ArcGIS 10 – Field Calculator and Python

Python has been more tightly integrated in the new release of ArcGIS 10, allowing scripting to occur directly through a Python process without even opening up ArcMap. Admittedly this was available before, but now everything is more tightly coupled and a lot cleaner in it’s implementation. However, what has really interested, and indeed confused me of late is how to use Python in the ‘field calculator’.

Field Calculator is a really useful tool, when you are looking at an attribute table for a shapefile in ArcGIS and you want to derive a value for each object in the file based on a function you can input the function into the field calculator and it will work it out for you row by row. Sometimes the value you want to derive is a bit more complicated than simple arithmetic and you need to write a script. Previously you could do this in VBA, but I always found it limited and confusing, now however you can do it in Python – much simpler!

There are a few pitfalls to using Python in ArcGIS field calculator, and so I’m going to specify how to write simple field calculator python scripts in ArcGIS from my early experience.

Firstly, for Python in field calculator the way to do it seems to be in write a Python function, and then call it for each row. In addition to this, because you are writing a function you have to give it the relevant parameters (i.e fields) with which to do the computation. Finally, and annoyingly you have to write your function in a little box, and use a consistent indentation standard (1 space works best for reasons of space) as Python requires.

Here is a basic recipe for achieving field calculations in ArcGIS using Python. Obvious this is overly simplistic as you do not need a script to do this calculation, but it serves as an introductory example.

1) Name a function and parameterise it with the fields to base the calculation on. Do this in the lower box.

In the image you can see I’ve input: density( !sum_pop!, !Area!) This means send the values in the fields called sum_pop and Area to the function called density.

2) Define the function you are calling in the larger upper box.

You define a function in python using the “def” command. In the image i have defined the “density” function by writing the line: def density( pop,area):

This function definition means: define a function called density which takes the parameters pop and area. The parameters could be called anything, but it is useful to call them something that makes sense for use in the function. These parameters are variable names that the function uses to identify the fields you have passed the function when you called it, as in 1).

Normally you’d do some sort of calculation within the function, however this example is so simple that all we need to do is “return” a value to the function call. This function is the density defined as population over area: pop/area.

Looking at the field calculator I have found that you are limited to the basic, math and datetime modules in python, without the ability to import other modules. You can however define several functions and call them from within your main function.

For details on the basic syntax of using python, this site is particularly good: