only when all fields to be concatenated have values

Question

I have two fields that I want to concatenate. Field1 contains null values and Field2 has values for every record. I want to concatenate only when both fields have values. I tried using a function with an if/else statement but that doesn't work as the resulting concatenation will return values from field2.

def concat_fields(field1, field2):
    if field1 == ""
        return 

    else:
        return field1 + field2

I know this is wrong. Any suggestions?

EDIT:

Answer 1

Based on your results, it looks like field1 may have a space (" ") in the field, instead of an empty string.

def concat_fields(field1, field2):
    if field1.strip() == "" or field2.strip() == "":
        return ""
    else:
        return field1 + field2

Answer 2

If you are mean empty strings "" instead of NULL, you are missing just a 'or' operator:

def concat_fields(field1, field2):
  if field1 == "" or field2 == "":
      return 

  else:
      return field1 + field2

Answer 3

I've the same problem. I've two fields to concatenate, exluding empty values. The fields are: AREA_HI AREA_LO

But arcgis return an error "000539 : Error message from Python."

Some suggestions? Thank's.

def concat_fields(!AREA_HI!, !AREA_LO!):
    if !AREA_HI!.strip() == "" or !AREA_LO!.strip() == "":
        return ""
    else:
        return !AREA_HI! + !AREA_LO!