An Introduction to Python for GIS Specialists and Data Managers

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

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

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Sessions

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

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Session 1 Overview

• Scripts and the Debugger
• Commenting
• Data Types
• Operators
• Decisions and Loops
• File Handling
• Modules, Functions and Classes

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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/

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Scripts and the Debugger

• Exercise
• type at the prompt gtgt'hello world'
• type at the prompt gtgtprint 'hello world'
• Run a script with the following code
• print 'hello world'

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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

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Exercise

• Add both types of comments to your script

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Data Types

• Strings
• Numeric
• Lists
• Tuples

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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
• strNull100 repeat
• str2.upper() string method call
• str102 indexing
• str1 str1str2 strings can't be modified
• 
  but can be reassigned

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Strings

• Special Strings
• \n newline
• \t horizontal tab
• \\ backslash

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Strings

• Type the following
• gtgtgta'Hello'
• gtgtgtb 'World'
• gtgtgt ab
• gtgtgt (ab)10
• gtgtgt a01
• gtgtgt a-4-1
• gtgtgta.upper()
• gtgtgta.lower()
• Add two strings
• Try using the two other string methods

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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

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Numeric Operators
Operation Result
x y sum of x and y
x - y difference of x and y
x y product of x and y
x / y quotient of x and y
x // y (floored) quotient of x and y
x y remainder of x / y
-x x negated
x x unchanged
abs(x) absolute value or magnitude of x
int(x) x converted to integer
long(x) x converted to long integer
float(x) x converted to floating point
complex(re,im) a complex number with real part re, imaginary part im. im defaults to zero.
c.conjugate() conjugate of the complex number c
divmod(x, y) the pair (x // y, x y)
pow(x, y) x to the power y
x y x to the power y
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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.

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Numeric Types - Exercises

• gtgt 1 1
• gtgta 2 set value to 2
• gtgtb 3
• gtgtba
• gtgtb/a still an integer
• gtgtvalue "10" not an integer
• gtgtc 1.27 floating point
• gtgtcb
• gtgtb/c
• gtgtd int(c) rounding

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Lists

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

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List - Examples

• d empty list
• d 1,2,3 list of three integers
• d 1,2,"3" two integers and a string
• d0 first element
• d 1,2, 3,3,4, 5 3 by 3 array
• len(d) 2
• len(d0) 3

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Lists - Exercises

• gtgtL1 1,2,3,4,5,6
• gtgtlen(L1)
• gtgt L1 0
• gtgt L1.pop() remove last element from stack
• gtgt L1 6 is removed
• gtgt L1 02 index 0-2
• gtgt L1 -1 index from end of list
• gtgtL2 5,6,7
• gtgt L1.append(L2) insert a new list
• gtgt L1
• gtgt L1.extend(L2) extend the current list
• gtgt L1

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Operators

• Operators compute a value when presented with a
  string or number
• Common Operators include
• xy assign
• xy true or false
• x lt y True or false
• xlty True or false
• xgty True or false
• xy, x/y, xy, x-y Basic math
• x1 index
• x110 slice
• x or y y evaluated only if x is false
• x and y y is evaluated only if x is true

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Operators - Exercises

• gtgtgtx 5
• gtgtgty 6
• gtgtgt x lt y
• gtgtgt x gt y
• gtgtgt x lt y or y gt 7
• gtgtgt x lt y and y gt 7

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Controlling Flow

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

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Decisions/Conditionals

• if condition
• action(s)
• elif condition
• action(s)
• else condition
• action(s)

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if Condition Example

• x 15
• if x gt 5 and x lt 10
• print The value is between 5 and 10
• else
• print The value is not between 5 and 10

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if Condition - Exercise

• gtgtgtx 7
• gtgtgtif x gt 5 and x lt 10
• print The value is between 5 and 10

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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)

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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

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For Exercises

• gtgtgtfor x in range(1,20)
• if x2 0
• print x, is even
• gtgtgtimport glob
• gtgtgtfiles glob.glob(.)
• gtgtgtfor file in files
• print file

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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)

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While Examples

• x 100
• while xgt0
• x x/2
• print x

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While Exercise

• gtgtgtx 20
• gtgtgty 0
• gtgtgtwhile xgty
• print y
• y 2

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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.

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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

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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()

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File Exercises

• gtgtgtfilename C\\temp\\test.txt
• gtgtgtoutfile open (filename ,w)
• gtgtgtoutfile.write(hello world)
• gtgtgtoutfile.close()
• gtgtgtinfile open(filename ,r)
• gtgtgtdata infile.read()
• gtgtgtprint filename, -, data
• gtgtgtinfile.close()

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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.

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Standard Modules

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

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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

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Instantiating Classes

• import module
• instance module.class(arguments)
• instance.method()
• Examples
• Import gzip import gzip module
• Zipfile gzip.GzipFile(datafile.zip) instantia
  te GzipFile class
• files Zipfile.read() Read zipfile and assign
  it to the files string

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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__

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Third Party Modules

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

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Exercises

• gtgtgtimport os
• gtgtgtdir(os)
• gtgtgtos.rename.__doc__
• gtgtgtimport shutil
• gtgtgtshutil.__doc__
• gtgtgtdir(shutil)