Python Scripting in ArcGIS10: 25 Potential Sources of Confusion

http://nrm.salrm.uaf.edu/~dverbyla

Python Scripting in ArcGIS10: 25 Potential Sources of Confusion

Dave Verbyla Professor of GIS/Remote Sensing

[email protected]

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Presentation Notes

Title slide (2001 AK Surveying and Mapping Conference Feb 24, 2011, Anchorage, AK)


Why learn ArcGIS Python

• Python scripting language installs with

ArcGIS

• Python window in ArcGIS

• Create your own script tools

• Arcpy.mapping (new with ArcGIS10)

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At version10, python is integrated into ArcGIS with a python command window, new arcpy.mapping module for working with mxd objects, etc.


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Here is an example where we want to rename all txt files in the folder to a simpler name by replacing the month integer with a 3-character month, and stripping away the string “ak.cr.temp.”.


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There is typically more than one way to solve a problem with arcgis Python scripting. Both these scripts solve the problem, the script in the blue box is more efficient, but the script in the red box may be more understandable? to a beginnner.


Source of Confusion• Python interpreter is case sensitive

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Remember all Python commands are lower case. A common convention is to name variables in mixed case such as myTheme, aList, intVariable


• Python command not understood without appropriate module loaded

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Typically you load the modules you need at the start of your script.


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Better to import module rather than from module import *


Python Keywords

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Python has reserved keywords that you can not use as variable names


“\” character\ is a special (\t means tab, \n means new line, etc.)

(use / or \\ for paths instead of \)

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the \ causes problems when dealing with paths and the \n newline character when reading/writing text files.


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The backslash \ character is a special character to the Python interpreter. There are 3 different ways you can specify paths: r’\....’,’/….’, or ‘\\....’


Newline = “\n”• Newline character at the end of every line in a text file

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The ‘\n’ is a carriage return or new line character that is invisible in the data file when you look at it in notepad.


Python Lists

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Remember that lists always start with element#0, and not element#11


Looping using range()

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The range function syntax is range(begin, up_to) So if you want to see all 3 items, use range(0,3) in the examples here.


: then indents• Indentation interpreted as loop or decision structure

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PythonWin or other editors such as Wing helps because it automatically maintains correct indentation.


Recycling Variables• Some Python methods returns result None

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Instead of fList = fList.sort(), it is always safer to create a new object like srtFlist = fList.sort()


Arcpy Site Package

• Python scripting language

• Arcpy geoprocessing

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The arcpy site package allows you access to all the geoprocessing tools and environments either in a stand-alone script or in the arcgis Python window


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Here the fields are represented by a string, a Python list, and a an arcpy value table.


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The geoprocessing tools are case-sensitive, but tool parameters are not case sensitive.


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One source of confusion is tha the Python current working directory is no the same as arcpy’s workspace!


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Also if your running a stand-alone geoprocessing script, it will not see the environments that are set by default in Arcmap.


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The GetCount_management tool returns the number of records as an Result object. Use the Python str(nRecs) to convert the Result to a string. If you need a numeric value then use int(str(nRecs))


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When working with Cursors, always remember to delete your row and Cursor objects to unlock the files so other applications can access them.


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The field calculator VB Script computes the field Root_Length as the square root of the Shape_Length of each polyline. The same operation fails when we use Python scripting instead!


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The error occurred because a !field_name! was referenced in the pre-logic script code….always pass field names to the Python function in the pre-logic script code.


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The table field calculator using Python really passes the calculations to the CalculateField_management tool.


Script Tools• Built-in Dialogs• Filtering to prevent

errors • Output to Arcmap

Data Frame• Portable (email the

.tbx file and script)• Toolbox, toolbar, or

context menus

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A script tool is a Python script that is run from an ArcGIS application such as ArcMap or ArcCatalog using the built-in dialog that most users are familiar with. With a script tool, you can filter so the user selects for example on Polygon feature classes, output is automatic to your Arcmap data frame. You can email your personal toolbox that contains the script tool(s) and your Python scripts.


