Geography 465

1
Geography 465

• Analytic Cartography
• Getting Started with Python Scripting

2
What is Geoprocessing?

• The application of GIS operations on a set of
  inputs for generating new information (or new
  data as appropriate to situation)
• All geoprocessing tools are accessed from
  ArcToolbox
• A tool can be a dialog, a script, or a model.

3
Why write scripts?

• Scripts supply the added benefit of decision
  making logic and looping functionality
• Automate a work flow, for example
• Copy all incoming data into a geodatabase
• Perform a project, clip, buffer operation on
  multiple data sets (iterate)
• Easily distribute code
• A script is a self-contained, single file

4
Why use Python for Scripting?

• An open-source, object-oriented, scripting
  language
• Offers IDE (integrated development environment)
  with debugging tools
• Modular, can be broken apart
• Ability to compile scripts
• Installed with ArcGIS 9 and ESRI samples provided

5
Writing code in Python

• Where to write Python code?
• Python command line
• IDE (integrated development environment)
  PythonWin
• IDE allows you to perform all jobs from one
  location
• Write,
• Save,
• Run, and
• Debug code

6
PythonWin Interface

• Script window
• Write and save code
• Interactive window
• Test lines of code
• Report messages
• Menus and Toolbars
• standard and debugging

7
Basics of Python

• Comment A non-executable line of code
• One number sign () for green and italicized
• Two number signs () for gray
• Name Tim Nyerges
• Date January 3, 2007
• Purpose To buffer a feature class
• import win32com.client
• gp win32com.client.Dispatch(esriGeoprocessing.G
  pDispatch.1)
• gp.Workspace C\\Python_Data\\SanDiego.mdb
• Gp.Buffer_analysis (Freeways, BuffFreeway,
  1000)
• Can comment and uncomment blocks of code

8
Variables in Python

• Variables are dynamically typed
• No declaration required
• No type assignment required
• fc C\\ProjectData\\SanDiego.mdb\\Freeways.shp
  
• Variables are case sensitive
• fc Freeways.shp
• Fc 20000
• Variables can hold different data types
• strings, numbers, lists, files

Two different variables
9
Strings

• Variables can hold strings
• folder c/Student
• Strings are surrounded in double () or single
  () quotes
• Pathnames use two back (\\) or one forward (/)
  slash
• One backslash (\) is a reserved escape character
  and a line continuation character

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Strings

• Strings can be combined together
• gdbPath c\\SanDiego.mdb
• fc Roads
• fullPath gdbPath \\ fc
• Strings are indexed
• strings are zero-based from the left and
  one-based from the right
• fc Streets.shp
• fc0 ---gt S S is in the 0 position
• fc13 ---gt tr start at 1st, up to not
  including 3rd
• fc-4 ---gt Streets get rid of the last 4
  charaters

C\SanDiego.mdb\Roads
11
Numbers and lists

• Variables can hold numbers and expressions
• num1 1.2
• num2 3 5
• Variables can hold lists
• numList 1, 2, 3
• fcList Roads, Streets, Parcels,
  Zipcodes
• Lists are indexed
• fc1 fcList1
• fc2 fcList02 ---gt Roads, Streets
  
• fc3 fcList0-1 ---gt Roads, Streets,
  Parcels
• fc4 fcList2 ---gt Parcels,
  Zipcodes

---gt Streets
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Variable naming conventions

• Upper case versus lower case
• First word lower case, capitalize each successive
  word
• tableFieldName Street
• - Acronym at the beginning, use lower case
  letters
• gdbPath C\\SanDiego.mdb
• Acronym in the middle or at the end, use upper
  case letters
• inputFC Streets.shp
• Avoid special characters (for example / \
  !)
• Use descriptive variable names

13
Line continuation

• Line continuation characters
• Parentheses ( ), brackets , and braces
• Backslash \
• Indentation is automatic
• fcList Roads, Climate, Streams,
• Zipcodes, Coastlines
• distanceValues 100, 200, 300, 400, 500, \
• 1000, 1500
• gp.Buffer_analysis(fcList2, BuffStreams1000,
  distanceValues5)

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Built-in functions

• Python has many built-in functions
• Built-in means that Python automatically makes
  these functions available they dont have to be
  imported like most other functions
• You can see the list of built-in functions by
  typing dir(__builtins__) into the interactive
  window of PythonWin

