Introduction to Python

C463 / B551 Artificial Intelligence

Introduction to Python

Python Overview

An interpreted, compiled, and interactive, object-oriented, dynamic, imperative, and open source programming language.
Created in early 90's by Guido von Rossum at Stichting Mathematisch Centrum in the Netherlands.
The name comes from the Monty Python and not from the snake.
There is a big community of Python programmers, with conferences and magazines:
http://pycon.org/
Web site: http://www.python.org.

Features of Python

Interactive: one can launch a Python console and run instructions directly it.
Portable: available on most existing systems. It only requires a C compiler to be ported to any new platform.
Structure: functions, classes, modules.
It is easy to embed Python with C and C++.
The user can write their own code in C or C++ and compile it as Python modules or functions. That makes Python extensible.
Usual applications: scripts including CGI scripts, GUIs, scientific computing.
Many existing libraries for all sort of purposes.

Syntax Rules

The syntax is designed to be simplified as compared to other languages like C/C++.
Every compound instruction ends with ":"
There are no blocks of code; blocks are implicitly created by indentation.
Expressions: usual arithmetic operators, named logic operators: and, or, not.
Assignments use the = sign but they don't have to end with ";"
Comments start with # as in shell scripting.
Variables are declared by assigning them a value and they are local to the block where they appear first.

Control Structures

Conditional:

if condition:
    instructions
elif condition: #*
    instructions
else: # optional
    instructions

Loops:

while condition:
    instructions
else:  # optional
    instructions
for var in S:
    instructions
else:  # optional
    instructions 
for i in range(n):
    instructions

Built-in Data Structures

Lists: linked lists implementing the subscript operator:
```
x = [1,2,3]
x.append(4)
print x[2]       # result: 3 
```
Tupples: constant kind of arrays
```
x = (1,2,3)
```

Dictionaries: association lists

x = {}
x["word"] = reference
for k in x.keys():
    print x[k]

Functions and Parameters

Function definition:

def function_name (par1, par2, ...):
    body of the function

It supports default values for parameters.
All parameters are value parameters.
Any variable storing a complex data structure contains a reference to it.
Any changes to the content of such a data structure in the function will affect the variable passed in the function call.
Assignments involving a complex data structure don't make a copy of it.

More Built-in Functions

Function type: returns the type of an object.
type(0) - returns <type 'int'>
Checking if something is an integer:
```
if type(x) == type(0): ...
```
Reading a value from the terminal: input()
```
x = input()
```
Returning a value from a function:
```
return True
```

Example of Conditional

def check_type(x):
    if type(x) == type(0):
        print x, "is an integer"
    elif type(x) == type(1.0):
        print x, "is a float"
    elif type(x) == type(""):
        print x, "is a string"
    elif type(x) == type([]):
        print x, "is an array"
    elif type(x) == type({}):
        print x, "is a dictionary"
    else:
        print x, "is probably an object of a class"

Example of while/else

def Euler(a, b):
    if b==0:
        return a
    r = a % b
    while r:
        a = b
        b = r
        r = a % b
    else:
        print "a divisible by b"
        return b
    return r

Booleans

Truth values: True and False.
False is equivalent with 0, and empty list [], an empty dictionary {}.
Anything else is equivalent to True.
Example:
```
x = 0
if not x:
print "0 is False"
```

Default Values for Parameters

def function (var1 = value, var2 = value, ...):

Just like in C++, all the parameters that have default values must be grouped at the end.
```
def GCD1(a=10, b=20): ...
GCD1() -> 10
GCD1(125) -> 5
GCD1(12, 39) -> 3
```

Variables and Scope

Module: one python file.
Global scope: exists in the module in which they are declared.
Local scope: local to the function inside which it is declared.
Global variables in a module can be accessed from somewhere else using the notation module.variable.
Example: string.digits contains '0123456789'.

Example Scope

def test_scope():
    for i in range(4):
        for j in range (3):
            x = i*10+j
            if x>20:
                print x,
    print x
test_scope()
21 22 30 31 32 32

Try - Except

try: attempts to execute an instruction.
If the operation is successful, it moves on.
If not, we have the option of doing something else with the instruction
except:
Another option:
except error_type:
which does something only for a particular type of exception.

def scope1():
    y = 15
y = 20
def scope2():
    y = 25
def scope3():
    try:
        print y
    except:
        print "cannot access global y"
    print days
    y = 25
    print y
days=["monday", "tuesday"]

More examples in the file examples1.py.