scripting in Python

OUTLINE
• Why Use Python?
• Running Python
• Types and Operators
• Basic Statements
• Functions
• Some programs
• Industry applications

WHY PYTHON?
 Python is a general-purpose, interpreted high-level programming
language
 It's free (open source)
Downloading and installing Python is free and easy
Source code is easily accessible
Free doesn't mean unsupported! Online Python community is huge
 It's portable
Python runs virtually every major platform used today
It just need you to have a compatible Python interpreter installed
 It's powerful
Dynamic typing
Built-in types and tools
Library utilities
Automatic memory management

Running Python
 $ python
 print 'Hello world'
 Hello world
# Relevant output is displayed on subsequent lines without the
>>> symbol
 >>> x = [0,1,2]
# Quantities stored in memory are not displayed by default
 >>> x
# If a quantity is stored in memory, typing its name will display it
 [0,1,2]
 >>> 2+3
 5
 >>> # Type ctrl-D to exit the interpreter
 $

 Suppose the file script.py contains the following
lines:
 print 'Hello world'
 x = [0,1,2]
To execute the script
 $ python -i script.py
 Hello world
 >>> x
 [0,1,2]
 >>>
 # “Hello world” is printed, x is stored and can be called later, and the
interpreter is left open
 >>> import script
 Hello world
 >>> script.x
 [1,2,3]

Types and operators: operations on
numbers
 Basic algebraic operations
Four arithmetic operations: a+b, a-b, a*b, a/b
Exponentiation: a**b
 Comparison operators
Greater than, less than, etc.: a < b, a > b ,a <= b, a>= b
Identity tests: a == b, a != b
 Bitwise operators
Bitwise or: a | b
Bitwise exclusive or: a ^ b # Don't confuse this with
exponentiation
Bitwise and: a & b
Shift a left or right by b bits: a << b, a >> b

Types and operators: strings and
operations
 Strings are ordered blocks of text
Strings are enclosed in single or double quotation marks
Examples: 'abc', “ABC”
 Concatenation and repetition
Strings are concatenated with the + sign:
>>> 'abc'+'def'
'abcdef'
Strings are repeated with the * sign:
>>> 'abc'*3
'abcabcabc'

 Indexing and slicing, contd.
s[i:j:k] extracts every kth element starting with index i
(inclusive) and ending with index j (not inclusive)
>>> s[0:5:2]
'srn'
Python also supports negative indexes. For example, s[-1]
means extract the first element of s from the end.
>>> s[-1]
'g‘
>>> s[-2]
'n‘
>>> s.upper()
STRING

 Indexing and slicing
Python starts indexing at 0. A string s will have indexes running
from 0 to len(s)-1 (where len(s) is the length of s) in
integer quantities.
s[i] fetches the i th element in s
>>> s = ‘string'
>>> s[1] # note that Python considers 't' the first element
't' # of our string s
s[i:j] fetches elements i (inclusive) through j (not inclusive)
>>> s[1:4]
'tri'
s[:j] fetches all elements up to, but not including j
>>> s[:3]
'str'
s[i:] fetches all elements from i onward (inclusive)
>>> s[2:]
'ring'

Types and operators: Lists
 Basic properties:
Lists are contained in square brackets []
Lists can contain numbers, strings, nested sublists, or nothing
Examples: L1 = [0,1,2,3], L2 = ['zero', 'one'],
L3 = [0,1,[2,3],'three',['four,one']], L4 = []
List indexing and slicing works just like string indexing

 Some basic operations on lists:
Indexing: L1[i], L2[i][j]
Slicing: L3[i:j]
Concatenation:
>>> L1 = [0,1,2]; L2 = [3,4,5]
>>> L1+L2
[0,1,2,3,4,5]
Repetition:
>>> L1*3
[0,1,2,0,1,2,0,1,2]
Appending:
>>> L1.append(3)
[0,1,2,3]
Sorting:
>>> L3 = [2,1,4,3]
>>> L3.sort()

