Batch define projection with Python

Question

Hi all,

I am fairly new to Python and I have been trying to make a script work-to assign a GC system to a number of shapefiles in a folder.(The GC system is currently Unknown so I cannot use the other tools in batch mode)

I am using Python 2.5 and Arc 9.3.1 but I keep getting errors 
( for fc in fcS:
TypeError: 'geoprocessing list object' object is not iterable)

that don't really tip me off about what it s going wrong (newbie)

My code is:
import arcgisscripting
gp = arcgisscripting.create()

gp.workspace = "P:\Proj"
gp.toolbox = "management"
coordsys = "Coordinate Systems/Projected Coordinate Systems/National Grids/British National Grid.prj"
fcs = gp.ListFeatureClasses() 
for fc in fcS:
gp.defineprojection(fcS, coordsys)

I am certain there are a lot of errors there, but if anyone can assist with the iteration/listing of the shapefiles in the folder it would be great.

Thanks in advance for any suggestions.
S.

JoeFlannery · Answer

Sofia:

Have you seen ESRI's Batch Define Coordinate System python script in the Samples Toolbox for ArcMap?

http://resources.arcgis.com/content/kbase?fa=articleShow&d=30873

SofiaStouki · Answer

It makes sense, especially seeing the differences in syntax for each version. It just makes me wish that the rules were more uniform, regardless of which product you are using.

Anyway, thanks for the links-the geoprocessor model is very useful to have as a reference.:)

Cheers,
S.

CedricWannaz · Answer

Hi Sofia and James,

You had the error because the Geoprocessor method ListFeatureClasses() returns a Python List, as described here. Under Syntax, you can see the type that is returned:object.ListFeatureClasses({wildCard} As String, {FeatureType} As String) As Python List

A way to scan a list in Python is as you do with your for loop:
for anElement in myList:
      do something ..

but lists don't have a Next() method that you initially tried to call with "fc = fcs.Next()". This is a confusion with iterable objects that, for example, methods for creating cursors usually return. The documentation for the SearchCursor() method available here states under syntax that the return is an object (and no more a simple Python List):
object.SearchCursor(InputValue As String, {WhereClause} As String, {SpatialReference} As Object, {FieldList} As String, {SortFields} As String) As Object

This object that is returned has methods, and one of them is called Next(), that returns the next element as an object. This is illustrated in the Geoprocessor Programming Model that available here as a PDF. The left column contains methods of the Geoprocessor, and the other "boxes" contain descriptions (properties, methods) of the objects that these methods return. If you lookup SearchCursor() in the left column, you will see that it returns an object that contains the rows, and that this object has a Next() method that returns another object: the next row. This object has a property called FieldName and a method called GetValue(), and so on ..

The Geoprocessor and all the objects that are returned by the different toolboxes are a quite complex structure, and it is almost impossible to guess (at least in the beginning) what is the type of the objects that we are dealing with, without using the help (the information about the type of returned values) and (to a lesser extent) the programming model.

Cheers,

Cedric