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Examples of Built-in functions

• len ( ) returns the length
• fc Streams.shp
• len (fc) ---gt 11
• max ( ) returns the maximum value
• xExtent (5210474.99, 7438807.99)
• max (xExtent) ---gt 7438807.99
• open ( ) opens a file
• coord open(C\\Mydata.txt, r).read( )
• ---gt holds the contents of the Mydata.txt file
• round ( ) Rounds a number
• xCoord 5210474.99
• round(xCoord) ---gt 5210475.0

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Accessing modules

• Most functions are imported from modules
• The math module
• import math
• math.sqrt(64) ---gt 8.0
• math.pi ---gt 3.14156
• The string module
• Import string
• string.split(75.7 -45.3) ---gt 75.7, -45.3
• string.upper(c\\student) ---gt C\\STUDENT

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Accessing modules

• The os.path module
• import os.path
• os.path.basename(C\\SanDiego.mdb\\Streets,shp
• ---gt Streets.shp
• os.path.dirname((C\\SanDiego\\Streets.shp
• ---gt C\\SanDiego

18
Defining your own functions

• Function is a named sequence of statements that
  performs a desired operation.
• This operation is specified in a function
  definition.
• def NAME( LIST OF PARAMETERS )
• STATEMENTS

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Defining your own functions - example

• def newLine()
• print

print "First Line." newLine() print "Second
Line."
def threeLines() newLine() newLine() newLine()
print "First Line." threeLines() print "Second
Line."
20
Defining your own functions with parameters-
example

• def printTwice(n)
• print n, n

printTwice(Geog465 project!) ---gt ?
n Geog465 project! printTwice(n)
21
Boolean Expressions

• A boolean expression is an expression that is
  either true or false.
• In Python an expression that is true has the
  value 1, and an expression that is false has the
  value 0.
• In this case the equal comparison (not
  assignment) operator is as follows.

gtgtgt 5 5 1 gtgtgt 5 6 0
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Boolean Expressions

• The operator is one of the comparison
  operators
• the others are
• x ! y x is not equal to y
• x gt y x is greater than y
• x lt y x is less than y
• x gt y x is greater than or equal to y
• x lt y x is less than or equal to y

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Logical Operators

• There are three logical operators and, or, and
  not.
• The semantics (meaning) of these operators is
  similar to their meaning in English.
• For example,
• x gt 0 and x lt 10
• is true only if x is greater than 0 and less
  than 10.

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Decision Statement Syntax

• Conditional statement ifelifelse
• if x 1
• print x is 1
• elif x 2
• print x is 2
• else
• print x is not 1 or 2
• Colons are used at the end of each condition
• Indentation defines what executes for each
  condition

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Looping syntax

• While loop
• x 1
• while x lt 10
• print x
• x x 1
• Colons used at end of each statement
• Indentation defines what executes for the loop

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Looping syntax

• Counted loop
• for x in range(1, 5)
• print x
• Counted loops increment and test a variable on
  each iteration of the loop
• The last value is not executed

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Looping syntax

• List loop
• x 1, 2, 3
• For a in x
• print a
• List loops iterate over each value in a list
• The loop will execute once for each value in the
  list

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Case sensitive rules

• Case sensitive
• Functions and statements
• Correct max len open print import if
• Incorrect Max LEN OpEn imporT IF
• Variable names
• Not case sensitive
• Path names
• C\\DATA c\\Data
• Geoprocessing properties and methds
• gp.BUFFER gp.buffer

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Exposing Geoprocessing Functions

• An ArcObjects component, the geoprocessor object,
  is the main vehicle for exposing the
  geoprocessing functions to scripting in Python
• It is an object that provides a single access
  point and environment for the execution of any
  geoprocessing tool in ArcGIS, including
  extensions.

