GEOG 485:
GIS Programming and Automation

4.1 Functions and modules

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One of the fundamentals of programming that we did not previously cover is functions. To start this lesson, we'll talk about functions and how you can use them to your benefit as you begin writing longer scripts.

A function contains one focused piece of functionality in a reusable section of code. The idea is that you write the function once, then use, or call, it throughout your code whenever you need to. You can put a group of related functions in a module so you can use them in many different scripts. When used appropriately, functions eliminate code repetition and make the main body of your script shorter and more readable.

Functions exist in many programming languages, and each has its way of defining a function. In Python, you define a function using the def statement. Each line in the function that follows the def is indented. Here's a simple function that reads the radius of a circle and reports the circle's approximate area. (Remember that the area is equal to pi [3.14159...] multiplied by the square [** 2] of the radius.)

>>> def findArea(radius):
... 	area = 3.14159 * radius ** 2
... 	return area
... 
>>> findArea(3)
28.27431

Notice from the above example that functions can take parameters, or arguments. When you call the above function, you supply the radius of the circle in parentheses. The function returns the area (notice the return statement, which is new to you).

Thus, to find the area of a circle with a radius of 3 inches, you could make the function call findArea(3) and get the return value 28.27431 (inches).

It's common to assign the returned value to a variable and use it later in your code. For example, you could add these lines in the Interactive Window:

>>> aLargerCircle = findArea(4)
>>> print aLargerCircle
50.26544

Please click this link to take a close look at what happens when the findArea(...)  function is called and executed in this example using the code execution visualizion feature of pythontutor.com. In the browser window that opens, you will see the code in the top left. Clicking the "Forward" and "Back" buttons allows you to step through the code, while seeing what Python stores in memory at any given moment in the window in the top right.

  • After clicking the "Forward" button the first time, the definition of the function is read in and a function object is created that is globally accessible under the name findArea.
  • In the next step, the call of findArea(4) in line 5 is executed. This results in a new variable with the name of the function's only parameter, so radius, being created and assigned the value 4. This variable is a local variable that is only accessible from the code inside the body of the function though.
  • Next, the program execution jumps to the code in the function definition starting in line 1.
  • In step 5, line 2 is executed and another local variable with the name area is created and assigned the result of the computation in line 2 using the current value of the local variable radius, which is 4.
  • In the next step, the return statement in line 3 sets the return value of the function to the current value of local variable area (50.2654).
  • When pressing "Forward" again, the program execution jumps back to line 5 from where the function was called. Since we are now leaving the execution of the function body, all local variables of the function (radius and area) are discarded. The return value is used in place of the function call in line 5, in this case meaning that it is assigned to a new global variable called aLargerCircle now appearing in memory.
  • In the final step, the value assigned to this variable (50.2654) is printed out.

It is important to understand the mechanisms of (a) jumping from the call of the function (line 5) to the code of the function definition and back, and of (b) creating local variables for the parameter(s) and all new variables defined in the function body and how they are discarded again when the end of the function body has been reached. The return value is the only piece of information that remains and is given back from the execution of the function.

A function is not required to return any value. For example, you may have a function that takes the path of a text file as a parameter, reads the first line of the file, and prints that line to the Interactive Window. Since all the printing logic is performed inside the function, there is really no return value.

Neither is a function required to take a parameter. For example, you might write a function that retrieves or calculates some static value. Try this in the Interactive Window:

>>> def getCurrentPresident():
... 	return "Barack Obama"
... 
>>> president = getCurrentPresident()
>>> print president
Barack Obama

The function getCurrentPresident() doesn't take any user-supplied parameters. Its only "purpose in life" is to return the name of the current president. It cannot be asked to do anything else.

Modules

You may be wondering what advantage you gain by putting the above getCurrentPresident() logic in a function. Why couldn't you just define a string currentPresident and set it equal to "Barack Obama?" The big reason is reusability.

Suppose you maintain 20 different scripts, each of which works with the name of the current President in some way. You know that the name of the current President will eventually change. Therefore, you could put this function in what's known as a module file and reference that file inside your 20 different scripts. When the name of the President changes, you don't have to open 20 scripts and change them. Instead, you just open the module file and make the change once.

