This document provides an outline for a Python programming workshop for ArcGIS. It discusses using Python to access ArcGIS commands, attribute tables, and geometries for geoprocessing. It covers Python data types like lists and tuples, string processing, functions, and accessing attribute tables using search cursors. Exercises demonstrate selecting features by attributes and location using cursors and layers.

1. Python Programming for Arcgis 2
Daniel Sheehan
dsheehan@mit.edu, gishelp@mit.edu
9:30AM-12:30PM
January 15, 2015
This class was originally developed by David
Quinn and taught by David and Daniel in IAP
2010 and 2011.

2. Goals for the workshop (from yesterday)
• Learning enough Python to
– Access Arcgis commands through Python
– Access individual records from attribute table
– Acccess individual geometries for use in
geoprocessing
• Develop the ability to record and document
your geoprocessing

3. Slides available at …
• http://web.mit.edu/dsheehan/www/
dataForPythonIAP2015.zip (same data as
yesterday)
• http://web.mit.edu/dsheehan/www/
PythonProgrammingforArcgis2.pdf

4. Outline
• More data types
• Python functions
• Accessing Attribute Tables
• Select by attributes and location

5. Lists = []
An ordered set of elements enclosed in square brackets.
Zero based (first element is accessed by typing list[0]).
# This is a list []
animals = [ 'dog ' , ' cat ' , 'horse ' , ' seal ' ]
print animals[3]
animals.append(‘owl’)
Lists are mutable

6. Tuples = ()
Zero base (first element of a non-empty tuple is
always tuple(0)
# This is a tuple
Countries = (‘Costa Rica’, ‘India’, ‘Abu Dhabi’)
# You cannot change a tuple (immutable)

7. String processing – slicing a string
# slicing a string
Name = ‘Massachusetts’
#Fenceposts – starting at index 1 and ending
# before index 2
Name[1:2]
# find the first 4 characters of Name
Name[:4]

8. Example: using length
len(Name) shows length of string
• Find the length of a string variable called
name with a value of ‘Massachusetts’
• Print the last 4 characters in the name string

9. Example Function 1
# Python uses dynamic typing (type is set when
a variable is assigned or argument is passed)
# Save in file called test.py
def print_value(argument):
# Print the passed argument
print argument
# default return is none for a function

10. Import ‘test.py’ - DIY
# Importing a script(that is stored in the same
folder, otherwise use full path)
import test
# print number from function
Answer = test.print_value(15)

11. Example Function 2
# Function will give an error if argument is not a
number
# Save this file as example.py
def change_value(number):
# Add documentation here
number += 8
return number

12. Import ‘example.py’ - DIY
# Importing a script (that is stored in the same
folder, otherwise use full path)
import example
# get new number from function
Answer = example.change_value(15)
print Answer

13. Exercise 1: Function to return shapefile
name
• Take in full file path string:
“C:Usersdsheehandesktopinterstatehigh
ways.shp”
• Return shapefile name:
“InterstateHighways.shp”
Should work for any full file path string, not one
case. You should have a second .py file to call the
function.
Examine python documentation http://doc.python.org;
in particular string.split(), len()

14. Warning: beware when copying from
slides
• Double quotes aren’t the same in PP as in IDLE
• Maintain indents so logic of the program
remains as intended
• And don’t open shapefiles with a cursor that
are open in Arcmap

15. Accessing the Attribute Table
def sumUpLength(shapefile):
# returns the length of all line segments in an attribute
table
import arcpy, os
sumLength = 0.0
try:
with arcpy.da.SearchCursor(shapefile, (“LENGTH”))
as cursor:
for row in cursor:
sumLength += row[0]
del cursor
except:
return -9999
return sumLength

16. Exercise 2
Call the function in the previous slide (called
accessAT.py) from a .py file. You should submit
a full path to the shapefile
(example -
“C:dsheehandesktopInterstateHighways.shp”)
as the argument to the function. Once this works,
add a where clause to the searchCursor function
to find only records with a length of 40 KM or
greater. Search help for Search Cursor and look for
AddFieldDelimiters.

17. Exercise 3
• Working with cursors
• Select by attributes
• Select by location

18. Exercise 3 - cursor
We are working with a shapefile, stepping
through the atttribute table of a FEATURE
CLASS with the cursor:
FC =
"C:UsersdsheehanDesktopkenyaDistrict2.shp
"
with arcpy.da.SearchCursor(FC, ("DISTRICT")) as cursor:
for row in cursor:
print row[0]
del cursor

19. Exercise 3 – selecting using attributes
Selecting by layer requires converting from a FEATURE CLASS to
LAYER but uses the same shapefile but opened a second time.
before loop
arcpy.MakeFeatureLayer_management("C:Usersdsheehan
DesktopkenyaDistricts.shp", "lyr")
Inside loop
arcpy.SelectLayerByAttribute_management("lyr",
"NEW_SELECTION", ' "DISTRICT" = '' + row[0] + ''')
Confused by mix of quotes, single and double?

20. Exercise 3 – select by location
Using the Layer, not the feature class, all inside
the loop:
arcpy.SelectLayerByLocation_management
("lyr", "BOUNDARY_TOUCHES", "lyr")
And checking the number selected:
theCount = arcpy.GetCount_management("lyr")
print theCount

21. Exercise 3 - result
import arcpy, os
try:
with
arcpy.da.SearchCursor("C:UsersdsheehanDesktopPython2015currentkenyaDistricts.
shp", ("DISTRICT")) as cursor:
arcpy.MakeFeatureLayer_management("C:UsersdsheehanDesktopPython2015current
kenyaDistricts2.shp", "lyr")
for row in cursor:
arcpy.SelectLayerByAttribute_management("lyr", "NEW_SELECTION", ' "DISTRICT" = '' +
row[0] + ''')
arcpy.SelectLayerByLocation_management ("lyr", "BOUNDARY_TOUCHES", "lyr")
theCount = arcpy.GetCount_management("lyr")
print row[0]
print theCount
arcpy.SelectLayerByLocation_management ("lyr", "BOUNDARY_TOUCHES", "lyr")
theCount = arcpy.GetCount_management("lyr")
print theCount
del cursor
except:
print arcpy.GetMessages()
raise

22. Try this at home:
• Add code to test whether any of the nearby
polygons contain a specified district
• If not, select additional neighbors and test
• Count how many iterations before you find
the specified district
• You should do this inside a while loop