Shell Programming & Scripting Languages

Dr. K. Sasidhar

UNIT – IV Contents Introduction to python language python-syntax statements functions, Built-in-functions and Methods Modules in python Exception Handling

UNIT IV Outcomes From the IV unit Student can

Understand python programming features and its statements Write and execute programs with basic concepts, functions,

Modules and Exception Handling. Analyze the problem statements and can write solution in the

form of a python program. Arrives to a conclusion about programming languages

principles by comparing python with other Programming languages already he/she learnt in the previous semesters.

Python

Python was developed by Guido van Rossum in the late

eighties at the National Research Institute for Mathematics

and Computer Science in Netherlands.

Python is derived from many other Programming

languages including, C, C++, Algol-68, Smalltalk, Unix

shell and other scripting languages

Why Python? Python is object-oriented

Structure supports Encapsulation, polymorphism, Abstraction, multiple inheritance

It's open source, so downloading and installing Python is free and easy

Source code is easily accessible Online Python community is huge to support the Python

programmers It's portable Python runs on every major platform

Python programs will run in the same manner, irrespective of platform

It's powerful Dynamic typing Built-in types and tools Library utilities Third party utilities (numeric, NumPy, SciPy) Automatic memory management Can design data structures, web applications, Database

programs, games etc…

Why Python?

Python's library is very portable and cross-platform compatible on UNIX, Windows, and Macintosh.

Support for an interactive mode which allows interactive testing and debugging the code.

Python can be linked to components written in other languages easily

Linking to fast, compiled code is useful to computationally intensive problems

Python is good for code steering and for merging multiple programs

Why Python?

WARP is implemented in a mixture of Python and Fortran Python programs are compiled automatically to an

intermediate form called bytecode, which the interpreter then reads

This gives Python the development speed of an interpreter without the performance loss inherent in purely interpreted languages

It's easy to learn also. Structure and syntax are pretty intuitive and easy to grasp

Why Python?

Running Python

Unix:  IDLE is the very first Unix IDE for Python. Windows: PythonWin is the first Windows interface for

Python and is an IDE with a GUI. Macintosh: The Macintosh version of Python along

with the IDLE IDE is available from the

main website, downloadable as either

MacBinary or BinHex'd files.

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source code: The sequence of instructions in a program. syntax: The set of legal structures and commands that can be used

in a particular programming language. output: The messages printed to the user by a program. console: The text box onto which output is printed. Some source code editors pop up the console as an external

window, and others contain their own console window.

Programming basics

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Compiling and interpreting

Many languages require you to compile (translate) program into a form that the machine understands.

Python is instead directly interpreted into machine instructions.

compile execute

outputsource codeHello.java

byte codeHello.class

interpret

outputsource codeHello.py

Reserved Words

And exec Not Assert finally or Break for pass Class from

print Continue global raise def if return del import try

elif in while else is with except lambda yield

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Expressions expression: A data value or set of operations to compute a value.

Examples: 1 + 4 * 3 Arithmetic operators we will use: + - * / addition, subtraction/negation, multiplication, division % modulus, a.k.a. remainder ** exponentiation precedence: Order in which operations are computed. * / % ** have a higher precedence than + -

1 + 3 * 4 is 13

Parentheses can be used to force a certain order of evaluation.

(1 + 3) * 4 is 16

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Real numbers Python can also manipulate real numbers.

Examples: 6.022 -15.9997 42.0 2.143e17 The operators + - * / % ** ( ) all work for real numbers.

The / produces an exact answer: 15.0 / 2.0 is 7.5 The same rules of precedence also apply to real

numbers:Evaluate ( ) before * / % before + -

When integers and reals are mixed, the result is a real number. Example: 1 / 2.0 is 0.5

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Math commands (import math) Python has useful commands for performing calculations.

To use many of these commands, you must write the following at the top of your Python program:

from math import *

Command name Descriptionabs(value) absolute valueceil(value) rounds upcos(value) cosine, in radiansfloor(value) rounds downlog(value) logarithm, base elog10(value) logarithm, base 10max(value1, value2)

larger of two values

min(value1, value2)

smaller of two values

round(value) nearest whole numbersin(value) sine, in radianssqrt(value) square root

Constant

Description

e 2.7182818...

pi 3.1415926...

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Variables variable: A named piece of memory that can store

a value. Usage:

Compute an expression's result, store that result into a variable, and use that variable later in the program.

assignment statement: Stores a value into a variable.

