Using nested If statement in Python parser of ArcGIS Field Calculator?

Question

I'm trying to use field calculator to assign values to a new field I created. I wan't to assign the value from the first field unless that field is empty, if it is empty use field 2, if that is empty use field 3. Here is what I have thus far. I feel it is way off but not sure where to go from here.

New_Size: My new field

wmDeviceEq: First field to check for new field name

wmDevice_4: Second ""

wmDevice_5: Third ""

Answer 1

In the codeblock, you can only use parameters, not field names. You'll need to create parameters for each of your field names, like so:

def Reclass(f1, f2, f3):
    New_size = f1 # this doesn't actually do anything
    if f1 == '':
        return f2
    elif f2 == ''
        return f3
    else: # is this cutoff?
        return f1 # New_size maybe?

and then

Reclass(!wmDeviceEq!, !wmDevice_4!, !wmDevice_5!)

What if f3 (wmDevice_5) is empty? Not sure if your picture is showing a cuttoff else statement.

Are the values in the fields actually empty strings? Shapefiles store a single space " " and feature classes in a geodatabase typically use <Null>, which in Python would be None.

If the fields are actually empty strings and only one field will ever have data, you don't need the code-block at all.

"".join((!wmDeviceEq!, !wmDevice_4!, !wmDevice_5!))

As an aside, a nested if statement would be something like this:

if result > 10:
    print("Greater than 10")
    if result < 100:
        print("and less than 100")
    else:
        print("It's big...")
else:
    print("Too small!")

Answer 2

@Paul

I edited the code and came up with this

def Reclass( ph1, ph2, ph3 ):
  New_Size = ph1
  if ph1 == ' ':
      return ph2
  elif ph2 == ' ':
      return ph3
  elif ph3 == ' ':
      print ("NULL")

and then

Reclass(!wmDeviceEq!, !wmDevice_4!, !wmDevice_5!)

When it runs my new field pulls values from only ph2... Also not sure about that final elif. it's meant to fill a space that has no value to pull, with a "Null".

I tried the "".join() function but it places spaces every time it moves to the next field, like so.

Won't work unless there is a way to remove those spaces before the values in the New_Size field.