Python�

Raje Ramrao Mahavidyalaya, Jath.

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Department of B.C.A.

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Name:-Miss Balikai J.I.

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Python Introduction

• 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: 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

Artificial Intelligence – D. Vrajitoru

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"

...

Artificial Intelligence – D. Vrajitoru

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

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Artificial Intelligence – D. Vrajitoru

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”

Artificial Intelligence – D. Vrajitoru

Default Values for Parameters

• Default values:
• 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

Artificial Intelligence – D. Vrajitoru

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’.

Artificial Intelligence – D. Vrajitoru

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

Artificial Intelligence – D. Vrajitoru

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.

Artificial Intelligence – D. Vrajitoru

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"]

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Artificial Intelligence – D. Vrajitoru