Syntax: name = value Examples: x = 5 gpa = 3.14 x 5 gpa 3.14 A variable that has been given a value can be used in expressions. x + 4 is 9

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print : Produces text output on the console. Syntax: print "Message"

print Expression Prints the given text message or expression value on the

console, and moves the cursor down to the next line. print Item1, Item2, ..., ItemN Prints several messages and/or expressions on the same line. Example: print "Hello, world!" age = 45 print "You have", 65 - age, "years until retirement" Output: Hello, world! You have 20 years until retirement

print

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Reads a number from user input. You can assign (store) the result of input into a variable. Example Program:

import sys

age = int(input("How old are you? "))

print "Your age is", age

print (“No. of years for retirement”, 65-age) Output: How old are you? 53 Your age is 53 No. of years for retirement: 12

input statement

Programming Basics Programs are composed of modules Modules contain statements Statements contain expressions Expressions create and process objects

Built-in Objects Make programs easy to write Components of extensions More efficient Standard part of language

Program statements You can run programs at command prompt or IDLE

Window print(“hello world!”)

Arithmetic operations: 5+4 or print(5+4) 5-4 or print(5-4) 5*4 or print(5*4) 5/4 or print(5/4) displays 1.25 5//4 or print(5//4) displays 1 5%4 or print(5%4)

Program Statements

2**8 displays 256 Name=‘sasidhar’ Name displays sasidhar ‘sasidhar|’ * 8 displays sasidhar|sasidhar| for 8 times Import os os.getcwd() displays the current python version with its path

Python ScriptType the following program in idle and save it

as script1.py

Import sys # Load a library module

print(sys.platform) Print(2**100)X=‘Sasidhar|’print(x*8)

Core data types & Built-in Objects in Python

Object type Example literals / creation Numbers 1234, 3.145, 3+4j Strings ‘anil’, ‘a\x01c’

Lists [1, [2, ‘three’], 4]Dictionaries {‘food’: ‘spam’, ‘taste’:}Tuples (1, ‘anil’, 77, ‘ecm’)Files myfile = open(‘ecm’, ‘r’)Sets set(‘abc’), {‘a’, ‘b’, ‘c’} Other core types Booleans, types, NoneProgram unit types Functions, modules, classesImplementation types Compiled code, stacktracebacks

Strings and its operations Strings are used to record textual information as

well as bytes. s=“anil” len(s) s[0] displays a s[4] or s[-1] displays l s[1:3] displays ni s + xyz anilxyz s.find(il) found at 2 s.replace(‘an’, ‘man’)

String operations s.split splits based on delimeter s.upper() displays in capital letters. s.lower() displays in small case letters s.isalpha() tests the text. s.rstrip() removes the whitespace characters on

right side

Lists (Data Structures) Lists are ordered collections of arbitrarily

typed objects and they have no fixed size. Lists are mutable. Lists supports all sequence of operations

similar to strings. The difference is the result will be displayed in lists.

Ex: L=[13, ‘anil’, 88] L[1] displays anil L[:-1] displays anil, 88 L + [4,5,6 ] displays [13,anil,88, 4,5,6]

Operations on Lists L.append(‘kk’) adds element kk at the end

of the list L displays anilkk L.pop (2) deletes the 2nd element from

the list L.sort() sorts in ascending order L.reverse() in reverse order.

List Operations len([1,2,3]) 3 [1,2,3] + [4,5,6] [1,2,3,4,5,6] [‘Anil’] * 4 [‘Anil’, ‘Anil’, ‘Anil’, ‘Anil’] str([1,2]) + “34” [1,2]34 [1,2] + list(“34”) [1,2,’3’,’4’]

Nesting Nesting of lists is possible in python. Application is

matrix. A = [ [1,2,3], [4,5,6], [7,8,9]]

>>> A displays [[1,2,3], [4,5,6], [7,8,9] ] >>> A[1] displays [4,5,6]>>> A[1,2] displays [6]

Comprehensions Useful to process the matrix like structures. Example1: to extract 2nd column from the

matrix: Col2= [ row[1] for row in A ] Col2 displays [2,5,8]

Example2: [row[1] + 1for row in A] Example3:

[row[1] for row in A if row[1]%2 ==0] [2,8]

List built-in functions cmp(list1, list2) Compares elements of both lists. len(list) Gives the total length of the list max(list) Returns item from the list with max value. min(list) Returns item from the list with min value. list(seq) Converts a tuple into list.

