1. Functions Reading/writing files Catching exceptions
Python language
Functions
Reading/writing files
Catching exceptions

2. Function Definition
def function_name (arg1, arg2, …) : # arg means argument or parameter!
body
# Sometimes we pass a known value for one of the arguments:
def function_name (arg1, arg2=default2, arg3): # e.g., power (x, y=3)

3. Function with docsting

4. Function modifies a string

5. Pass arguments by position

6. Casting user’s entered value

7. Function traverses a string with a loop

8. Using datetime functions
The age function uses the input function to get the birth year.
It then imports the datetime module
Gets the current year form the now () function.
Calculates the age, and returns it

9. Built in Numeric Functions
abs(), divmod(), float(), hex(), long(), max(), min(), oct(), pow(), round()
# there are a lot more for trigonometry, etc.
‘none’ is a special data type for empty value
e.g., when a function does not return a value
It is used as a placeholder

10. Find maximum of any unknown set of arguments
Notice that the function is called with different numbers of arguments. The * does the trick!

11. Defining a global variable
Notice that calling the local variable outside of the function leads to error!

12. Assigning the returned value of a function to a variable

13. Tools in Arcpy are function!
Tools have name, which is similar to the tool’s label in ArcGIS (without space), for example:
AddField for the Add Field tool
The alias for a tool is not the same as its name or label.
It is a short version of it, for example:
‘management’ is the alias for Data Management toolbox
Clip_management is the Clip tool in the Data Management toolbox

14. Two ways to access a tool in ArcGIS
All tools are available as function in Arcpy
Each tool (function) does a specific task
To access a tool, we can:
1. Call its corresponding function
arcpy.toolname_toolboxalias (arguments)
For example: call the Clip tool:
>>> import arcpy
>>> arcpy.env.workspace = “C:/Data”
>>> arcpy.Clip_analysis (“stream.shp”, “study.shp”, “result.shp”)
Another example:
>>> arcpy.env.workspace = “C:/Data/study.gdb”
>>> arcpy.Buffer_analysis (“roads”, “buffer”, “100 METERS”)

15. Functions in ArcGIS – GetMessages()
When a tool is run in ArcGIS, messages write about its success or failure
To get the message, we use the following functions:
>>> arcpy.GetMessages ()
>>> print arcpy.GetMessages (0) # 1st message
>>> count = arcpy.GetMessageCount ()
>>> print arcpy.GetMessage (count-1)
# prints the last message

16. 2. Tools are also in modules that match the toolbox alias name
arcpy.toolboxalias.toolname (arguments)
For example:
>>> import arcpy
>>> arcpy.env.workspace = “C:/Data”
>>> arcpy.analysis.Clip (“stream.shp”, “study.shp”, “result.shp”)
These two methods are equivalent! It depends on our preference.

17. Other examples
>>> import arcpy >>> arcpy.env.workspace = “C:/Data” >>> infc = “streams.shp” # fc stands for feature class >>> clipfc = “study.shp” >>> outfc = “result.shp” >>> count = arcpy.GetCount_management (infc, clipfc, outfc) >>> print count # prints the count for streams >>> result = arcpy.analysis.Clip (infc, clipfc, outfc) >>> print result # prints an object # will print the path to the result shape file, i.e., : C:/Data/result.shp

19. Filesystem
Filesystem refers to naming, creating, moving, or referring to files.
Depends on the operating system.
Pathname: String that refers to the location of the file in a directory
Filesystem in all operating systems is a tree structure, made of a root (disk), directory, subdirectory, and its branches.
In Windows, the \ character is used to separate the file or directory names from each other.
There is a separate root for each drive (A:\, C:\, D:\)
For example: C:\myDirectory\myfile

20. Paths in Python In Python, backslash (\) is used for escape.
For example, \n and \t for line feed and tab
Therefore, we avoid using \ in Python paths. Instead:
We use forward slash;
C:/Data/Exercise01.txt
Or, we use double backslash (\\)
C:\\Data\\Exercise01.txt
OR, use a string litteral: #put ‘r’ before the string. r means raw string
r”C:\Data\Exercise01.txt”
Paths are stored as string in Python!
For example, the following assigns the path to a shape file as a string for the inputfc variable (fc is feature class in ArcGIS)
>>> Inputfc = “C:/Data/lakes.shp”

21. Absolute path vs. relative path
Absolute path is the exact address of the file in the filesystem, starting from the root.
C:\Geoinformatics\myFile.txt
A:\Courses\Python\Programming.html
Relative path writes it with respect to another point in the file system
Python\Functions\map.txt
We can either append a relative path to an existing absolute path.
Or, we can implicitly reference the current working directory where a Python program may consider to be during an execution.

