Geoprocessing IV:

1
Geoprocessing IV

• Creating ArcTools

2
Sharable scripts

• Scripts are often created to be shared with
  others.
• Shared scripts should have a user-friendly
  interface and be robust
• 3rd party users will not necessarily know how to
  work with Python or troubleshoot a problem.
• Importing a script into ArcToolbox provides a
  user-friendly interface and allows you to provide
  thorough documentation and instruction.
• Creating a robust script needs to be done before
  bringing the script into ArcToolbox.

3
Handling errors

• Nobody likes it when a program crashes without an
  explanation.
• Python has tools that allow you to catch
  errors
• deal with them and continue running the script
  if possible
• or exit the script and provide an explanation for
  the failure if possible.

4
Try / except

• The try / except statement is the main tool for
  handling errors in Python.
• The script attempts to run code in the try part.
  If an error occurs, the script immediately skips
  to the except part and runs the associated code.
• Can trap any type of error or trap specific types
  of errors.

5
Example try / except
Try adding a field
If unsuccessful, print message

• The above example is the simplest way to address
  the issue of adding a field when there is a
  possibility that it may already exist but what
  are the potential problems with this solution?
• An alternative is to use ListFields and then
  check to see if the field is present in the table
  and only attempt to add the field if it is not
  present.

6
Error messages and ArcTool scripts

• Scripts run from ArcToolbox can be difficult to
  troubleshoot
• Make sure the script works before importing it.
• ArcGIS will not know the reason for a script
  failure unless the script tells it.
• Send the error message to the ArcTool progress
  window

7
Getting messages from the geoprocessor

• ArcTools always generate a message when they are
  finished running
• Indicates the success or failure of the operation
• Indicates the reason for a failure if known.
• The script can access ArcTool messages with the
  geoprocessors GetMessages method.
• Retrieves the message generated by the last run
  ArcTool

8
Example catching error messages from the
geoprocessor
Try reclassification
If unsuccessful, print ArcTool message
9
Sending messages to the ArcMap progress window

• The print statement does not print to the script
  progress window in ArcMap.
• To send messages to the progress window, use the
  geoprocessors AddMessage, AddError, or
  AddWarning methods.

10
Progress window messages
11
User-specified parameters

• A 3rd party user will want to specify certain
  parameters when running a script tool
  user-specified parameters.
• These parameters are specified when the script is
  run.
• There are two ways to have the script request the
  parameters
• the python way
• the geoprocessor way

12
Requesting parameters before script execution -
the python way

• The argv tool in the sys module allows a script
  to request a parameter at run-time
• The sys module must be imported before hand.
• The parameter number is specified in brackets.
• This method will work when running scripts
  through the Python interface or through the
  ArcGIS interface.

13
Requesting parameters during script execution -
the python way

• Parameters can be requested during script
  execution using the raw_input statement
• In the interactive window (or Python Shell), user
  will be prompted with specified string
• User can the type in an appropriate value
• Note this will not work if script is run through
  ArcGIS

14
Script path name

• Another use for sys.argv is to get the file path
  name of the script
• Script can set workspaces using script folder.

15
Requesting parameters the geoprocessor way

• The geoprocessor has two tools for requesting
  run-time parameters
• Parameter number is specified in parenthesis.
• These methods will only work when running a
  script through ArcGIS.

16
Importing a script into a toolbox
17
Specify tool properties
18
Specify file path name of script
19
Parameter name
Data type
Parameter properties
20
Parameter name

• Specify parameter names in the same order that
  they are numbered in the script.
• Name will be displayed in the tool dialog window.

21
Parameter data type
22
Parameter properties type

• The type property specifies if parameter is
  required, optional, or derived.
• The derived option is used when a parameter value
  is derived within the script. Usually the
  parameter is an output that needs to be exposed
  for use in a model.

23
Parameter properties direction

• Specify if the parameter is an input or an output.

24
Parameter properties multi-value

• Specify if parameter can have multiple values.
  The values will be placed in a string with values
  separated by

25
Parameter properties default

• Specify a default value for the parameter.

26
Parameter properties environment

• Set parameter equal to value of an environment
  setting (e.g. set parameter value equal to the
  cell size specified in the environment)

27
Parameter properties domain
28
Parameter properties dependency

• Specify if the parameter is dependant on the
  value specified for another parameter. For
  example, a field parameter should have dependency
  on an input dataset parameter.

29
Help on setting parameters

• Use question cursor to get help on listing
  parameters or setting parameter properties.

30
The script tool

• After a script is imported into a toolbox
• it can be run from the dialog window like a
  model.
• It can be documented like a model
• Dialog window documentation
• Help page documentation
• Script tools can be used in models.

31
In-class practical

• Download the Demo script sharable scripts
  through the syllabus
• Convert this script to a sharable script
• Identify model parameters
• Set up variables for model parameters if needed
• Use sys.argv, GetParameter, GetParameterAsText,
  or raw_input to get values for parameters at
  runtime.
• Import script into ArcToolbox