Control statementsif conditionif expression: statementif-else statementif expression: statement else: statement

elif statementif expression1: statementelif expression2: statementelif expression3: statementelse: statement

Looping Statements while loop

while expression: statements

Example:count = 0 while (count < 10): print ‘Count is:', count count = count + 1

for loop for var in sequence: statements

Tuples tuple = ( 'abcd', 786 , 2.23, 'john', 70.2 ) tinytuple = (123, 'john') print tuple # Prints complete list print tuple[0] # Prints first element of the list print tuple[1:3] # Prints elements starting from

2nd till 3rd print tuple[2:] # Prints elements starting from

3rd element print tinytuple * 2 # Prints list two times print tuple + tinytuple # Prints concatenated

lists

Tuples vs. Lists Lists slower but more powerful than tuples

Lists can be modified, and they have lots of handy operations and mehtods

Tuples are immutable and have fewer features To convert between tuples and lists use the list()

and tuple() functions:li = list(tu)tu = tuple(li)

Dictionaries Python's dictionaries are kind of hash table type. They work like associative arrays or hashes and consist of

key-value pairs. A dictionary key can be almost any Python type, but are

usually numbers or strings. Values, on the other hand, can be any arbitrary Python

object. Dictionaries are enclosed by curly braces ({ }) and values

can be assigned and accessed using square braces ([]).

Dictionaries Example dict = { } dict['one'] = "This is one" dict[2] = "This is two" tinydict = {'name': 'john','code':6734, 'dept': 'sales'} print dict['one'] # Prints value for 'one' key print dict[2] # Prints value for 2 key print tinydict # Prints complete dictionary print tinydict.keys() # Prints only keys: name,code,dept print tinydict.values() # Prints all the values

Working with Time namespace import time >>> ticks = time.time() >>> print("no.of ticks since 12am, march31, 2015:", ticks) Getting current time: import time; localtime = time.localtime(time.time()) print "Local current time :", localtime Formatted time: localtime =

time.asctime( time.localtime(time.time()) )

Working with calendar namespace in python import calendar cal = calendar.month(2016, 3) print "Here is the calendar:" print cal;

Python’s Statements Statement Role Example Assignment Creating references a, *b = 'good', 'bad', 'ugly' Calls and other expressions Running functions log.write("spam, ham") print calls Printing objects print('The Killer', joke) if/elif/else Selecting actions if "python" in text: print(text) for/else Sequence iteration for x in mylist: print(x) while/else General loops while X > Y: print('hello') Pass Empty placeholder while True: pass break Loop exit while True: if exittest(): break Continue Loop continue while True: if skiptest(): continue

Python’s Statements … def Functions and methods def f(a, b, c=1, *d): print(a+b+c+d[0]) return Functions results def f(a, b, c=1, *d): return a+b+c+d[0] yield Generator functions def gen(n): for i in n: yield i*2 global Namespaces x = 'old' def function(): global x, y; x = 'new' import Module access import sys from Attribute access from sys import stdin class Building objects class Subclass(Superclass): staticData = [] def method(self): pass

Statements of Python try/except/ finally Catching exceptions try: action() except: print('action error') raise Triggering exceptions raise EndSearch(location) assert Debugging checks assert X > Y, 'X too small' with/as Context managers (2.6+) with open('data') as myfile: process(myfile)

Command Line arguments in Python import sys print ('Number of arguments:', len(sys.argv),

'arguments.‘) print 'Argument List:', str(sys.argv)

Functions

Function blocks begin with the keyword def followed by the function name and parentheses ( ( ) ).

Any input parameters or arguments should be placed within these parentheses. Can also define parameters inside these parentheses.

The first statement of a function can be an optional statement - the documentation string of the function or docstring.

The code block within every function starts with a colon (:) and is indented.

The statement return [expression] exits a function, optionally passing back an expression to the caller.

A return statement with no arguments is the same as return None.

def printme( str ): "This prints a passed string into this function" print str return

Functions

Function calling # function definition def display(str): "this prints sasidhar“ # value for str

argument print(str); return; # function calling display("hi this is sasidhar"); display("second time sasidhar");

Pass by reference All parameters (arguments) in the Python language are

passed by reference.

If you change the parameter that refers to within a function, the change also reflects back in the calling function.

Example # function definition def changelist( mylist ): # "This changes a passed list into this function"

mylist.append([1,2,3,4]); Print( "Values inside the function: ", mylist) return #call changelist function mylist = [10,20,30]; changelist( mylist ); print( "Values outside the function: ", mylist)

Output: Values inside the function: [10, 20, 30, [1, 2, 3, 4]] Values outside the function: [10, 20, 30, [1, 2, 3, 4]]

Example for passed by reference and overwritten inside the function. mylist = [1,2,3,4]; # global variable def changeme( mylist ): print "Values inside the function: ", mylist return # Now call changeme function mylist = [10,20,30]; changeme( mylist ); print "Values outside the function: ", mylist Output: Values inside the function: [10, 20, 30] Values outside the function: [10, 20, 30]

Function Arguments Required arguments Keyword arguments Default arguments Variable-length arguments

Required arguments Required arguments are the arguments passed to a

function in correct positional order. Here, the number of arguments in the function call should match exactly with the function definition.

