Field calculator if and elseif statement with <Null> values in ArcGis 10.1

Question

Trying to figure out how to get this "Headwater" column to indicate "Yes" (or 1) if the value of "NextDown_Copy" is "" and "No" (or 0) if it has any other value (eg. 7100381010).

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I attempted to try figuring out the Python code to do this using 1 and 0 coding (although I'd rather use Yes and No) (as illustrated below) but keep getting errors. I consulted a few other posts (another one), but still wasn't able to find the problem.

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• Sara Barnes: Looks like you are missing the indentation in the Code box.
• Dalal_EL_Hanna: Sara Barnes, where should the indentation have been?

Answer 1

Here's a simple expression with no need to define a function in the codeblock:

Headwater =

0 if !NextDown_Copy! else 1

This checks if each value of NextDown_Copy is truthy (i.e. not a null, 0, empty string), and returns a 0 if so, otherwise it returns a 1.

• Dalal_EL_Hanna: Thanks! This pretty much worked, although it returned 1s for Null values and left Null in all the rows for which NextDown_Copy had values. Any ideas why?
• nmpeterson: That's very odd. What if you try 1 if !NextDown_Copy! is None else 0?
• Dalal_EL_Hanna: Hum! Good idea. Unfortunately still didn't work!
• nmpeterson: Is Headwater a text field? If so, try "0" if !NextDown_Copy! else "1"
• Dalal_EL_Hanna: Works like a charm! Thanks. (Looks like today I was making a big mistake by not selecting Python while running the codes... I just realized this, which is likely what was causing the problem, sorry about this!)

Answer 2

Python uses None to denote null values. I'd replace your Null with None (no quotes or brackets)and see what you get. Your script is essentially looking for a text string value of null. Furthermore, it's not contained in single quotes, so you're throwing a syntax error twice over.

• Dalal_EL_Hanna: Tried this out with no luck. But thanks, as its great to know that Python denotes nulls as None. I am totally unfamiliar with Python for the moment and was doing guess work, so indeed, I was trying to get it to search for a string value! Good to know I shouldn't have been doing that.