Python Model Builder Python and Model Builder Overview
Python & Model. Builder
Python and Model. Builder Overview • Python/Model. Builder Concepts – – – The Geoprocessor Checking some environment variables Providing feedback from your model/script Starting a script with a model Bringing a completed script into Arc. Toolbox • Exercises Continuing Education
Python and Model. Builder Why model or write scripts? • • • Automate workflows Test hypotheses Reproducibility Extend Functionality Simplification Quality Control Continuing Education
Python and Model. Builder The Geoprocessing Object • • • Also known as the "geoprocessor" Has many methods Manages layers and selections Provides access to all of Arc. GIS's tools Provides status of Arc. GIS licensing Handles errors and warning messages Continuing Education
Python and Model. Builder The Geoprocessing Object gp. Clip_analysis("Roads_2007", "Counties", "Mercer_Roads. shp") Continuing Education
Python and Model. Builder 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 • Arc. Toolbox will check, but scripts run from the command line will not Continuing Education
Python and Model. Builder 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 Continuing Education
Python and Model. Builder Describe Method • gp. describe returns an object with numerous properties based on the type of data passed – – Feature Classes (shapefiles, geodatabase layers) Feature Datasets Workspaces Raster Data • gp. describe provides you with GIS-specific information about your data Continuing Education
Python and Model. Builder Using the Describe Method • Use Describe to ensure that data passed to your model fits certain criteria – – 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 Continuing Education
Python and Model. Builder Check License Availability • Models and scripts might be dependent on the level of Arc. GIS available • The gp. Check. Product("level") returns "Available" or "Not. Licensed" depending on the availability of the product level passed • The gp. Set. Product("level") tells the geoprocessor to use that specific level Continuing Education
Python and Model. Builder Handling Input • Arc. GIS 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 Continuing Education
Python and Model. Builder Providing Feedback • Geoprocessor methods to provide feedback – gp. Add. Message(message string) – gp. Add. Warning(message string) – gp. Add. Error(message string) • Adds messages to the message list • Message list is simply that – stores all the errors, warnings and messages generated by the script Continuing Education
Python and Model. Builder Providing Feedback • gp. Get. Message() and gp. Message. Count • Get. Message(x) returns message number x in the list • print gp. Get. Message(gp. Message. Count - 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 Continuing Education
Python and Model. Builder 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 Continuing Education
Python and Model. Builder 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. Continuing Education
Python and Model. Builder 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 Continuing Education
Python and Model. Builder 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() Continuing Education
Python and Model. Builder Start in Model. Builder Continuing Education
Python and Model. Builder Export to Python Continuing Education
Python and Model. Builder 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 • Split. Multi. Inputs will clean up a line of code and return it as a list Continuing Education
Python and Model. Builder Adding Split. Multi. Features • Add the function and split the input string and the clippers string passed by the Toolbox to our script Continuing Education
Python and Model. Builder 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 Continuing Education
Python and Model. Builder Completed Script Continuing Education
Python and Model. Builder 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 Continuing Education
Python and Model. Builder Add Your Script • Open Arc. Toolbox • Right click on the Toolbox you want your script to be located • Add > Script… Continuing Education
Python and Model. Builder 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 • Multi. Value = Yes Continuing Education
Python and Model. Builder County Clipper • We’ll take a small Model. Builder 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 Continuing Education
Python and Model. Builder 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 Continuing Education
Python and Model. Builder Basic Model to Python Script loop just a portion of the model Continuing Education
Python and Model. Builder Exported Python Script Continuing Education
Python and Model. Builder 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 Continuing Education
Python and Model. Builder Looping Our Model loop just a portion of the model Continuing Education
Python and Model. Builder Looping Our Model • For each county in the model – – – 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 Continuing Education
Python and Model. Builder 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'" Continuing Education
Python and Model. Builder 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: Continuing Education
Python and Model. Builder 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 Continuing Education
Python and Model. Builder Changing the Output Feature Continuing Education
Python and Model. Builder Finalizing Our Loop Input and Output County Layer Our For Loop Modified SQL Statement Create Output Path Geoprocessing Tasks Continuing Education
Python and Model. Builder USGS Quad Boundaries Clipped Continuing Education
Python and Model. Builder 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 • Multi. Value = Yes Continuing Education
Python and Model. Builder Clip To Counties Tool • If the parameters are set correctly, the dialog box will look similar to the one at right • Our County. Clipper now ready to handle multiple layers, clipping each by county Continuing Education
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