Python Programming for ArcGIS: Part I

Python Programming for Arcgis 1 Daniel Sheehan dsheehan@mit.edu, gishelp@mit.edu 9:30AM-12:30PM January 14, 2015 This class was originally developed by David Quinn and taught by David and Daniel in IAP 2010 and 2011.

Arcgis 10.2.2 and Python • You need a normal install of Arcgis 10.2.2. This install includes Python 2.7 and includes the IDLE interface, which we will use. • Other installs of Pythons will not have the ‘arcpy’ module and will not work.

Download data for workshop • http://web.mit.edu/dsheehan/www/ dataForPythonIAP2015.zip • http://web.mit.edu/dsheehan/www/ PythonProgrammingforArcgis.pdf

Goals for the workshop • 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

Outline • Introduction to Python and Arcgis • Programming Principles and Modules • Model Builder • Reading and Writing data

Schedule Working with Python and geoprocessing tools today, working with attribute tables and individual geometries tomorrow

Python Python is a language that lets you work more quickly and integrate your systems more effectively 1 Documentation at http://docs.python.org and look for Python 2.7 (used in Arcgis 10.2.2) 1 http://www.python.org

Python + Arcgis • Python can interact with Arcgis and be used to repeat many types of analyses. • Why Python? • It is an integral part of Arcgis • Easy to read syntax • Large user community • Useful for scripts to control other programs

How does Python work with Arcgis • At Arcgis 10.2.2 – Fully integrated into Arcgis – Largely Geoprocessing functions • Automated mapping is not possible, yet

Logistics • We will be using the IDLE programming environment • Windows: START -> Programs -> Arcgis -> Python 2.7 -> IDLE • We are using Arcgis 10.2.2 on lab computers and assume that you are using 10.2.2 if you are using your own laptop

Learn by doing • Try every line of code in the slides • Create a new file for each different slide to create a record for yourself (optional) • Use the Python Help and Arcgis Help and ask questions

Programming concepts • Variables • Control Structures (IF statements and FOR loops) • Functions Python is case sensitive and reads whitespace for defining programming blocks – use space bar, not tabs.

The Print Function and Strings # this is a comment print “hello world” “”” Alternative Commenting Style “””

The Print function and Strings # this is a comment print (“hello world”) # this is a variable that contains a string name = “Daniel” print (name)

Integers and Floats # declare variables int_sample = 10 float_sample = 10.0 # printing variables # cast non-string variable as a string using str() print “The value of this integer is: “ + str(int_sample) print “The value of this float is: “ + str(float_sample)

if statement x = 2 # Condition checks if statement is true if x == 1: print ‘x is 1!’

if / elif / else statement x = 2 # Condition checks if statement is true if x == 1: print ‘x is 1!’ elif x == 2: print ‘x is 2!’ else: print ‘x is not known’

for loop for i in range(3): # convention is to use 4 spaces to indent # python reads whitespace at the beginning of a line print i Python, like most programming languages, uses arrrays that are zero based.

while loop # define j j = 1 # ‘while’ less than some condition while j < 3: print j # increment j j += 1

Three ways to access a folder # Accessing a folder path = “C:folderName” path = “C:/folderName/” path = r”C:folderName”

Importing Modules Use the import command: # count the number of files in a directory import os path = “c:...” files = os.listdir(path) print len(files) A module is a list of Python programs that can be accessed. Commonly used modules are os, sys, glob.

glob import glob # use the glob module path = “C:UsersdsheehandesktopPython2015current ” # loop through all files ListOfFiles = glob.glob(path + “*”) for i in ListOfFiles: print i Try replacing ‘*’ with ‘*.shp’

Importing the Arcgis module At 10.2.2 import arcpy

Exercise 1: Reading folder contents • Download zip file from http://web.mit.edu/dsheehan/ dataForPythonIAP2015.zip and unzip • Using the glob module, print out: – a list of all of the files – a list of shapefiles

Exercise 2: ModelBuilder Using ModelBuilder: • Buffer interstateHighways.shp (500 meters) – Units of data is meters • Clip schools.shp with buffer • Export model as ‘Python’

Catching exceptions try: <your code> except: print arcpy.GetMessages() raise

Overwriting files • from arcpy import env • env.overwriteOutput = True

Exercise 3: Convert ModelBuilder Code into a loop • Using the code from ModelBuilder • Iterate through a loop 5 times, buffering 500 meters, 1000, etc meters • Intersect cambridgeSchools.shp with buffer and make 5 new pairs of shapefiles

Writing to a text file # Create a file (‘w’ means create a new file, ‘a’ appends to an existing file, will create it if it doesn’t already exist) f = open(“C:usersdsheehantest.txt”, ‘w’) # Write to a file f.write(“Contents of file” + “n”) f.flush() # flushes buffer f.close() # closes file

Exercise 4: File Manipulation Create a folder called “temp_folder”: • Make 5 text files in this folder called File1.txt, File2.txt, etc. • Write a string in each file

Python Programming for ArcGIS: Part I

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Python Programming for ArcGIS: Part I