You may remember that you've already worked with some of Python's built-in modules. The Hi Ho! Cherry O example in Lesson 2 imported the random module so that the script could generate a random number for the spinner result. This spared you the effort of writing or pasting any random number generating code into your script.

You've also probably gotten used to the pattern of importing the arcpy site package at the beginning of your scripts. A site package can contain numerous modules. In the case of arcpy, these modules include Esri functions for geoprocessing.

As you use Python in your GIS work, you'll probably write functions that are useful in many types of scripts. These functions might convert a coordinate from one projection to another, or create a polygon from a list of coordinates. These functions are perfect candidates for modules. If you ever want to improve on your code, you can make the change once in your module instead of finding each script where you duplicated the code.

Creating a module

To create a module, create a new script in PythonWin and save it with the standard .py extension; but instead of writing start-to-finish scripting logic, just write some functions. Here's what a simple module file might look like. This module only contains one function, which adds a set of points to a feature class given a Python list of coordinates.

# This module is saved as practiceModule1.py

# The function below creates points from a list of coordinates
#  Example list: [[-113,23][-120,36][-116,-2]]

def createPoints(coordinateList, featureClass):

    # Import arcpy and create an insert cursor  
    import arcpy

    with arcpy.da.InsertCursor(featureClass, ("SHAPE@",)) as rowInserter:

        # Loop through each coordinate in the list and make a point    
        for coordinate in coordinateList:
            point = arcpy.Point(coordinate[0],coordinate[1])
            rowInserter.insertRow((point,))

The above function createPoints could be useful in various scripts, so it's very appropriate for putting in a module. Notice that this script has to work with insert cursors and point objects, so it requires arcpy. It's legal to import a site package or module within a module.

Also notice that arcpy is imported within the function, not at the very top of the module like you are accustomed to seeing. This is done for performance reasons. You may add more functions to this module later that do not require arcpy. You should only do the work of importing arcpy when necessary, that is, if a function is called that requires it.

The arcpy site package is only available inside the scope of this function. If other functions in your practice module were called, the arcpy module would not be available to those functions. Scope applies also to variables that you create in this function, such as rowInserter. Scope can be further limited by loops that you put in your function. The variable point is only valid inside the for loop inside this particular function. If you tried to use it elsewhere, it would be out of scope and unavailable.

Using a module

So how could you use the above module in a script? Imagine that the module above is saved on its own as practiceModule1.py. Below is an example of a separate script that imports practiceModule1.

# This script is saved as add_my_points.py

# Import the module containing a function we want to call
import practiceModule1

# Define point list and shapefile to edit
myWorldLocations = [[-123.9,47.0],[-118.2,34.1],[-112.7,40.2],[-63.2,-38.7]]
myWorldFeatureClass = "c:\\Data\\WorldPoints.shp"

# Call the createPoints function from practiceModule1
practiceModule1.createPoints(myWorldLocations, myWorldFeatureClass)

The above script is simple and easy to read because you didn't have to include all the logic for creating the points. That is taken care of by the createPoints function in the module you imported, practiceModule1. Notice that to call a function from a module, you need to use the syntax module.function().

Readings

To reinforce the material in this section, read Zandbergen 12.1 - 12.5, which talks about creating Python functions and modules.

Practice

Before moving ahead, get some practice in PythonWin by trying to write the following functions. These functions are not graded, but the experience of writing them will help you in Project 4. Use the course forums to help each other.

  • A function that returns the perimeter of a square given the length of one side.
  • A function that takes a path to a feature class as a parameter and returns a Python list of the fields in that feature class. Practice calling the function and printing the list. However, do not print the list within the function.
  • A function that returns the Euclidean distance between any two coordinates. The coordinates can be supplied as parameters in the form (x1, y1, x2, y2). For example, if your coordinates were (312088, 60271) and (312606, 59468), your function call might look like this: findDistance(312088, 60271, 312606, 59468). Use the Pythagorean formula A ** 2 + B ** 2 = C ** 2. For an extra challenge, see if you can handle negative coordinates.

The best practice is to put your functions inside a module and see if you can successfully call them from a separate script. If you try to step through your code using the debugger, you'll notice that the debugger helpfully moves back and forth between the script and the module whenever you call a function in the module.