topic Need help using Python in a Calculate Field operation in Python Questions

Need help using Python in a Calculate Field operation

GlennKammerer — Sat, 08 Sep 2012 13:30:16 GMT

Windows XP Pro SP3
ArcView 10.0 SP2

I have a pretty simple task that would be well suited for the attribute table's Calculate Field functionality, but I don't even know where to start as far as doing this with Python. I've written dozens of Python standalone scripts, so I don't think it's a language issue. I just don't know how to do this in the Calculate Field window.

I have five fields involved in this calculate.

Fields A1, A2, B1 and B2 are Long Integer fields. This is the logic:

if (A1 == B1 and A2 == B2):
return "A AND B IDENTICAL"
else:
return "OK"

I would like the returned value to appear in field C.

Now how in the heck would this be done in the Calculate Field window?

ALSO...is there a trove of Python code block examples somewhere?

Re: Need help using Python in a Calculate Field operation

FabianBlau — Sat, 11 Dec 2021 12:32:37 GMT

Read the help, there are many examples and tips:
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Calculate_Field_examples/00170000004s000000/

In the Field Calculator check "Show Codeblock" and insert the code:

if ([A1] == [B1] and [A2] == [B2]):
    output = "A AND B IDENTICAL"
else:
    output = "OK"

For "<fieldname> = " insert:

output

Re: Need help using Python in a Calculate Field operation

KimOllivier — Sat, 11 Dec 2021 12:32:40 GMT

If you are going to use Python to calculate fields with lots of logic then it is much easier to use an UpdateCursor. The FieldCalculator wraps a cursor around your expression anyway so it is the same thing. I regard the FieldCalculator as a prop to use in ModelBuilder only.

The benefits of using a cursor are many:

The logic is easier to write and understand
It is easier to test the result and debug
You can handle exceptions and unexpected input
The syntax is simpler
If you are using a join, then use a python dictionary instead for speed.

with arcpy.da.UpdateCursor(feature_class,['A1',' B1',' A2', 'B2',' C']) as cur:
      for row in cur:    
         if (row[0] == row[1] and row[2] == row[3]):  
              row[4] = "A AND B IDENTICAL"  
         else:  
             row[4] = "OK"
         cur.updateRow(row)‍‍‍‍‍‍‍