22. Current working directory - cwd
Python knows the current directory of the file it is working on (just like you know when you work on a file that you open from a directory).
This directory is called the ‘current working directory” or cwd for a program.
Do the following to get the cwd of Python on your computer

23. Opening files in read mode

24. Opening a file in the write mode
We pass ‘w’ instead of ‘r’ in the 2nd argument
This will first erase data in the file before writing.
NOTE: If we want to append to the file, use ‘a’ instead of ‘w’.

25. Open File - example
>>> f = open ("C:/Data/mytext.txt", "w") >>> f.write ("GIS") >>> f = open ("C:/Data/mytext.txt", "r") >>> f.read() 'GIS‘

26. Read the number of lines in a file
Assume the multi_line.txt file has five words written in five lines

27. Screen input/output
Input () is a built-in method to prompt for input; returns a string

29. Throwing exceptions (Error handling)
Are used to handle unusual circumstances (e.g., errors) encountered during the execution of the program.
We have to realize that errors will happen, and write code to detect and handle them.
This is done by returning a status value by a function to indicate whether a function was executed successfully or not

30. Raising (throwing) an exception
try : # if there is no problem, do your operations here # … except : # execute this block if there is any exception (problem) # handle possible problem here else : # If there is no exception, execute this block finally : # this always will execute. # used to manage resources (e.g., # close files).

31. General try/except
try: statements # Run this main action first except name1: statements # Run if name1 is raised during the try block except (name2, name3): statements # Run if any of these exceptions occur except: statements # Run for all (other) exceptions raised else: statements # Run if no exception was raised during try block

32. try : body # is executed first
try : body # is executed first. # If successful (no error), the else block is executed, and try is finished # If exception is thrown in try, the 1st except clause is searched except exception_type1 as var1 : # if this clause matches the error, # the error is assigned to the variable var1 after the exception type, # and the exception code below the associated type is executed exception code1 # handle the raised exception, otherwise: except exception_type2 as var2 : # comes to here if the first except clause did not match. exception code2 # this may process if it matches … # and so on else : # if there is no error, come here, this is optional, and rarely used else_body finally : # optional. If it exists, it always executes finally_body # clean up by closing files or resetting variables

33. Common exception errors and examples
IOError If the file cannot be opened or read.
print (“Error while reading the file”)
ImportError If python cannot find the module
print (“no module found”)
ValueError Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value
print (“non-numeric data found”)
KeyboardInterrupt Raised when the user hits the interrupt key (normally Control-C or Delete)
print (“you cancelled the operation”)
EOFError Raised when one of the built-in functions (input() or raw_input()) hits an end-of-file condition (EOF) without reading any data
print (“end of file”)

34. Handle ValueError

35. Assign exception to variable

36. Handle ValueError …
Example: Define a function to get a number from a user.
Convert it into float
If it cannot be converted to float, send a message

37. Exception is an object (as in OOP)
Exceptions can be raised by any function and given to a variable
>>> list = [2, 4, 6]
>>> x = list[2]
>>> print x 6
>>> x = list[4] # this raises the ‘list index out of range’ error
Traceback (most recent call last): File "<interactive input>", line 1, in <module>IndexError: list index out of range
The error is handled by Python by writing the above message, which in this case terminates the program because there is no exception handler!

38. Catching when knowing the type of error

39. Catching multiple error types

40. IOError

41. Handle opening file issues
We use try/except for quality assurance (QA)

42. IOError …

43. IOError …

44. Catch all exception types

45. Python programs
Up until now you have interactively used Pythonwin to communicate with the Python interpreter.
A set of Python statements saved in a file constitutes a Python program or script.
The program usually has a controlling function that can be called to run the program.