Keyword arguments Keyword arguments are related to the function calls.

When you use keyword arguments in a function call, the caller identifies the arguments by the parameter name.

This allows you to skip arguments or place them out of order because the Python interpreter is able to use the keywords provided to match the values with parameters.

Example for keyword argument def printme( str ): "This prints a passed string into this function" print str; return; # Now call printme function printme( str = "My string");

Default arguments A default argument is an argument that assumes a default

value if a value is not provided in the function call for that argument. The following example gives an idea on default arguments, it prints default age if it is not passed:

Example def printinfo( name, age = 35 ): "This prints a passed info into this function" print "Name: ", name; print "Age ", age; return; # Now you can call printinfo function printinfo( age=50, name="miki" ); printinfo( name="miki" );

Variable Length Arguments To process a function for more arguments than specified

while defining the function.

These arguments are called variable-length arguments

An asterisk (*) is placed before the variable name that

holds the values of all non-keyword variable arguments.

Example for variable length arguments def printinfo( arg1, *vartuple ):

print "Output is: " print arg1 #“This prints variable passed arguments"

for var in vartuple: print (var) return;

# Now call printinfo function printinfo( 10 ); printinfo( 70, 60, 50 );

Anonymous functions : lambda The functions that are not declared in the standard manner by

using the def keyword are called anonymous functions. The lambda keyword is used to create small anonymous

functions. Lambda forms can take any number of arguments but return just

one value in the form of an expression. They cannot contain commands or multiple expressions.

An anonymous function cannot be a direct call to print because lambda requires an expression.

Lambda functions have their own local namespace and cannot access variables other than those in their parameter list and those in the global namespace.

Anonymous functions : lambda These functions are not like inline functions in

c++ Syntax: lambda [arg1

[,arg2,.....argn]] :expression

sum=lambda arg1, arg2: arg1+ arg2 print(" value of total:", sum(10,22)) print("value of total:", sum(20,20))

Return statement def sum( arg1, arg2 ):

total = arg1 + arg2 print "Inside the function : ", total return total

total = sum( 10, 20 ); print( "Outside the function : ", total )

Scope of variables Variables declared inside the function are

local and can be accessible with in that function

Variables declared outside the function are global and can be accessible from the calling or outside of the function.

Module A module allows to logically organize the Python code.

Grouping related code into a module makes the code easier to understand and use. A module is an object with arbitrarily named attributes that you can bind and reference. A module can define functions, classes and variables.

# Import built-in module math import math content = dir(math) print content;

Simple calculator Program Using functions """This function adds two numbers""" def add(x,y): return x + y def subtract(x, y): """This function subtracts two numbers""" return x - y def multiply(x, y): """This function multiplies two numbers""“ return x * y def divide(x, y): """This function divides two numbers""" return x / y # take input from the user print("Select

operation.") print("1.Add") print("2.Subtract") print("3.Multiply") print("4.Divide") choice = input("Enter choice(1/2/3/4):")

num1 = int(input("Enter first number: ")) num2 = int(input("Enter second number: ")) if choice == '1': print(num1,"+",num2,"=", add(num1,num2)) elif choice == '2': print(num1,"-",num2,"=", subtract(num1,num2)) elif choice == '3': print(num1,"*",num2,"=",

multiply(num1,num2)) elif choice == '4': print(num1,"/",num2,"=",

divide(num1,num2)) else: print("Invalid input")

Files I/O Reading input from keyboard:

Input, raw_input The raw_input([prompt]) function reads one

line from standard input and returns it as a string (removing the trailing newline).

Example: str = raw_input("Enter your input: "); print "Received input is : ", str

Opening files A file can be opened using open() function. Syntax:

file object = open(file_name [, access_mode][, buffering])

access mode = file has to be opened in read or write or append mode.

f1 = open(“ecm.txt", "wb") print "Name of the file: ", f1.name print "Closed or not : ", f1.closed print "Opening mode : ", f1.mode

File open modesMode Description

r Opens a file for reading only. The file pointer is placed at the beginning of the file. This is the default mode.

rb Opens a file for reading only in binary format. The file pointer is placed at the beginning of the file. This is the default mode.

r+ Opens a file for both reading and writing. The file pointer is placed at the beginning of the file.

rb+ Opens a file for both reading and writing in binary format. The file pointer is placed at the beginning of the file.