 More list operations:
Reversal:
>>> L4 = [4,3,2,1]
>>> L4.reverse()
>>> L4
[1,2,3,4]
Shrinking:
>>> del L4[2]
Making a list of integers:
>>> range(4)
[0,1,2,3]
>>> range(1,5)
[1,2,3,4]

Types and operators: arrays
 Similarities between arrays and lists:
Arrays and lists are indexed and sliced identically
Arrays and lists both have sort and reverse attributes
Differences between arrays and lists:
With arrays, the + and * signs do not refer to concatenation or repetition
Examples:
>>> ar1 = array([2,4,6])
>>> ar1+2 # Adding a constant to an array adds the constant to each
term
[4,6,8,] # in the array
>>> ar1*2 # Multiplying an array by a constant multiplies each term
in # the array by 2
[4,8,12,]

Contd:
Adding two arrays is just like adding two vectors
>>> ar1 = array([2,4,6]); ar2 = array([1,2,3])
>>> ar1+ar2
[3,6,9,]
Multiplying two arrays multiplies them term by term:
>>> ar1*ar2
[2,8,18,]
Same for division:
>>> ar1/ar2
[2,2,2,]
Assuming the function can take vector arguments, a function
acting on an array acts on each term in the array
>>> ar2**2
[1,4,9,]

Basic Statements: The if statement
 If statements have the following basic structure:
 # inside the interpreter # inside a script
 >>> if condition: if condition:
... action action
...
>>>
4 spaces
>>> x=1
>>> if x< 2:
…. Print ‘Hello world’
….
Hello world

Basic Statements: While statement
 >>> while x < 4 :
…. Print x**2 # square of x
…. x = x+1
….
1
4
9 # sqaure of 4 is not there as 4 is not included

Basic Statements: For statement
 For statements have the following basic structure:
for item i in set s:
action on item i
 # item and set are not statements here; they are merely intended to
clarify the relationships between i and s
 Example:
>>> for i in range(1,7):
... print i, i**2, i**3, i**4
...
1 1 1 1
2 4 8 16
3 9 27 81
4 16 64 256
5 25 125 625
6 36 216 1296

 Example 2 :
>>> L = [0,1,2,3] # or, equivalently, range(4)
>>> for i in range(len(L)):
... L[i] = L[i]**2
...
>>> L
[0,1,4,9]

Functions
 Usually, function definitions have the following basic structure:
 def func(args):
return values
 >>> def f1(x):
... return x*(x-1)
...
>>> f1(3)
 def f2(x,y):
... return x+y,x-y
...
>>> f2(3,2)
(5,1)

 def avg (a,b):
return (a+b) /2
>>> avg(1,1)
1
 def f3():
 ... print 'Hello world'
 ...
 >>> f3()
 Hello world

Note:
 >>> a = 2 # a is assigned in the
interpreter, so it's global
 >>> def f(x): # x is in the function's
argument list, so it's local
 ... y = x+a # y is only assigned inside
the function, so it's local

Programs:
# A program to covert temperature from
Celsius to Fahrenheit
def main():
... Celsius = input("What is the Celsius temperature? ")
... Fahrenheit = (9.0 / 5.0) * Celsius + 32
... print "The temperature is", Fahrenheit, "degrees
Fahrenheit."
...
>>> main()

 A program to compute the value of an investment
carried 10 years into the future
vi inv.py
def main():
print "This program calculates the future value
print "of a 10-year investment."
principal = input("Enter the initial principal: ")
apr = input("Enter the annual interest rate: ")
for i in range(10):
principal = principal * (1 + apr)
print "The value in 10 years is:", principal
$ python –i inv.py
>>> main()

Industry applications:
 web programming (client and server side)
 ad hoc programming ("scripting")
 steering scientific applications
 extension language
 database applications
 GUI applications
 education

Who is using it?
 Google (various projects)
 NASA (several projects)
 Industrial Light & Magic (everything)
 Yahoo! (Yahoo mail & groups)
 Real Networks (function and load testing)
 RedHat (Linux installation tools)

scripting in Python

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