Introduction to Python

Basics of the Language

Install Python• Find the most recent distribution for your

computer at:http://www.python.org/download/releases/

• However, for this class, download and execute Python MSI installer for your platform– For version 2.7

• This version works with your Python textbook!

– You may need to be logged in as administrator to your computer to run the installation

Modes of Interaction with Python• You can interact with the Python interpreter through two built-in modes:

– Basic (command line)– IDLE (may need to download other software!)

• Command line mode:– You can find and double click the Python.exe executable (e.g., python27) in the

downloaded folder

– Make sure to right click it, and then create a shortcut to it, and pin it to the taskbar!

– Click the icon on your taskbar to start the command line

You will get the >>> Python promptType exit at the command prompt to get out of it!>>> exit //or ctrl z

– Note: you can use the Home, Pg up, Pg dn, and End keys, to scroll through previous entries, and repeat them by pressing the Enter key!

Basic Command Line

IDLE

• This is the integrated development environment for Python

• Involves interactive interpreter with editing and debugging capabilities

• You can find and double click the Pythonw.exe executable (e.g., python27) in the downloaded folder

• Pin it to the taskbar

Python IDLE Shell Window

• The IDLE (Python shell window) is more convenient than the basic python command line

• Provides automatic indentation and highlights the code

• Can use the Home, Pg up, Pg dn, and End keys to search the buffer in your session

Learn Basic parts of Python• Like other languages, Python has many data types– These can be manipulated with operators, functions, and

methods

• We can also make our own types (classes) and instantiate and manipulate them with operators, functions, and our own methods

• It also has conditional and iterative control, e.g.:– if-elif-else– while loop– for loop

Python Naming Style• Function name is lower case

def add (): #adds numbers; returns the result; ends with :

• Variable name is lower case (are case sensetive)inputFieldNamefc #holds a feature class as in: fc=“Roads.shp”Note: variables Road and road are different!

• Class name is UpperCamelCaseClass StrikeSlipFault

• Constant is all caps, e.g. PI, SPREADING_RATE

• Indentation: 4 spaces per level, do not use tabs– IDLE does it for you!

Data types - Practice

10/3 = 3, but 10/3.0 = 3.33333The // operator is for integer division (quotient without remainder). 10//3 = 3 and 4//1.5 = 2, and 13//4 = 3

Use Built in functions

Strings

Strings …

Strings …

Formatting strings with thestring modulus operator %

Booleans

List

x [:-2] -> [‘hydroxide’, ‘phosphate’, ‘carbonate’] # up to, but not including index -2, i.e., gets rid of the last twox [2:] -> [‘carbonate’, ‘oxide’, ‘silicate’] # values from position 2 to the end (inclusive)

List Index

P Y T H O N

0 1 2 3 4 5-6 -5 -4 -3 -2 -1

6

lang = [‘PYTHON’] # the whole ‘lang’ listlang [-2] = ‘O’ # at -2lang [2:5] = ‘THO’ # between 2 and 5, not including 5lang [:3] = ‘PYT’ # up to 3, not including 3lang [-2:-4] = ‘TH’ # from -4 and -2lang [-3:] = ‘HON’ # from -3 to the end

Lists have mixed types

• Lists are like Java arrays

List’s Built-in Functions

Sets: unordered collection of objects

Sets are unordered collection of objects and can be changedDuplicates are removed, even when you add themThe ‘in’ keyword checks for membership in the set

Sets

if-elif-else

>>> if condition1: # use colon after any statement!body1 # do this if condition is true. Notice the indentation

elif condition2: # notice the colonbody2 # otherwise do this

… # put more elifs if needed else condition:

bodyn # otherwise, if no condition was true except this, do bodyn

The While Loop

>>> while (condition): # NOTE: condition expression evaluates to true or falsebody # If condition evaluates to true (x<y in the example), it will

executeelse: # optional, rarely used!

post-code # If condition evaluates to false, post-code will execute

• Note: When typing in the IDLE, press enter after (x=x+2) (while the statement is highlighted), and then press backspace key to go to the beginning of line, then press enter to execute the loop!

While loop …

The r and n are said to be local variables because they are declared inside the function.If you want them to be available to other functions outside, we can declare them to be global, for example:

global r = 1

The For Loop

• It iterates in order over the items in the iterable sequence (e.g., list, tuple, string)

for item in sequence: # starts from the first element of the sequencebody # if there is an item, body will execute; it goes to the second item

else: # is optional (rarely used as in while loop!)post-code

The For Loop

• The loop searches the list one item at a time. For each item in the list it checks to see if it is an integer, if not an integer, it skips it, and then checks to see if it divides by eight without a remainder, if it divides by 8, then it prints the message and gets out of the loop!

Loop with specific indices

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)

body

Function …

input () function

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