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INTRODUCTION TO PYTHONGETTING STARTED
WHAT IS PYTHON?
An elegant and robust programming language
which delivers powerful and general applicable
traditional compiled languages with the used of
simpler scripting and interpreted languages.
 Running python:

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3 ways
Starting the interpreter interactively , entering one line of
Python at a time for execution
 Running a script written in Python – this is done by
invoking the interpreter on your script application
 Run from graphical user interface (GUI) from within an
integrated development environment (IDE/IDLE)

INTERACTIVE INTERPRETER FROM THE
COMMAND LINE
Python can be start coded using an interactive
interpreter by starting it from the command line.
 This can be done from Unix, DOS etc.
 Unix

At the Unix prompt (% or $), the interpreter can be
started by invoking the name python (jpython), as in
the following $python
 Once Python started, the interpreter startup message
will indicate the version and platform and be given
the interpreter prompt “>>>” to enter Python
commands.
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CONTINUE..
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Windows/DOS
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From a DOS window (either DOS /Windows), the
command to start python is same as Unix, the
difference is that the prompt is c:\>python
Command-Line Options

Additional options may be provided to the interpreter
such as listed below:
-d  provide debug output
 -o  generate optimized bytecode resulting in .pyo files
 file  Run Python script from given file
 - c cmd  Run Python script sent in as cmd string
 Etc…
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AS A SCRIPT FROM THE COMMAND LINE
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Unix- a Python script can be executed by
invoking the interpreter from the command line
as in the following
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$ python script.py
Same as Unix
AN INTEGRATED DEVELOPMENT
ENVIRONMENT (IDE)
Need GUI application on your system that
supports Python.
 Have graphical user interface (GUI), source code
editor , trace and debugging facilities.
 IDLE is the first Unix IDE for Python.
 PythonWin is the first Windows interface for
Python and is an IDE with a GUI.
 Usually installed in the same directory as
Python.
 PythonWin features a color editor, a new and
improved debugger, interactive shell window,
COM (component object extensions and more.

CONTINUE..
Python extension is .py
 IDLE –also available on the Windows platform
 In windows, it look similar to Unix
 From Windows, IDLE can be found in the
Lib\idlelib subdirectory, where Python
interpreter is found, c:\Python2x.
 To start IDLE from a DOS command window,
invoke idle.py
 To start IDLE from a Windows environment, just
double click on idle.pyw.

CONTINUE..
Files ending with .pyw will not open a DOS
command window to run the script on it instead
it is created a shortcut to
c:\Python2x\Lib\idlelib\idle.pyw
 Other IDEs and execution environment

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Open source
IDLE –http://python.org/idle/
 PythonWin +Win32 Extensionshttp://startship.python.net/crew/skippy/win32
 Etc……

WRITING A SIMPLE PYTHON PROGRAM
Prompt in Python “>>>”  primary prompt
“…” secondary prompt
 Primary prompt – interpreter is expecting the
next python statement
 Secondary prompt – interpreter is waiting for
additional input to complete the current
statement.
 There are two primary ways that Python “does
things”: 1. statements
2: expression – functions, equations, etc
 A statement is a body of control which involved
using keywords.

CONTINUE..
Statements may/may not lead to a result or
output.
 Example use print statement to print Hello
World

>>> print ‘Hello World!’
Output – Hello world
Expressions – do not use keywords.
 Example of equations- simple equations that
comes together with mathematical operators, can
be a functions which are called with parentheses.
 They may/may not take input, and they may/may
not return a meaningful value.

