An Introduction to Python for GIS Specialists and Data Managers

1. An Introduction to Pythonfor GIS Specialists and Data Managers Jacob Morgan Brent FrakesNational Park ServiceFort Collins, COApril, 2008

2. Session Goals • Overview of Python • See power of the language • Understand basic syntax • References to other sources • Too little time to really cover everything

3. Sessions • One: Basic Python • Two: Using the ESRI Geoprocessor • Three: Formatting Data and Python Scripting • Each one hour • Examples and practice

4. Session 1: Overview • Scripts and the Debugger • Commenting • Data Types • Operators • Decisions and Loops • File Handling • Modules, Functions and Classes

5. Scripts and the Debugger • General Setup and Notes • All code is text-based (*.py extension) • Once compiled, a new file will have a *.pyc extension. • Python is VERY picky about indentation • Python is case sensitive • IDLE • Free • GUI interface for development • Compiler included with ArcGIS • Provides debugging but doesn't allow for breakpoints • Pythonwin • Free • GUI interface for development • Good debugger with breakpoints • http://sourceforge.net/projects/pywin32/

6. Scripts and the Debugger • Exercise • type at the prompt >>'hello world' • type at the prompt >>print 'hello world' • Run a script with the following code: • print 'hello world'

7. Commenting • Why? • Make your code understandable to you and others • How to use code • Why code was written • Debugging (comment out parts of script) • Ways to Comment • Single Line: use the pound (#) sign before the comment text. # single line commented text • Multiple Lines: use triple quotes before and after a text block. “ “ “ These two lines of text Are being commented out “ “ “

8. Exercise • Add both types of comments to your script

9. Data Types • Strings • Numeric • Lists • Tuples

10. Strings • An ordered collection of characters used to represent text-based information • Common String Literals and Operations • strNull = ' ' #empty string • str1 = "metadata" #double-quotes • str2 = '.xml' #single quotes • str3 = str1 + str2 #concatenate • strNull*100 #repeat • str2.upper() #string method call • str1[0:2] #indexing • str1 = str1+str2 #strings can't be modified but can be reassigned

11. Strings • Special Strings • \n #newline • \t #horizontal tab • \\ #backslash

12. Strings • Type the following >>>a='Hello' >>>b = 'World' >>> a+b >>> (a+b)*10 >>> a[0:1] >>> a[-4:-1] >>>a.upper() >>>a.lower() • Add two strings • Try using the two other string methods

13. Numeric Types • Four types: • Integers • Numeric type providing a container for whole numbers. Plain integers are precise up to 32 bits (dependant on platform and compiler). • Floating Point Numbers • Numbers those with a decimal component. These are called floating point since the decimal point can be positioned anywhere in the digit string by a signed integer exponent- • Long Integers • Complex Numbers

14. Numeric Operators

15. Numeric Considerations • Integer calculations yield integers (including division!) • Floating point calculations yield floating points • The constructors int(), long(), float(), and complex() can be used to produce numbers of a specific type. However, converting numbers between types may result in rounding or truncating digits.

16. Numeric Types - Exercises >> 1 # 1 >>a= 2 #set value to 2 >>b= 3 >>b+a >>b/a #still an integer >>value = "10" #not an integer >>c = 1.27 #floating point >>c+b >>b/c >>d = int(c) #rounding

17. Lists • An ordered collections of arbitrary objects • Fully indexed • Variable in length, heterogenous, and arbitrarily nestable • Can be altered • Always in brackets • Aka Arrays!

18. List - Examples d = [] # empty list d = [1,2,3] #list of three integers d = [1,2,"3"] #two integers and a string d[0] #first element d = [[1,2, 3],[3,4, 5]] #3 by 3 array len(d) # 2 len(d[0]) #3

19. Lists - Exercises >>L1 = [1,2,3,4,5,6] >>len(L1) >> L1 [0] >> L1.pop() #remove last element from stack >> L1 #6 is removed >> L1 [0:2] #index 0-2 >> L1 [-1] #index from end of list >>L2 = [5,6,7] >> L1.append(L2) #insert a new list >> L1 >> L1.extend(L2) #extend the current list >> L1

20. Operators • Operators compute a value when presented with a string or number • Common Operators include: • x=y #assign • x==y #true or false • x < y #True or false • x<=y #True or false • x>y #True or false • x*y, x/y, x+y, x-y #Basic math • x[1] #index • x[1:10] #slice • x or y #y evaluated only if x is false • x and y #y is evaluated only if x is true

