topic Re: Calculate Field with Python in Python Questions

Calculate Field with Python

AaronSchnydrig — Sun, 07 May 2017 19:20:02 GMT

Hello everyone

I have a question concerning Python in ArcMap:

What I want to do: I want to write a "2" in the field "Hauptgesellschaft_Gew" if the field "Hauptgesellschaft" contains the number "1235" (both fields are in the table "Wald_Poly"). This is part of an exercise given to us in which we have to calculate a suitability map.

What I have tried in the Tool "Calculate Field":

Input Table: Wald_Poly

Field Name: Hauptgesellschaft_Gew

Expression: reclass(!Hauptgesellschaft!)

Expression Type (optional): PYTHON

Code Block (optional): inTable = "Wald_Poly"
      fieldName = "Hauptgesellschaft_Gew"
      expression = "reclass(!Hauptgesellschaft!)"
      codeblock = """def reclass(Hauptgesellschaft):
      if Hauptgesellschaft == '1235':
      return '2'"""

The field formats are text/string for "Hauptgesellschaft" and short integer for "Hauptgesellschaft_Gew".

Unfortunately it does not work, the programm runs, but there are no numbers written into the field "Hauptgesellschaft_Gew". We never had an introduction to Python so my (not working) solution is based on different examples I found on the internet.

The reason for this rather complicated way of solving this problem is, that I have to do everything in the model builder and that there are more complicated codes following. So if any of you know, how I could integrate an OR (like in if Hauptgesellschaft == '1235' OR Hauptgesellschaft == '1236' OR Hauptgesellschaft == '1237' ) I would be glad too.

I would be really happy, if somebody could help me. I tried for hours, but I could not get it working.

P.S. This is the first time I post something on this board, I am sorry, if I did something wrong.

Re: Calculate Field with Python

DanPatterson_Retired — Sun, 07 May 2017 20:07:38 GMT

some issues... first you said the number 2 should be written into the new field... '2' is not a number but a text representation of the number.... second, you don't want anything to happen if the condition isn't met? That may be fine, it will leave null values in a geodatabase field, but it will leave 0 for shape fields.

So, check that the field type is indeed a number field (ie long, short for no decimals) or make sure it is a text field if a string.

Also, test your code block in a sample field calculation to ensure that your indentation and everything is correct, then copy and paste and enclose in the triple quotes.... use python9.3 as well

All for now

Re: Calculate Field with Python

AaronSchnydrig — Mon, 08 May 2017 09:48:15 GMT

First of all, thank you very much for your answer.

"Hauptgesellschaft" is a text (so I continue writing '1235'), "Hauptgesellschaft_Gew" is a short integer (so I write 2 instead of '2')

I do want something to happen if the condition isn't met, but first I will try to understand with one condition.

What do you mean by "test your code block in a sample field calculation to ensure that your indentation and everything is correct"? How can I do that? But I am quite sure, that my indentation is NOT correct. As I said, I really have no clue at all with Python, that is why I came here, to ask what part of my code is wrong.

If I run my programm, get this error message:

ERROR 000539: Error running expression: reclass(u"1835")
Traceback (most recent call last):
File "<expression>", line 1, in <module>
NameError: name 'reclass' is not defined

Failed to execute (Calculate Field).

Re: Calculate Field with Python

DanPatterson_Retired — Mon, 08 May 2017 12:08:05 GMT

you seem to be trying to use the calculate field tool, which is normally used in a script, although exclusively. I was referring to the field calculator which you access from an open table as per usual procedures

Re: Calculate Field with Python

DanPatterson_Retired — Mon, 08 May 2017 13:42:54 GMT

This may help when trying to see the association between the field calculator expressions and how to use the CalculateField tool

Re: Calculate Field with Python

AaronSchnydrig — Thu, 11 May 2017 11:15:44 GMT

Thank you very much, with your help I was able to solve this problem. One main mistake was that I did not type enough spaces in front of the "if" and "return" lines.

My final solution, just in case somebody will have the same problem in the future:

Input Table: Wald_Poly

Field Name: Hauptgesellschaft_Gew

Expression: reclass(!Hauptgesellschaft!)

Expression Type (optional): PYTHON_9.3

Code Block (optional):

def reclass(crit):
    if (crit == '1'):
        return 0
    elif (crit == '2' or crit == '1235' or crit == '2300' or crit == '3207' or crit == '3212' or crit == '3225' or crit == '4400' or crit == '4465' or crit == '4500' or crit == '4565' or crit == '4600' or crit == '4601' or crit == '4606' or crit == '4607' or crit == '4608' or crit == '5720' or crit == '6900' or crit == '7000' or crit == '7102' or crit == '7108' or crit == '7200'):
        return 2
    else:
        return 1

Re: Calculate Field with Python

XanderBakker — Sat, 11 Dec 2021 09:01:54 GMT

A little shorter version could be this:

def reclass(crit):
    if crit == '1':
        return 0
    elif crit in ['2', '1235', '2300', '3207', '3212', '3225', '4400',
                  '4465', '4500', '4565', '4600', '4601', '4606', '4607',
                  '4608', '5720', '6900', '7000', '7102', '7108', '7200']:
        return 2
    else:
        return 1‍‍‍‍‍‍‍‍‍