PROGRAM OUTPUT, THE PRINT
STATEMENT, AND “HELLO WORLD!”
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>>> myString = ‘Hello World!’
>>> print myString
Hello World!
>>> myString
‘Hello World!’
In this example, use a string variable, then use print to
display its contents.
Given only the name reveals quotation marks around the
string.
Why? This is to allow objects other than strings to be
displayed in the same manner as this string.
Means that using the name of the variable, you can
print/display a printable string representations of any
object not just strings.
CONTINUE..
Python’s print statement, paired with the string
format (%), supports string substitution similar
to printf() function in C.
>>> print “%s is number %d!” % (“python”,1)
Python is number 1
 %s means to substitute a string while %d
indicates an integer should be substituted.
 %f is for floating point numbers.
 The print statement also allows its output
directed to a file. (will look into this matter later)

CONTINUE..
Program input and the raw_input() built-in
function.
raw_input – to get user input from the command
line.
 read from standard input and assigns the string
value to the variable you designate.
 fucntion int() can be use to convert any numeric
input string to an integer representation.
EXAMPLE
>>> user=raw_input(‘Enter login name: ‘)
Enter login name: root
>>> print ‘Your login is :’, user
Your login is: root

This example is using text input
EXAMPLE 2- A NUMERIC STRING INPUT
>>> num=raw_input (‘Now enter a number:’)
Now enter a number:1024
>>> print ‘Doubling your number: %d’
%(intnum)*2)
Doubling your number :2048

The int () function converts the string num to an
integer so that the mathematical operation can
be performed.
COMMENTS
Used the symbol (#) (hash or pound ) to indicate
comments.
 It begin with (#) and continue until the end of
line.
>>># one comment
print ‘Hello world!’ # another comment
Hello World!
 Special comments called documentation strings
or “doc strings”
 Unlike regular comments, doc strings can be
accessed at runtime and used to automatically
generate documentation.

OPERATORS-STANDARD MATHEMATICAL
Operators-symbol
Operators-name
+
Addition
-
Subtraction
*
Multiplication
/
Division(classic division)
%
Modulus
//
For floor division (rounds down
to the nearest whole number)
**
Exponentiation
An operation that raises some given constant (base)
to the power of other number (exponent).
CONTINUE..
Classic division means that if the operands are
both integers, it will perform floor division
 For floating numbers, it represents true division.
 If true division is enabled, then the division
operator will always perform that operator,
regardless of operand type.
 More on this later.

OPERATOR PRECEDENCE
**
Unary +/%
//
/
*
+/_
>>> print -2*4 +3**2
1
In this example, 3**2 is calculated first followed by (-2*4).
STANDARD COMPARISON OPERATORS

This operators return a Boolean value indicating
the truthfulness of the expression.
Operators
(symbol)
<
<=
>
>=
==
!=
<>
EXAMPLE
>>> 2 < 4
True
>>> 2 == 4
False
>>> 2 > 4
False
6.2 <= 6.2
True
>>> 6.2 <= 6.20001
True
CONJUNCTION OPERATORS
Operators
and
or
not
• can use these operations to chain together arbitrary expressions and
logically combine the Boolean results:
>>> 2 < 4 and 2 == 4
False
>>> 2 > 4 or 2 < 4
True
>>> not 6.2 <= 6
True
>>> 3 < 4 < 5
True
>>> 3 < 4 < 5 is a short way of saying >>> 3<4 and 4<5
VARIABLES AND ASSIGNMENTS
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Variables in Python is similar to other high level
languages.
Variables are simply identifier names with an alphabetic
first character , upper or lowercase letters including the
underscore (_).
Other alphanumeric or underscore.
Python is case sensitive means that “CASE” is not same as
“case”.
Python is dynamically typed means that no pre-declaration
of a variable or its type is necessary.
The type (and value) are initialized on assignment.
Assignments are performed using the equal sign.
EXAMPLE
>>> counter =0
>>> miles =1000.0
>>> name =‘Bob’
>>> counter = counter +1
# the integer
# the floating point
# string
#an incremental statement for
integer
>>>kilometers = 1.609*miles #floating point operation and
assignment
>>> print “%f miles is the same as %f km’ %(miles,kilometers)
1000.000000 miles is the same as 1.609.000000 km
In this example, there are 5 variable assignments.
CONTINUE..
Python also supports augmented assignments –
statement that both refer to and assign values to
variables.
Example
n =n *10  shortcut n *= 10
 Python does not support increment/decrement
operators like in C : n++ /--n. Because +, -- are
also unary operators, Python will interpret --n as
-(-n) == n and the same is true for ++n.

