Scripting Basics and Introduction to Python

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Scripting Basics and Introduction to Python

• Geo599
• Week 2, Oct. 5, 2009

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What is a script?

• An ordered series of statements that instruct the
  computer to complete a task
• Sound familiar? Definition of geoprocessing from
  last week Organized sequence of steps to
  accomplish a spatial analysis task

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Statements

• Statement A discrete instruction that tells the
  computer to do one specific thing
• Statements generally consist of one line in the
  script
• In Python, statements are distinguished by line
  breaks (no special characters)

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Types of Statements

• Assignment
• Print
• Comment
• Run a method or function
• Combination of assignment and method or function
• Start a loop
• Test a condition
• Other special purpose statements

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Variables

• Variables are named containers for data
• Provide a way for you to refer to data when you
  want the computer to do something with it
• Variables can contain many different kinds of
  data
• The specific data stored in a variable can change
  over the course of a script

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Data Types

• Once a variable is assigned a value, its data
  type is set
• The actions you are allowed to do with data
  depend on its type
• Basic data types
• Integers
• Doubles
• Strings
• Python data types
• Dictionaries
• Lists

• ArcGIS data types
• Feature classes
• Tables
• Cursors
• Row objects

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Pair and Compare

• Statements
• Variables
• Data Types

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Assignment

• Assignment statements assign a value to a
  variable
• The syntax for an assignment statement is
• ltVariableNamegt ltValuegt
• Examples
• FirstName Tracy ? FirstName is a string
  variable
• CreditHrs 12.0 ? CreditHrs is a double

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Assignment, cont.

• After assigning a value to a variable, you can
  use the variable name to refer to the data stored
  in it
• Variable names can include alphanumeric
  characters and underscores (no spaces!)
• (Python is dynamically typed so you do not need
  to declare variables before using them.)

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More complex assignments

• Can assign results of an operation
• Sum 23
• ? Sum is now an integer variable with value 5
• Operations can include other variables
• Diff Big - Small
• Assignment is also used to store results of
  functions and methods

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Print Statements

• Print statements print something to the
  interactive window
• The syntax for a print statement is
• print ltvaluegt
• Examples
• print Hello, my name is
• print FirstName
• Each print statement starts on a new line

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Simple Concatenation

• Concatenation is a string operation that allows
  you to put several strings together
• Pythons basic concatenation operator is
• Example
• FullName FirstName LastName

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A very short script

• FirstName Tracy
• LastName Kugler
• print Hello, my name is FirstName
  LastName
• What variables are used in this script?
• What data type are they?
• What types of statements are used?
• What does the script do?
• What kinds of modifications could be made?

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Comments

• Comments are special statements that are not
  executed by the computer
• Comments are used to explain what the script is
  doing
• The syntax for comments is
• ltcommentgt OR
• ltcommentgt
• (The second version is typically used for blocks
  of comments, such as at the beginning of a script)

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Use of Comments

• Comments should be used liberally to allow people
  looking at your script to understand what it does
• Each script should include a header comment block
  that includes
• A summary of what the script does
• The scripts inputs and outputs
• Who wrote the script and when
• Comments should also be used throughout the
  script to describe each functional set of
  statements

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IDLE

• IDLE is the interactive development environment
  that is packaged with Python in ArcGIS
• IDLE is used to write and to run scripts
• Geoprocessing scripts can be run in IDLE without
  ArcGIS running
• IDLE consists of two types of windows
• Python Shell (interactive window)
• Scripts

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Python Shell

• Python Shell opens when you start IDLE
• Serves as an interactive window

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Python Shell as an Interactive Window

• Statements entered at the gtgtgt prompt will be
  executed immediately
• Variable values assigned here persist until you
  close the window
• The results of print statements appear here
• Good way to quickly test statements

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Script Window

• Scripts (.py) are essentially text files
• IDLE color-codes
• Orange Python key words
• Red Comments
• Green Strings
• Purple Function names

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Script Window Menu Highlights

• File gt Recent files
• Also in interactive window
• Quick access to recently opened scripts
• Run gt Run module (F5)
• Run the script
• Format gt Comment/Uncomment
• Useful for turning off sections of script
• Format gt Indent/Dedent
• Define code blocks

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Activity

• Use the interactive window to play with some
  assignment and print statements
• Try different data types (e.g., strings,
  integers)
• Try creating variables that store combinations of
  other variables (e.g., concatenation or
  calculation)
• What happens when you try to concatenate a string
  and an integer?
• Try combining some of these statements in a
  simple script in the script window and running it

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Functions

• Functions are used to do things that are more
  complex than operations
• Have inputs and outputs
• Inputs are called arguments or parameters
• Outputs are returned

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Parameters

• Parameters are passed to a function
• The syntax for calling a function is
• ltFunctionNamegt(ltparametersgt)
• Ex Str(Int)
• ? Converts an integer into a string
• Functions may have multiple parameters, separated
  by commas
• With multiple parameters, ORDER MATTERS
• Parameters are expected to be a specific data type

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Function example

• Suppose there was a function to do what the Clip
  tool does (there actually isnt, quite)

• What are the parameters? What data type are
  they?
• What do you do with the output in a script?
• What would a call to this function look like in a
  script?

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Function Example

• ClippedLayer Clip(InputLayer, ClipLayer)