Python

Python is a...
- popular,
- high-level,
- flexible,
- open-source
- programming language.
Python runs on many platforms.
- To download the latest version, go to Python's Home Page.
- Note that in the class we will be using Python 3.2, which differs in a couple of significant ways from Python 2.*.
Python's philosophy: make it clear and simple
- A "Hello, world!" program in Java (must be compiled before running)
```
public class HelloWorld
{
    public static void main (String[] args)
    {
        System.out.println("Hello, world!");
    }
}
```
- A "Hello, world!" program in Python (no need to compile before running)
```
print("Hello, world!")
```

Python can be run as an interpreted language.
- You can enter Python expressions on a command line in a shell and see what they evaluate to.
```
>>> print("Hello, world!")
Hello, world!
>>> 31234 * (762 + 181)
29453662
```

A Python program

A Python program is a set of expressions that are executed in order.
The program normally resides in one more more files ending in .py.
When you run a Python program, the files are compiled into "byte code", a more efficient form (files ending in .pyc).
Compilation happens automatically.
Writing a program usually means editing one or more .py files containing the source code for the program.
You can use any text editor, but you'll want to use an editor that knows how to indent Python code and to use color to highlight different kinds of text.
You run a Python program by typing
```
python <file>
```
in a shell, where <file> is a file containing the program. Note that if you have multiple versions of Python running on your computer, you may have to specify which one:
```
python3.2 <file>
```
You can also make a Python program into an executable script than can be run without invoking Python directly.

IDLE

IDLE: an integrated development environment that comes with Python
Opens with a window called "Python shell", where the Python interpreter runs
IDLE's shell has...
- auto-completion (type "TAB" to complete a command)
- auto-indentation
- syntax highlighting
- automatic display of function arguments
- possibility of moving up and down within the expressions already entered (<ctrl>-p to go up, <ctrl>-n to go down)
- many Emacs-like keyboard shortcuts
An editor with features like the shell
After editing and saving a file with Python code, execute it by selecting "Run Module" in the "Run" menu
Features you expect from a text editor (undo, search, replace, cut, copy, paste, etc.)
Convenient ways to indent, dedent, comment, or uncomment stretches of code

Playing around in the interpreter

When we type expressions into the interpreter, we are in interactive mode.
Arithmetic
- Arithmetic operators: +, -, /, //, *, **, %.
  - With appropriate operands, each operator returns a number.
```
>>> 5+20
25
>>> 50.7 / 2.3
22.04347826086957
>>> 8**3
512
>>> 9 / 2
4.5
>>> 21 % 4
1
```
  - Note that you don't need =. This operator has a completely different meaning in Python.
```
>>> 21 + 3 =
SyntaxError: invalid syntax
>>> 21 + 3 = 24
SyntaxError: can't assign to operator
```
  - Operators have their usual precedence. You can override this with parentheses.
```
>>> 3 * 4 + 5
17
>>> 3 * (4 + 5)
27
```
Types
- Numbers in Python come in two basic types, integers (int) and floating-point numbers (float).
```
>>> type(3)
<class 'int'>
>>> type(3.0)
<class 'float'>
```
- Every Python object has a type, or class.
```
>>> type(int)
<class 'type'>
>>> type(type)
<class 'type'>
```

Strings

Sequences of characters are represented by strings (type str), which begin and end with single or double quotes.

>>> 'Hello, world!'
'Hello, world!'
>>> type('Hello, world!')
<class 'str'>
>>> type("12")
<class 'str'>
>>> type('type')
<class 'str'>

+ and * also work with strings.

>>> 'Hello, world!' + " This is me."
'Hello, world! This is me.'
>>> "no" * 3
'nonono'
>>> '1' + '2'
'12'
>>> '1' + 2
Traceback (most recent call last):
  File "<pyshell#260>", line 1, in <module>
    '1' + 2
TypeError: Can't convert 'int' object to str implicitly

Lists

Like a string, a list is a sequence, but its elements can be any objects, including other lists.

Lists start and end with brackets, and the list elements are separated by commas. + and * work with lists too.

>>> ['a', 'list', 'of', 'strings']
['a', 'list', 'of', 'strings']
>>> 
>>> ['a', 'list', 'of', 'strings'] + [';', 'another', 'list']
['a', 'list', 'of', 'strings', ';', 'another', 'list']
>>> [['x', 4], ['y', 5], ['heading', 0]]
[['x', 4], ['y', 5], ['heading', 0]]
>>> [+, -]
SyntaxError: invalid syntax

Looking ahead
- When we write a program, one of the first things we have to figure out is how to represent the things that the program will deal with — numbers, people's names, sentences, databases, places on the screen, music and other sounds, etc.
- To do this, we select from the data types that the programming language makes available data types that have the properties we care about.
- For example, we could represent a natural language word with a string: 'hello', 'أهلاً', 'ሰላም', 'こんにちは'
- For example, we could represent a sentence as a list of words (strings): ['hello', 'world'], ['ለዓለም', 'ሁሉ', 'ሰላም']