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Python Calculate Field If Statement

Ryan_Galbraith

Just wondering if the Python-Geoprocessing Community could help me. I am trying to write the following code block into a calculate field called weight from the values in another field called Symbol.

def Reclass( !Weight!):
if ( !SYMBOL! == 6):
    return 1.2
elif ( !SYMBOL! == 85):
    return 1.5
elif ( !SYMBOL! ==2):
    return 1
elif ( !SYMBOL! ==42):
    return 1.8
elif ( !SYMBOL! ==58):
    return 2

It just won't work. help...

model.jpg

script.jpg

Attributes.PNG

SarmisthaChatterjee_0-1608335744079.png

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Anonymous User

You're gonna have to give more info than that. And no, that method won't work.

DanPatterson_Retired

enclose your code in code blocks (# in the html editor window) so that indentation can be checked. Include a screen grab so the whole field calculator dialog window can be checked for missing elements (like how you pass the field name to the function and to make sure that the Python radiobutton is checked on)
also any further details on the field's properties (perhaps a wrong field type, precision or scale)

Anonymous User

Original User: rdg129

You're gonna have to give more info than that. And no, that method won't work.

This was what I was trying, but really lost track of what to put where.

calculcatepython.jpg

ChrisSnyder

Just a suggestion, but if this is part of a Python script, you are WAY better off using an updatecursor instead. The benefit is readability, codeability, and debugging. The code block for the FieldCalculator is notoriously hard to write code for, while the updatecursor has very clear and clean syntax: http://forums.esri.com/Thread.asp?c=93&f=1729&t=262236#806786

Anonymous User

Original User: niklas.norrthon

This goes in the code block:

symbol_weights = {
    6: 1.2,
    85: 1.5,
    2: 1.0,
    42: 1.8,
    58: 2.0,
    }

And in the expression field you put:

symbol_weights[!SYMBOL!]

NiklasNorrthon

Just a suggestion, but if this is part of a Python script, you are WAY better off using an updatecursor instead. The benefit is readability, codeability, and debugging. The code block for the FieldCalculator is notoriously hard to write code for, while the updatecursor has very clear and clean syntax

I disagree. Calculate field with short snippets of python code as in this case are perfect in most situations. The one place to avoid this tool and use an update cursor instead is within a python script, becuase of the quoting issues.

But as a stand alone tool, or in a model builder model, CalculateField is a very useful tool.

Anonymous User

Original User: csny490

If you are going the Python route, I still assert that a cursor is better because:
1. It's faster and easier to write the code (no need to post to the forums per the pain in the ass \\n\\n\\n shinanigans).
2. When you print the code it's actually understandable by a human, where as the codeblock generally comes out in one big line and is generally unintelligible.

For example (yes this is VB, but you get my drift):

KLAScodeblock = "If [CMPAREA] <= 1000 Then\\nklasse = 100\\n\\nElseif [MIN] <= "+MDV+"  Then \\nklasse = 1\\n\\nElseif [MIN] <= "+MDA+" Then \\nklasse = 2\\n\\nElse \\nklasse = 3\\n\\nEnd  If\\n\\n\\n"

Ryan_Galbraith

This goes in the code block:

symbol_weights = {
    6: 1.2,
    85: 1.5,
    2: 1.0,
    42: 1.8,
    58: 2.0,
    }

And in the expression field you put:

symbol_weights[!SYMBOL!]

It looks good, but I am still getting an error.

Here is what I put in.

ERROR

000539: Error running expression: SYMBOL_weights[2] <type 'exceptions.NameError'>: name 'SYMBOL_weights' is not defined

Failed to execute (Calculate Field).

Failed at Wed Feb 16 13:17:20 2011 (Elapsed Time: 1.00 seconds)

error.jpg

DanPatterson_Retired

let us examine how Python is used in the field calculator as indicated from the help files....

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//005s0000002m000000.htm

There is an Expression box, and a Code block, in the code block there is a section for variables and/or imports to be denoted (in this case an import statement) and a Python function, as denoted by the def
If the syntax is correct and the fieldname is enclosed in double !'s then it should work

Parser:
Python

Expression:
getRandomValue()

Code Block:
import numpy.random as R

def getRandomValue():
    return R.random()

can you get this to work in a field? The error in the previous example is that SYMBOL_weights is not a function

Anonymous User

Original User: niklas.norrthon

I tested it and got the same error, and I still don't understand why. But I found an easy workaround: Rename the dictionary to something else (without underscore) like symweights. Do that both in the code block and the expression field and it works.

Ryan_Galbraith

I tested it and got the same error, and I still don't understand why. But I found an easy workaround: Rename the dictionary to something else (without underscore) like symweights. Do that both in the code block and the expression field and it works.

That works perfect. Like Python should! Amazing. I choose this method, because I wanted to use the ModelBuilder. Not sure if a cursor would work in this case.

Anonymous User

Original User: niklas.norrthon

That works perfect. Like Python should! Amazing. I choose this method, because I wanted to use the ModelBuilder. Not sure if a cursor would work in this case.

A cursor would work just as well (untested):

symbol_weights = {
    6: 1.2,
    85: 1.5,
    2: 1.0,
    42: 1.8,
    58: 2.0,
    }

cur = arcpy.UpdateCursor(my_feature_class)
for row in cur:
    row.Weight = symbol_weights[row.SYMBOL]
    cur.UpdateRow(row)

Ryan_Galbraith

Cursor works better!

AndyOmmen

Hi Ryan,

When you define the method within the Python codeblock, use a variable for the Reclass method's parameter and use that variable within the method's logic:

def Reclass(weight):

if (weight == 6):

return 1.2

elif (weight == 85):

return 85

.....

Then when you call the Reclass method in the Calculate Field Expression parameter, you will then pass in the SYMBOL field:

Reclass(!SYMBOL!)

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