Week 6: Introduction to Programming for GIS and Spatial Data Analysis GEO GEO A

Transcription

1 Week 6: Modules Procedures and Functions Introduction to Programming for GIS and Spatial Data Analysis GEO GEO A

2 Review: Sequences of Instructions Strict sequential flow: Step One Two Three Ta-Da!

3 Review: Modules Module

4 A Module by Any Other Name: Functions vs. Subroutines In many programming languages there s a distinction: Functions return something Subroutines just package commands However, in Python, functions do both Functions can return None (i.e. NULL, Empty) or any other data object

5 In Python, What is a Function? almost everything! Most built in i or base Python functionalities i are functions Functions take arguments, perform some commands, and may return something

6 Anatomy of a Python Function: The Basic Syntax def function_ name(<arguments>):... return <object or None> ## Then we can call our function: function_name(<arguments>) ## Or if it returns something, store the ## return value in an object: return_val = function_name(<arguments>)

7 Functions That Return Objects Using return explicitly tells thefunction what to return You may return any kind of data object But tif your function just ends without t an explicit return, the function (module!) returns None (e.g. Null)

8 Anatomy of An R Function: Definition vs. Execution def my_ function(): print("hello World!") >>>my_function ## vs. >>>my_function()

9 Anatomy of An R Function: Arguments ## A function with a single, ## required argument: def my_function(name): print( "Hello " + name ) my_function("forrest") my_function(name="twigs") my_function()

10 Anatomy of An Python Function: Required vs. Optional Args ## A function with two arguments, ## one required, one optional: def my_function(name, greeting = "Hello"): print( greeting +" " + name ) my_function("forrest") my_function("twigs", "Yo") my_function(greeting="'sup", name="twigs")

11 Anatomy of An R Function: Order of Arguments ## Make sure required arguments are listed ## first, otherwise you will get a syntax error: def my_function( name, greeting = "Hello" ): print(greeting + " " + name) ## vs. def my_function( greeting = "Hello", name ): print(greeting + " " + name)

12 Returning and Printing From def my_ function(): print(5 + 10) return 2 x = my_function() Functions x

13 Global vs. Local Scope ## This variable is set outside ## the scope of the function, and is has global scope: x = 5 def my_function(): #print(x) ## And it can't be accessed, error... x = 10 ## If x is assigned here it is local scope print(x) my_function() print(x) ##... only access global scope

14 Explicit Global Scope ## This variable is set outside ## the scope of the function, ## and is has global scope: x = 5 def my_function(): #print(x) ## And it can't be accessed, error... global x print(x) ## but now it can... x = 10 ## If x is assigned here it is global scope my_function() print(x)

15 But Python Passes Objects by Reference to Arguments! ## This variable is set outside ## the scope of the function: x = [1, 2, 3] print("x before: " + str(x)) def my_function( list ): list.append( 4 ) print("argument inside: " + str(list)) my_function( x ) print("x after: " + str(x))

16 However, assignment inside a function overrides reference: ## This variable is set outside ## the scope of the function: x = [1, 2, 3] print("x before: " + str(x)) def my_function( list ): list = [1, 2, 3, 4] print("argument inside: " + str(list)) my_function() print("x after: " + str(x))

17 Functions Why create functions? Provides modularity to your code You can solve small problems using functions Self contained Keeps your code much cleaner You can distribute and re use your functions Recursion Functions that call themselves!

18 How do we distribute/re use use code? We save our Python code that contains instructions and/or function definitions in its own file We can then use : exec(open("./filename").read()) Or we can create a package out of our code, and use the import command to load functions For next week