Programming Concepts

1
Programming Concepts

• GEO 599
• Week 4, Oct. 19, 2009

2
Objects

• Python is an object-oriented language
• Technically speaking, an object has
• Identity (location in memory)
• Type
• Value
• In Python everything is an object
• Strings, integers floats
• Lists, dictionaries, geographic layers
• Functions

3
Attributes

• Some objects have attributes
• Attributes are properties or pieces of the
  object
• Ex Point objects have attributes for
• ID
• X coordinate
• Y coordinate
• Many attributes can be assigned values
  individually
• Attribute values can be assigned to variables

4
Methods

• Many objects have methods
• Methods do things to objects
• Have already seen some methods of lists
• Append adds a single element to the end of a list
  object
• Extend adds a list of new elements to the end of
  a list object
• Methods typically have parameters
• Results of methods can be assigned to variables

5
Dot Notation

• Attributes and methods are both accessed using
  dot notation syntax
• ltobjectgt.ltattributegt
• ltobjectgt.ltmethodgt(ltparametersgt)

6
Examples

• Assign a value to an attribute
• CenterPt.x Xvalue
• Assign an attribute value to a variable
• Yvalue CenterPt.y
• Use a method to modify an object
• NameList.append(Bob)
• Assign the results of a method to a variable
• Name TableRow.GetValue(FULL_NAME)

7
Geoprocessor Object

• ESRI has constructed a big geoprocessor object
• Python interacts with ArcGIS through this object
• Geoprocessor object is created and assigned to a
  variable at the beginning of scripts
• Geoprocessor object has methods that parallel
  tools in ArcToolbox

8
Other types of objects

• Feature Class objects, with attributes including
• ShapeType
• HasSpatialIndex
• Table objects, with attribute
• Fields (as a Python list)
• Row objects, with methods
• GetValue(fieldName)
• SetValue(fieldName, value)
• Geoprocessor Programming Model shows many, many
  more (see links page on course web site)

9
Modules

• Python has a limited set of built-in functions
• Additional functions are available in modules
• Modules are imported at the beginning of scripts
• import ltmodulegt
• After a module is imported, its functions are
  accessed using dot notation
• ltmodulegt.ltfunctiongt(ltparametersgt)
• See Python Library for long list of available
  modules

10
Activity

• Open ArcGIS Desktop Help
• Find the topic Describe method
• Copy and paste the example at the bottom into a
  new Python script window
• Change the 3rd line to point to your geodatabase
• (e.g. "C\\GEO599_Python\\GEO599_Python_ExData.gd
  b)
• Change the 4th line to get the UrbanizedAreas
  feature class
• Delete the line dealing with Topology Name
• Use the format menu to Comment out the section
  dealing with Relationship Class Names (4 lines,
  through for loop)
• Save and run the script

11
Activity, cont.

• Work with a partner to figure out the script and
  add comments to it
• Include a header
• Add a comment for each line before the series of
  print statements
• A single comment should be sufficient to describe
  the series of print statements
• Download the question sheet from the course
  website and answer the questions

12
Debugging Getting scripts to work

• Use print statements to keep track of your script
• Status reports
• Created new feature class
• Updated values in table
• Now processing...
• Track variable values
• A buffer will be created at str(BuffDist)
• The UA code for MSAname is UAcode
• Remove most of these statements once the script
  is working

13
Error Handling

• Errors that are not handled cause a script to
  stop
• When debugging, stopping the script might not be
  a problem
• May not want the script to continue anyway
• Error message is usually informative
• In finished scripts, problems should be dealt
  with more gracefully

14
Try...Except Structure

• Syntax
• try
• ltcode blockgt
• except ltexceptionTypegt
• lterror handlinggt
• If a problem is encountered in the ltcode blockgt,
  execution jumps immediately to lterror handlinggt
• lterror handlinggt typically includes a print
  statement indicating the problem
• May have multiple except statements for different
  types of exceptions

15
Try...Except Example

• try
• StAbrev StAbrevDictMSAname
• except KeyError
• print MSAname was not found in the state
  abbreviation dictionary
• KeyError is the type of exception that occurs
  when a key is not found in a dictionary

16
Exception Types

• Exception types are Python key words
• Accessing non-existent key in a dictionary ?
  KeyError
• Searching for a value that doesnt exist in a
  list ? ValueError
• Referencing a non-existent variable ? NameError
• Trying to do something to the wrong type of data
  ? TypeError

17
Try...Except in Loops

• Often useful to enclose the code block of a loop
  in a try code block
• Except statement placed at end of loop code block
  
• Using the key word continue as the last line of
  the error handling block will restart the loop
  with the next item in the list

18
Syntax for Try...Except in a Loop

• for ltloopVargt in ltListgt
• try
• ltstatements to repeatgt
• except
• lterror handlinggt
• continue
• ltfirst statement outside loopgt

19
GetMessages

• Method of the geoprocessor object
• Returns ArcGIS-specific messages
• Often used in error handling
• print ltGPobjectgt.GetMessages(ltseveritygt)
• Severity
• 0 message
• 1 warning
• 2 error
• None specified returns all levels

20
Exercise 2 additions

• Add print statements tracking progress
• Incorporate the try...except structure
• 5 additional points