NUMBERS

Python support 5 basic numerical types
int
0101, 84, -247, 0x80,017,-982
long
29979064258L,DECADEDEBEEF
bool
True, false
float
3.1412,4.2E-10,-90. , 6.022
complex
6.23+1.5j, -1.23-875J
Boolean values are a special case of integer.
If put in a numeric context such as addition with other numbers,
true is treated as the integer with value 1 and false as value 0.
Complex number , number that involve the square root of -1, so
called “imaginary” numbers.
More on numbers in later chapter
STRINGS
String in Python are identified as a contiguous
set of characters in between quotation marks.
 Python allows 8 pairs of single /double quotes.
 Triple quotes (three consecutive single/double
quotes) can be used to escape special characters.
 Subsets of strings can be taken using the index
([ ] ) and slice ([ : ]) operators, which works with
indexes starting at 0 in the beginning of the
string and working their way from -1 at the end.
 The plus (+) sign is the string concatenation
operator, and the asterisk (*) is the repetition
operator.

EXAMPLE
>>> pystr = ’PYTHON’
>>> iscool = ‘is cool!’
>>> pystr[0]
‘P’
>>> pystr[2:5]
‘tho’
# the output start at 2 up to 4 not include the character at end.
>>> iscool[:2]
Counting forward
'is‘
>>> pystr + iscool
0
1
2
3
4
5
‘PYTHON is cool!'
P
Y
T
H
O
N
>>> pystr*2
-6 -5 -4
-3 -2
-1
‘PYTHONPYTHON'
>>> iscool[-1]
'!‘
More on string in coming chapter
Counting backward
LISTS AND TUPLES
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Lists and tuples can be though of as generic “ arrays” with
which to hold an arbitrary number of arbitrary Python
objects
The items are ordered and accessed via index offsets,
similar to array except that lists and tuples can store
different types of objects.
Lists are enclosed in brackets ([ ]) and their elements and
size can be changed.
Tuples are enclosed in parentheses (()) and cannot be
updated.
Tuples can be thought of for now as “read-only” lists.
Subsets can be taken with the slice operator ([] and [ : ] in
the same manner as string.
EXAMPLES
>>> alist =[1,2,3,4]
>>> alist
[1, 2, 3, 4]
>>> alist[0]
1
>>> alist[2:]
[3, 4]
>>> alist[:3]
[1, 2, 3]
>>> alist[1]=5
>>> alist
[1, 5, 3, 4]
EXAMPLES
>>> atuple=('robots',77,93,'try')
>>> atuple
('robots', 77, 93, 'try')
>>> atuple[:3]
('robots', 77, 93)
>>> atuple[1]=5 # not allowed to change the element in tuple
Traceback (most recent call last):
File "<pyshell#25>", line 1, in <module>
atuple[1]=5
TypeError: 'tuple' object does not support item assignment
CODE BLOCKS

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Code blocks are identified by indentation rather than using
symbols like {}
Indentation clearly identifies which block of code a statement
belongs to.
if statement
if expression :
if _suite
If
the expression is non-zero or true, then the statement if_suite is
executed, otherwise execution continues on the first statement after.
Suite
is the term used in python to refer to a sub-block of code and
can consists of single or multiple statements.
Example:
>>> if x < .0:
print '"x must be at least 10!'
CONTINUE..

Python also support an else statement that is
used with if :
if expression:
if_suite
else:
else_suite

Python has an “else-if” spelled as elif :
if expression1:
if_suite
elseif expression2:
elif_suite
else:
else_suite
WHILE LOOP
while expression:
while_suite

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The statement while_suite is executed continuously in a loop until
the expression becomes zero or false.
Execution then continues on the first succeeding statement.
counter =0
while counter <3:
print 'loop #%d'% (counter)
counter +=1
loop #0
loop #1
loop #2
FOR LOOP

Python’s for takes an iterable (such as sequence
or iterator) and traverses each element once.
>>> print 'i like to use the internet for:'
i like to use the internet for:
>>> for item in ['e-mail','net-surfing','homework','chat']:
print item
e-mail
net-surfing
homework
chat

More on if and loops in coming chapter