Welcome!

What we will cover this morning

• Why Python?
• What is programming?
• Variables and arithmetic
• Statements and Error Messages
• Development Environment Setup

Why Python?

• Suitable for beginners, yet used by professionals
• Readable, maintainable code
• Rapid rate of development
• Variety of applications
• Vast, welcoming community

What is Python used for?

• System Administration (Fabric, Salt, Ansible)
• 3D animation and image editing (Maya, Blender, Gimp)
• Data Science (pandas, sklearn, nltk, tensorflow)
• Web development (Django, Flask)
• Game Development (Civilization 4, EVE Online)
• Web scraping (selenium, scrapy)

Who is using Python?

• Disney
• Dropbox
• Buzzfeed
• Google
• NASA
• (many, many others)

What is programming?

• Teaching the computer to do a task
• A program is made of one or more files of code, each of which solve part of the overall task.
• The code that you program is human readable but translates into a form that the computer can understand and run.
• Don't focus on what's "under the hood" for now. We will "drive the car" first.

NOTE: Computers need simple, clear instructions

Photo credit: baboon cc

Thinking like a programmer

Computers are great at processing. They are bad at understanding.

When you write a program, you must break down every step into simple pieces.

Example: Draw a Square

1. Find a whiteboard and a dry erase marker.
2. Uncap the dry erase maker.
3. Hold the marker in your hand.
4. Place the marker against the whiteboard.
5. Move your hand 1 foot to the right.
6. Stop.
7. Move your hand 1 foot down...

Example: Make a Sandwich

Source: Harvard Students Making Sandwich: CS 50 Algorithm Intro

Basically, computers can be hard to work with!

Photo credit: Dan Hatton cc

How does Python work?

Photo credit: Open Source for Geeks

If the interpreter finds code it doesn't understand, it stops running and creates an error message.

Command line, Python Shell, Text Editors, Notebooks

Example Text Editors

Let's Develop It!

Let's setup our computer for Python programming

• Install Sublime Text 3 - A text editor
• Install Python 2.7
  (This step is for Windows users only. GNU/Linux and Mac OS X come with Python installed!)
• Windows Users: After installing Python, open the "powershell" program and type:
  [Environment]::SetEnvironmentVariable("Path", "$env:Path;C:\Python27", "User")
• Locate and run the terminal program.
• More setup instructions are available: here
  (You can, but don't have to install the other text editors recommended in this guide)

Let's Develop It!
Working in the Python Shell

Open up your terminal and type python

Follow along with the examples in the upcoming slides.
Just type them right in!

Feel free to explore, as well.
You will not accidentally break things!

Variables and Arithmetic

3 + 4
2 * 4
6 - 2
4 / 2
            

>>> a = 2
>>> b = 3
>>> print a + b
5
>>> c = a + b
>>> print c * 2
10
            

>>> a = 0
>>> a = a + .5
>>> print a
0.5
>>> a
0.5
            

Data Type: Strings

Python provides some tools to manipulate different data types

For the data type string, we can print words in all uppercase, lowercase, or capitalize the first letter.

>>>'hello'.upper()
'HELLO'
>>>'welcome'.capitalize()
'Welcome'
>>'GOOD BYE'.lower()
'good bye'
            

Operators for strings

Strings can also be manipulated with a set of operators

A "string" can be used with the operators: +, *

a = 'Hello '
b = 'World'
c = a + b
print c
            

a = "Spam "
b = a * 4
print b
            

a = 'spam '
b = 'eggs'
c = a * 4 + b
print c
            

Data Types

• Variables are used to store data for future use
• Data always has a "type"
• The type of a piece of data helps define what it can do
• The type can be found using: type()
• type() is a function. We call it by using parenthesis and pass it an object by placing the object inside the parenthesis

  
  a = 4
  print type(a)
  print type(4)
  
  print type(3.14)
  
  b = 'spam, again'
  print type(b)
  print type("But I don't like spam")
              

Data types - continued ...

• Each data type has a specific set of operators
• We saw strings use the + and * operators
• An "int" or "float" can be used with any of: +, -, *, /
• What happens if we try to use division or subtraction with a string?

>>> print "Spam" / "eggs"
Traceback (most recent call last):
File "", line 1, in 
TypeError: unsupported operand type(s) for /: 'str' and 'str'
        

Errors

• There are different kinds of errors that can occur.
  We've seen a few already.
• A "runtime error" results in an Exception, which has several types.
  Each type gives us some information about the nature of the error and how to correct it
• One type of exception is a SyntaxError. This results when our code can not be evaluated because it is incorrect at a syntactic level.
  In other words, we are not following the "rules" of the language.
• Some other examples are the TypeError and NameError exceptions.

Errors - continued ...

# SyntaxError - Doesn't conform to the rules of Python.
# This statement isn't meaningful to the computer
4spam)eggs(garbage) + 10

# NameError - Using a name that hasn't been defined yet
a = 5
print b
b = 10

# TypeError - Using an object in a way that its type does not support
'string1' - 'string2'
          

There are also semantic errors.
These are harder to catch because the computer can't catch them for us.

Comments

You can leave comments in your code—notes that people and future you (!) can read (and appreciate) but computers will ignore.

# This is a sample comment.

Let's Develop It

We'll practice what we've learned in the shell.

Practice commands you didn't fully understand before.

Play with strings, arithmetic, variables, type.

Ask the teacher, TAs, and students around you for help!

Using the Terminal

Creating a folder

We need a folder to save and run our code from.

$ cd
$ pwd
/home/username ( or /User/username or similar)
$ mkdir gdipython
$ cd gdipython
$ pwd
/home/username/gdipython
            

The Text Editor

• Open sublime text.
• Click "File", then "Open". Navigate to the gdi-intro-python folder we just created and click "Open"
• In the text editor, enter the following:

  
  print 'Hello World!'
              

• Click "File", then "Save As...". Type "class1.py" and click "Save".
• Open a terminal and navigate to the gdi-intro-python folder.
  If you don't already have this folder open in a terminal.
• Type python class1.py
• You should see the terminal print "Hello World!"

User Input

To obtain user input, use raw_input()

Change the class1.py text to the following and run it again

name = raw_input("What is your name?")
age = raw_input("How old are you?")
age_int = int(age)
print "Your name is" + name + " and you are " age + " years old."

type(age)
type(age_int)
            

The user's input is always a string. Keep that in mind when working on other problems!

Let's Develop It!

Write your own program that uses raw_input
and does something else with the value

You can use float() to treat the input as a number if you need a number,
or use the input directly as a string.

Questions?

Exercises

Write a program that could replace the initial conversation you have at the checkout line. It should do the following:

>> Did you find everything ok?
(wait for input)

>> What's your name?
(read in a name from the user)

>> Hi, (User's Name) do you want paper or plastic?
(wait for input)

>> What is your total before tax?
(read in a total)

>> Your total with sales tax is: (total with sales tax)
(wait for input before exiting)