21. Operators - Exercises • >>>x = 5 • >>>y = 6 • >>> x < y • >>> x > y • >>> x < y or y > 7 • >>> x < y and y > 7

22. Controlling Flow • Decisions/Conditionals are statements that determine if a series of conditions is true. • Most common • if • for • while

23. Decisions/Conditionals if condition: action(s) elif condition: action(s) else condition: action(s)

24. “if” Condition Example x = 15 if x > 5 and x < 10 : print “The value is between 5 and 10” else: print “The value is not between 5 and 10”

25. “if” Condition - Exercise >>>x = 7 >>>if x > 5 and x < 10 : … print “The value is between 5 and 10”

26. “for” loop • Supports repeated execution of a statement, or block of statements, controlled by an iterable express for target in iterable: statement(s) else: #optional on normal termination statement(s)

27. “for” Examples for x in range (1,30,2): print x for letter in “hello world”: print letter infile = open (“dataset.txt”,”r”) for line in infile: print line

28. For Exercises >>>for x in range(1,20): … if x%2 == 0: … print x, “ is even” >>>import glob >>>files = glob.glob(“*.*”) >>>for file in files: … print file

29. “while” loop • allows looping to occur only while a specified condition is true. WHILE evaluates the condition each time the loop completes and terminates the loop when the condition evaluates to false while expression: statement(s) else: #optional for normal termination statement(s)

30. While Examples x = 100 while x>0: x = x/2 print x

31. While Exercise >>>x = 20 >>>y = 0 >>>while x>=y: … print y … y += 2

32. File Handling • Ways to open, read, write, and close files File2read = open (filename, mode = 'r') filename - string containing filename and extension (e.g., 'metadata.xml') file modes - indicates how to read or write the file 'r' - file must already exist and is read only 'w' - creates a new file for writing. Overwrites any existing filename 'a' - Write only mode. If file already exists, information is appended. 'r+' - File must already exist and is opened for both reading and writing. 'w+' - New file is created. Open for reading and writing. 'a+' - File open for reading and writing. If file already exists, data is appended.

33. Useful File Methods • file.read() - reads up to the end of file and returns it as a string. • file.readline() - reads one line from file (up to the '\n') and returns a string • file.seek(pos, how = 0) - Move to a position in the file. 'How' is the reference point for moving (0 = start of file, 1 = current position; 2 = end of file). • file.write() - Writes a string to a file • file.writelines(line) - writes a line to a file • file.close() - # must be closed for it to be unlocked by Windows

34. File Examples • infile = open(“d.txt”, 'r') • data = infile.read() #the string data is assigned the file contents • outfile = open(“e.txt”,'w') # assign outfile to the file object for writing • outfile.write(data) # write the contents to the file • infile.close() • outfile.close()

35. File Exercises >>>filename = “C:\\temp\\test.txt” >>>outfile = open (filename ,’w’) >>>outfile.write(“hello world”) >>>outfile.close() >>>infile = open(filename ,’r’) >>>data = infile.read() >>>print filename, “-”, data >>>infile.close()

36. Modules, Functions, and Classes • Module • Single Python file • Provides definitions of functions, variables or classes • Corresponds to a specific thema (e.g., read, edit, and write zipfiles) • Composed of functions and/or classes. Functions can be stand-alone or are part of a class. Whether a function or class, both accomplish tasks.

37. Standard Modules • Supported by Python - integral to many applications • Index can be found at: http://docs.python.org/lib/modindex.html • Importing is simple: • >>import module • (e.g., import os)

38. Accessing Functions import module module.function(arguments…) Examples import os os.getcwd() #get current directory import glob glob.glob(“*.py”) #get a list of all files with the .py extension

39. Instantiating Classes import module instance = module.class(arguments) instance.method() Examples Import gzip #import gzip module Zipfile = gzip.GzipFile(“datafile.zip”) #instantiate GzipFile class files = Zipfile.read() #Read zipfile and assign it to the files string

40. Getting Documentation About Modules dir() – List of available functions and methods module.__doc__ - module documentation module.function.__doc__ - function/method documentation Example import os dir(os) os.rename.__doc__

41. Third Party Modules • Many others have developed modules and posted them to the Python Package Index (http://pypi.python.org/pypi/)

42. Exercises >>>import os >>>dir(os) >>>os.rename.__doc__ >>>import shutil >>>shutil.__doc__ >>>dir(shutil)