Python & ModelBuilder
Python and ModelBuilder
Overview
• Python/ModelBuilder Concepts
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The Geoprocessor
Checking some environment variables
Providing feedback from your model/script
Starting a script with a model
Bringing a completed script into ArcToolbox
• Exercises
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Why model or write scripts?
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Automate workflows
Test hypotheses
Reproducibility
Extend Functionality
Simplification
Quality Control
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The Geoprocessing Object
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Also known as the "geoprocessor"
Has many methods
Manages layers and selections
Provides access to all of ArcGIS's tools
Provides status of ArcGIS licensing
Handles errors and warning messages
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The Geoprocessing Object
gp.Clip_analysis("Roads_2007", "Counties", "Mercer_Roads.shp")
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Check Data Existence
• gp.exists() returns a boolean depending on whether or not
the feature class exists
• Check most user-submitted data for existence
– if gp.exists(data):
# continue working
else:
# warn user about missing data
• ArcToolbox will check, but scripts run from the command line
will not
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Defining a Workspace
• When the geoprocessor has a workspace defined, all
processing results will be stored in that folder, geodatabase,
or SDE gdb
• gp.workspace = "C:\\Student\\”
– Backslash is an escape character; ie a “tab” is “\t”, new line is “\n”
For just a simple backslash, you need to escape the escape character,
which is why it’s doubled.
• Once defined, we do not need to specify absolute paths to
datasets or outputs located in that workspace
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Describe Method
• gp.describe returns an object with numerous properties
based on the type of data passed
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Feature Classes (shapefiles, geodatabase layers)
Feature Datasets
Workspaces
Raster Data
• gp.describe provides you with GIS-specific information about
your data
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Using the Describe Method
• Use Describe to ensure that data passed to your model fits
certain criteria
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Prevent line or point features being used as Clip Features
Add an area and perimeter field to shapefiles if not present
Convert rasters to 8-bit unsigned if not already
Use Python to get a list of .shp files, then process only the polygon
shapefiles
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Check License Availability
• Models and scripts might be dependent on the level of ArcGIS
available
• The gp.CheckProduct("level") returns "Available" or
"NotLicensed" depending on the availability of the product
level passed
• The gp.SetProduct("level") tells the geoprocessor to use that
specific level
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Handling Input
• ArcGIS passes information to your Python script through
sys.argv
• For example, a clip script would need to listen for three inputs
– Input Feature / sys.argv[1]
– Clip Feature / sys.argv[2]
– Output Feature / sys.argv[3]
• Gets tricky when we ask for multiple inputs
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Providing Feedback
• Geoprocessor methods to provide feedback
– gp.AddMessage(message string)
– gp.AddWarning(message string)
– gp.AddError(message string)
• Adds messages to the message list
• Message list is simply that – stores all the errors, warnings
and messages generated by the script
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Providing Feedback
• gp.GetMessage() and gp.MessageCount
• GetMessage(x) returns message number x in the
list
• print gp.GetMessage(gp.MessageCount - 1)
– prints the last message generated
• Print messages at the end of script or when an
error is caught
• Print before and after a lengthy processing step
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Defining a Function
• A Python Function allows you to write a block of code that
accepts arguments and returns results
• Functions allow you to write something once and reuse it as
needed
• Helps debugging
• Keeps code clear and concise
• Easier to program
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Using Functions
• We use the def statement to start a block of code that will
only be run when our function is called
• We want our functions to return values we can use in our
geoprocessing
• The last statement in our function should be return.
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Basic Function
• A simple function that adds 1 to value passed
• def addone(x):
return x + 1
• print addone(2)
>>> 3
• var = addone(3)
print var
>>> 4
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Multiple Clips
• Like Exercise #2 from last week, we will provide two lists to a
script
– List of Input Features
– List of Clip Features
• We also need to tell the script where to place the output,
using gp.workspace()
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Start in ModelBuilder
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Export to Python
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Our First Function
• We need a custom function that will take a string of text, split
it into pieces and return those pieces as a list
• SplitMultiInputs will clean up a line of code and return it as a
list
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Adding SplitMultiFeatures
• Add the function and split the input string and the clippers
string passed by the Toolbox to our script
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Two Loops
• We will need another loop for each of the inputs
• The "inputs" loop contains the "clippers" loop
• The loops are structured like so:
for input in inputs: (runs m times)
for clip in clippers: (runs n times)
• The script will then perform mn clips
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Completed Script
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Back to the Toolbox
• Once our script is complete, we can test it in IDLE and on the
command line
• What we'd really want is to run our script like any other tool
in the Toolbox
• We can add the script back to the Toolbox
• We need to specify how to talk to the script
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Add Your Script
• Open ArcToolbox
• Right click on the
Toolbox you want your
script to be located
• Add > Script…
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Add Your Script
• Our model takes two
input parameters
– Input Feature Classes
– Clipping Feature Classes
– Output Workspace
• Note that parameters
can be set for multiple
values
• MultiValue = Yes
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County Clipper
• We’ll take a small ModelBuilder model that clips one input
using the statewide Counties layer
• Using Python, we’ll enhance the model to clip the input 21
times for each of the separate counties
• Add functionality to clip multiple inputs by each county
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Setting Up Our Model
• We need a County layer as our clip feature, and a userdefined layer as our input feature
• Make Feature Layer
• Select By Attributes
• Clip (of course)
• User-defined Output Workspace
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Basic Model to Python Script
loop just a portion of the model
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Exported Python Script
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Adding County Tuple
• We need a static list of County names to pass as part of the
Select by Attributes “where” clause
• Put all the County names into a tuple
• Use a for loop to loop over the Select by Attributes and Clip
steps with different Counties each time
– for county in counties
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Looping Our Model
loop just a portion of the model
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Looping Our Model
• For each county in the model
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prepare a SQL statement to select County
run the selection to create our clipping features
use selected features to clip
create 21 output feature classes
each output layer named input_county
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SQL Statement
• SQL statements use single and double quotes to delimit field
and value names
• Python also uses quotes to delimit strings
• Keep track of which quotes are being used
• Escape quotes using backslash (\') or (\")
• SQL: "COUNTY" = 'MERCER'
Python: "\"COUNTY\" = 'MERCER'"
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Looping Over SQL Expressions
• Each time the loop is run, we want to change the SQL
statement to select the current county in the loop
• We can concatenate the parts of the SQL statement with the
county name
• Example:
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Changing the Output Feature
• Each output feature name needs to be unique
• Append the current County name
• Wetlands.shp becomes
– Wetlands_Atlantic.shp
– Wetlands_Bergen.shp
– …
• Some string methods are needed
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Changing the Output Feature
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Finalizing Our Loop
Input and Output
County Layer
Our For Loop
Modified SQL Statement
Create Output Path
Geoprocessing Tasks
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USGS Quad Boundaries Clipped
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Remember Multiple Inputs
• How will we modify the
script to accept multiple
inputs?
• We'll need another for
loop in the script
• Set the Input parameter
to accept multiple
values
• MultiValue = Yes
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Clip To Counties Tool
• If the parameters are
set correctly, the dialog
box will look similar to
the one at right
• Our CountyClipper now
ready to handle
multiple layers, clipping
each by county
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