File access modesMode Description

w Opens a file for writing only. Overwrites the file if the file exists. If the file does not exist, creates a new file for writing.

wb Opens a file for writing only in binary format. Overwrites the file if the file exists. If the file does not exist, creates a new file for writing.

w+ Opens a file for both writing and reading. Overwrites the existing file if the file exists. If the file does not exist, creates a new file for reading and writing.

Mode Description

wb+Opens a file for both writing and reading in binary format. Overwrites the existing file if the file exists. If the file does not exist, creates a new file for reading and writing.

a Opens a file for appending. The file pointer is at the end of the file if the file exists. That is, the file is in the append mode. If the file does not exist, it creates a new file for writing.

a+ Opens a file for both appending and reading. The file pointer is at the end of the file if the file exists. The file opens in the append mode. If the file does not exist, it creates a new file for reading and writing.

Closing, reading and writing files close() method of file object closes the file Syntax: fileObject.close(); write() and read() methods are used to read

and write files. fileObject.read([count]); Passed parameter is the number of bytes to be read from

the opened file. This method starts reading from the beginning of the file and if count is missing, then it tries to read as much as possible, maybe until the end of file.

Write() is used to write the data into the file. fileObject.write(string); fo = open(“python.txt", "wb") fo.write( "Python is a good language.!\n"); # Close opend file fo.close()

Closing, reading and writing files

fo = open(“python.txt", "r+") str = fo.read(10); print "Read String is : ", str # Close opend file fo.close()

Closing, reading and writing files

File Positions

tell() method tells you the current position within the file seek(offset[, from]) method changes the current file position. The offset argument indicates the number of bytes to be

moved. The from argument specifies the reference position from

where the bytes are to be moved.

Example # Open a file

fo = open(“ecm.txt", "r+") str = fo.read(10); print "Read String is : ", str

# Check current position position = fo.tell(); print "Current file position : ", position

# Reposition pointer at the beginning once again position = fo.seek(0, 0); str = fo.read(10); print "Again read String is : ", str

# Close opend file fo.close()

Renaming and Deleting Files

rename() method takes two arguments, the current filename and the new filename.

os.rename(current_file_name, new_file_name) Example: import os # Rename a file from test1.txt to test2.txt os.rename( "test1.txt", "test2.txt" )

remove() Method remove() method is used to delete files by supplying the

name of the file to be deleted as the argument. os.remove(file_name) Example: import os

# Delete file test2.txt os.remove("text2.txt")

Exception Handling An exception is an event, that occurs during the execution of a

program and that disrupts the normal flow of the program Execution.

An exception is a Python object that represents an error. When an exception raises in python program, it must either

handle the exception immediately otherwise it terminates and quits.

Handling an Exception If any suspicious code that may raise an exception, then, place

the suspicious code in a try: block. After the try: block, include an except: statement, followed by

a block of code which handles the problem as elegantly as possible.

try and except blocks try: <block of statements> #main code to run except <name1>: #handler for exception <block of statements> except <name2>,<data>: #handler for exception <block of statements> except (<name3>,<name4>): #handler for exception <block of statements> except: #handler for exception <block of statements> else: # optional, runs if no exception occurs <block of statements>

Example >>>try: action() except NameError(): … except IndexError(): … except KeyError(): … except (AttributeError,TypeError,SyntaxError):… else: …. General catch-all clause: add empty except. It may catch unrelated to your code system

exceptions. It may catch exceptions meant for other handler

(system exit)

Example programdef division(x,y): try: return x/y except ZeroDivisionError: print("division by zero")

elseelse is used to verify if no exception occurred in trytry.

You can always eliminate elseelse by moving its logic at the end of the trytry block.

However, if “else statement” triggers exceptions, it would be misclassified as exception in try block.

try/else

try/finally In try/finallytry/finally, finallyfinally block is always run

whether an exception occurs or not

try: <block of statements> finally: <block of statements>

Ensure some actions to be done in any case It can not be used in the trytry with exceptexcept and

elseelse.

Example for try .. finally def division(x,y): try: return x/y except ZeroDivisionError: print("division by zero")

finally: print("it executes compulsory")

Built-in Exceptions

Built-in Exceptions

Built-in Exceptions

Raise raise triggers exceptions explicitly raise <name> raise <name>,<data> # provide data to handler raise #re-raise last exception >>>try: raise ‘zero’, (3,0) except ‘zero’: print “zero argument” except ‘zero’, data: print data Last form may be useful if you want to propagate

cought exception to another handler. Exception name: built-in name, string, user-defined

class

def result(marks ): if marks < 40: raise “fail!", marks