An Introduction to Python

python-lists-variables-functions-arithmetic.md

An Introduction to Python

Introduction

What is Python?

• A programming language that is compiled when you run it. (Explain JIT vs. Compiled languages like C)

How can You use Python?

• Sort files on your computer
• Do complex math
• Create websites
• Make games

Who else uses Python?

• NASA - Curiosity log parsing
• bit.ly - Data analysis
• Django Framework - rapid web application development (think of it as Rails for Python)
• reddit.com - python is the backbone of one of the most visited sites in the world

Resources

• python.org
• docs.python.org
• /r/learnpython
• Dive Into Python Free book!
• Codeacademy Course

The interpreter vs. running files

• Use the interpreter ie. the python shell idle or others
• Create a text file and run that

Commenting Code

• This is important. It helps you and others figure out what you were trying to do.
• Denote comments with #
• # This is a comment, it won't be run

Arithmetic

• Basic Operations

  +, -, /, *

• More Complex Operations

  • Order

    (1 + 2) * 3 = 9

    1 + 2 * 3 = 7

Assignment vs. Equality

• Assignment is by =
• Comparison is by ==

Variables

• Variables can be called anything, but it is best to be descriptive for you.
• Must begin with an _ or letter
• Case Sensitive
• Some words are reserved: and, del, if, not, print etc.

x = 3

x = raw_input("Enter your name") Python 2

x = input("Enter your name") Python 3 use just input instead

Strings

• Strings are for you not, for the computer.

• 1 vs. '1'

  x = 1

  y = '1'

  print x

  1

  print y

  1

  3 + y

  Traceback (most recent call last): File "", line 1, in TypeError: unsupported operand type(s) for +: 'int' and 'str'

  3 + x

  4

• String concatenation

  • Use it to join things

    ` string1 = 'Hi'

    ` string2 = 'there'

    print string1 + string2

Lists

• Analogous to variables that hold multiple values. You might recognize them as arrays if you know another language.

• Can be indexed by their position in the.

• Indexes start with 0 not 1.

  list1 = ['a', 'b', 'c', 'd']

  print list1

  ['a', 'b', 'c', 'd']

• Reference items in the list by their index.

  print list1[3]

  d

• Use the built-in len function to count how many items are in a list.

  len(list1)

  4

• Integers in lists

  list2 = [2, 4, 6, 8]

  print list2[0] + list2[1]

  6

• Another type of list.

  list3 = ['apples', 'oranges', 'bananas']

  list3 = [0:2]

  'apples', 'oranges'

  [0:2] means a range not including the last index.

Iteration

• This is where Python's indentation is very important.

• IF something meets our criteria, then do this.

  x = 5

  y = 6

  if x + y == 11:

  print '5 + 6 DOES equal 11'

  5 + 6 does equal 11

Functions

• Take the form

  def functionname(value1, value2, …)

  do something here

• A function that adds two numbers together:

  def number_adder(x, y): print x + y

• Utilize the function by supplying arguments:

  number_adder(5, 3)

  8

An exercise

• Write a program that asks the user to enter their age, and name, prints it back to them.
• Expand on this and add a friend's age. Now calculate the difference between your friend and your age and tell the user.

A couple more really good resources:

http://learnpythonthehardway.org/
http://www.udacity.com/overview/Course/cs101/CourseRev/apr2012