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You can easily test-drive a script tool to figure out what the various script tool parameters represent.


#Python script to message to userimport arcpyTheme = arcpy.GetParameterAsText(0) #user selects a feature class #output using arcpy AddMessage function:Message = 'User-selected input featue class: ' + str(Theme) arcpy.AddMessage(Message)arcpy.AddMessage('arcpy AddMessage function.....Good Bye')#output using Python print command:print 'Python print command...All Done'

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Notice that the Python print statement does not appear in the output, but the arcpy.AddMessage() does appear in the ouput.


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Here is a simple script that runs the CopyFeatures tool and displays the resulting output to Arcmap.


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The same geoprocessing operation does NOT display the results to Arcmap when using a script tool.


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One work around is to have the user specify the output feature class as a script tool parameter.


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You will not see a list of fields in your script tool unless you specify the parameter to obtain the fields from…


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Triple-quote Python functions when using the CalculateField_managment tool


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Arcpy.mapping…new with ArcGIS10. This is more object-oriented compared to other arcpy methods. Here we first make a map doucment object, then get a list of grames in the arcmap object, get the fourth data frame, make this the active frame and then use the active frame to create the map document thumbnail….


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Another example, turning the parcels layer off. Many arcpy.mapping methods return lists, so using method( )[0] returns the first item from the list.


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Here the Schools layer is no longer in the city_parcels dataset, so the arcmap layer has a broken data source. We use the arcpy.mapping replaceDataSource method to specify the new data source…no problem


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We try the same thing with our new data source Schools.shp in c:\temp. The required parameters are workspace path (r”C:\temp”, the keyword representing type of workspace “SHAPEFILE_WORKSPACE”, and the datasetname of the new souce (“Schools.shp”). This yields a value error. The correct syntax is to specify dataset_name as “Schools” since the method already knows it is a shapefile from the “SHAPEFILE_WORKSPACE” parameter.


Beginner Websites(hyperlinks)

• Non-Programmer’s Tutorial for Python 2.6• Live Wires Python Course• Instant Hacking• Dive Into Python• Python Programming for Beginners• Python 101• Learn Python in 10 Minutes• Learn Python • 5 Minutes With Python (video)• Online Python resources (video)

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There are many websites aimed at beginner Python scripting.

http://en.wikibooks.org/wiki/Non-Programmer%27s_Tutorial_for_Python_2.6

http://www.livewires.org.uk/python/home

http://web.archive.org/web/20080707093731rn_1/hetland.org/writing/instant-hacking.html

http://diveintopython.org/

http://www.linuxjournal.com/article/3946

http://www.rexx.com/~dkuhlman/python_101/python_101.html

http://www.korokithakis.net/tutorials/python?

http://learnpythonthehardway.org/static/LearnPythonTheHardWay.pdf

http://www.python.org/doc/av/5minutes/

http://showmedo.com/videotutorials/video?name=pythonOzsvaldIntroToPyResources1&fromSeriesID=27


ESRI Resources

Using Python in ArcGIS10Free Web Course http://training.esri.com• Creating basic Python scripts with correct syntax• Arcmap Python window• Python scripting in field calculator• Creating basic script tools in ArcGIS10

Getting Started With Python in ArcGIS10Video http://resources.arcgis.com/gallery/video/geoprocessing/• Arcpy site package• Python window• Script tools• Arcpy mapping automation• Tool design and validation

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The free virtual campus course is a good place to start with arcpy.


ESRI Resource Center http://resources.arcgis.com/gallery/file/geoprocessing/

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The UC 2010 Technical Workshop is a good source at the Resource Center.


ESRI Resource Center

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You can download 20 excellent arpy.mapping script tools!


ESRI Resource Centerhttp://resources.arcgis.com/gallery/video/geoprocessing

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A good video presentation on arcpy.mapping

Python Scripting in ArcGIS10: 25 Potential Sources of Confusion

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