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Exposing Geoprocessing Functions

• GpDispatch is a COM wrapper for the geoprocessor
  making it possible for Python to access more than
  450 available tools
• Example
• import win32com.client
• gp win32com.client.Dispatch("esriGeoprocessing.G
  pDispatch.1")
• use Clip_Analysis tool
• gp.Clip_analysis ("C\\World\\Cities.shp",
  "C\\World.mdb\\Yemen\\Yemen",
  "C\\World.mdb\\Yemen\\ClipCitiesScript")

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Using COM Objects with Python

• To use COM objects
• import win32com.client
• o win32com.client.Dispatch(Object.Name")
• Example
• import win32com.client
• o win32com.client.Dispatch("Excel.Application")
• o.Visible 1
• o.Workbooks.Add()
• o.Cells(1,1).Value "Hello"

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Interacting with Geoprocessor

• Geoprocessor, like any ArcObject, has properties
  and methods
• Property characteristic of an object (an
  adjective)
• Method something the object knows how to do (a
  verb)
• Interact with Geoprocessor through properties and
  methods
• to buffer use Buffer method
• to clip use Clip methods
• to delete feature class use Delete method

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Accessing the Geoprocessor from Python

• Tell Python you want to use COM objects
• import win32com.client
• Create the instance of Geoprocessor
• gp win32com.client.Dispatch(esriGeoprocessing.
  GpDispatch.1)

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Syntax for Properties and Methods

• To assign a value to property
• Object.Property value
• gp.Workspace C\\Data
• To get a value of a property
• Object.Property
• print The name of the workspace is
  gp.Workspace
• To use a method
• Object.Method(argument, argument, )
• gp.Buffer_analysis (Freeways,
  FreewaysBuffer, 100)
• - parentheses around arguments
• - arguments separated by commas

35
Running Scripts in Python

• Use of ArcGIS help system to find
• Usage, command syntax, and scripting examples of
  standard ArcToolbox tools,
• Usage and syntax of Geoprocessor properties and
  methods, which are only accessible through
  scripting

36
Running Python Scripts Three Modes

• 1) Running scripts in PythonWin
• 2) Running scripts as script tools in ArcGIS
• 3) Running scripts as embedded model components
  in ArcGIS

37
Running Scripts in Python

• Problem with the schema lock
• Cannot manipulate with the same file open in
  ArcGIS and in PythonWin at the same time

38
Running scripts in PythonWin

• Example of a simple script to buffer a feature
  class
• Name Tim Nyerges
• Date January 3, 2007
• Purpose To buffer a feature class
• import win32com.client
• gp win32com.client.Dispatch("esriGeoprocessing.G
  pDispatch.1
• gp.Workspace "C\\Python_Data\\SanDiego.mdb
• gp.Buffer_analysis ("Freeways",
  "BufferedFreeways", 1000)

39
Debugging the Code

• Test code in PythonWin
• A check for syntax errors
• Do not test a script from ArcToolbox
• Test code in PythonWin, then add it to ArcToolbox

40
Running scripts in PythonWin

• Run the script from the script window of
  PythonWin
• Check the message in the lower left corner of the
  interactive window in PythonWin
• While the script is running you will see the
  message
• running script lt.gt.py
• The indication of the successful run is the
  message
• Script lt.gt returned the exit code 0
• Display the result in ArcCatalog

41
Running scripts in PythonWin

• Example of a simple script to clip a feature
  class
• Name Tim Nyerges
• Date January 3, 2007
• Purpose To clip a feature class
• import win32com.client
• gp win32com.client.Dispatch("esriGeoprocessing.Gp
  Dispatch.1")
• gp.Workspace "C\\Python_Data\\SanDiego.mdb"
• gp.Clip_analysis ("MajorAttractions",
  "SDdowntown", "SDdowntownAttractions")

42
Running scripts in PythonWin

• Example of a simple script to buffer and clip
  a feature class
• Name Tim Nyerges
• Date January 3, 2007
• Purpose To buffer SD freeways first and then
  to clip the downtown section
• of freeways
• import win32com.client
• gp win32com.client.Dispatch("esriGeoprocessing.G
  pDispatch.1")
• gp.Workspace "C\\Python_Data\\SanDiego.mdb"
• if gp.Exists(gp.Workspace "\\bufferedFreeways")
  
• gp.Delete_management(gp.Workspace
  "\\bufferedFreeways")
• gp.Buffer_analysis ("Freeways",
  "bufferedFreeways", "500")
• gp.Clip_analysis ("bufferedFreeways",
  "SDdowntown", "downtownFreeways")

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Gene will get you started with hands-on Python in
lab session