Welcome to Python

Open Source, Easy to Learn, Portable

Created by Guido van Rossum, a former resident of the Netherlands, whose favorite comedy group at the time was Monty Python's Flying Circus.

The Terminal

  • Open a terminal window
  • Type python
  • You're now in the Python Interpreter
  • When you see >>>, you know you're in the interpreter

Your computer speaks a different language.

This is how your computer wishes you could speak to it:

This is what an interpreter does

Interpreter vs Translator(Compiler)

  • Interpreter
    1. goes line by line
    2. only interprets what's needed when it's needed
    3. will only let you know of errors when it encounters them, if it does
Translator
  • everything up front
  • will do all the work before returning it
  • can let you know of errors before returning the final work
  • may even translate useless stuff
  • gives you a finished product i.e. Word, Excel...

Let's start talking

Type in "hello". Don't forget the quotes.

You get 'hello' back

Try 'hello'

Try 'How are you?'

Ok, that doesn't make for fun conversations

Let's start calculating then!

Enter 2

You get 2 back!

Enter 2+2

Woohoo! We get 4. The Pi is longer just repeating what we say!

Try:
200000000 + 3
or
592387438274 -92837847

Try 4 * 8

Try 9 / 4

Oh oh. What happened here?

Let's try 9.0 /4 or 9/4.0

Let's try more stuff

Try "2"+"2" and compare with 2+2

Try "hello,"+"world"

Try "hello"+2

Try "wow!"*5

Strings vs Integers vs Floats

A string is a collection of characters one after the other

Strings vs Integers vs Floats

An integer is a whole number, just like we learnt in Math

But what's a float?

But what's a float?

But what's a float?

Also known as a decimal number

Types

A type is "what it is"

type(2)
type("Hello")
type(3.14159)

Let's go back to strings

What can we do with strings?

We can tie one at the end of the other

firststring+secondstring
"hello,"+"world"

We can repeat a string

but you should never repeat a lie

"I will not lie."*200

Ok, give me something useful

try "this is cool".lower()

oops! Try this instead:
"this is cool.".upper()

Or even
"this is cool".upper()+"!"*3

More fun

Try:

  1. "the lord of the rings".title()
  2. "the lord of the rings".capitalize()
  3. "the lord of the rings".upper()

Variables

Try:
dogname = "Fido"
dogname.upper()

You just yelled at your dog!

String Formatting

Try the following:

dogname = "Brutus"
catname = "Felix"
runs = "{} runs after {}"
runs.format(dogname,catname)

Try inversing dogname and catname

Boxing!

Variables are basically boxes that contain something.You can refer to the content of the box by naming the label of the box

Put 4 in a box labelled x 
x = 4
put 3 in a box labelled y. 
y = 3
Take the contents of box x and box y, add them together, and put the result in box z 
z = x + y
Show z 
z
x = 10
z
The content of z isn't changed when we change the content of x. We have to recalculate z if that's what we need

What if I forget?

If you don't remember what you put in a box, you can always type it

x="something I'll soon forget"
type(x)

Find x in 
x = 3 + 1 
4 = x + 1 
x = x + 1 

x = x + 1
In computer speak, this means "take the content of box x, add 1 to it, and put it back into box x"

Let's leave the interpreter for now

Start IDLE

There's IDLE and IDLE3

two versions of Python: 2.7 and 3.2 (or 3.4)

We will use 2.7

Woah! Here's the >>> prompt again!
Yup. This is another way to access the interpreter. Try it out

Programs in Files

or how to keep your work

We are no longer having a conversation with the interpreter. But we're still not dealing with a translator/compiler either.

We can keep our work on disk, and then send it to the interpreter.

Entering text in a file

You know the drill, File/New, File/Open, File/Save As.., File/Save

Enter 2+2, save as firstprogram.py, and hit F5

Save under Documents/Python Projects

F5 means Save and Run

So what did we get?

nothing at all

Go back to your file, and type print 2+2 and try again with F5

NOT a conversation

Now we have to tell the interpreter every single thing we want to see.

Otherwise the interpreter stays quiet.

(it did the work though, it's just not telling you)

Functions

A function is a piece of code that's already written

It can be called whenever you want

Functions look like this:

function_name(list_of_parameters_separated_by_comma):

Example: len("I like my dog")

pow(2,8)

print

print is a function, but it's an exception

in Python 3, it's no longer an exception, they fixed it

for now we use: print "I like my dog"

import

Many functions are defined in libraries

You have to import the library before you can access the function


import math
math.sqrt(16)