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 == '':
elif f2 == ''
else: # is this cutoff?
return f1 # New_size maybe?
Reclass(!wmDeviceEq!, !wmDevice_4!, !wmDevice_5!)
wmDevice_5) is empty? Not sure if your picture is showing a cuttoff
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
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")