ch 12d_python fundamentals

8/8/2019 Ch 12d_Python Fundamentals

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Python Fundamentals

Scripting Languages


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References

http://www.linuxjournal.com/article/3882 Why Python? by EricRaymond in the Linux Journal, May 1st 2000.

http://www.tcl.tk/doc/scripting.html Scripting: Higher Level

Programming for the 21st Century, John K. Ousterhout, IEEEComputer, March 1998.

http://www.geeksaresexy.net/2008/07/25/the-ascent-of-scripting-languages/ The ascent of scripting languages by Chip. July 25,2008. From the site Geeks are Sexy

Python tutorial:

Python web page: http://www.python.org/


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What is a Scripting Language?

Wikipedia: A scripting language, script

language orextension language, is a

programming language that controls asoftware application. "Scripts" are often

treated as distinct from programs", which

execute independently from any other

application.


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Early Scripting Languages

Controlled the actions of system programs

Job Control Language (JCL):controlled batch job

executions on IBM mainframes

Shell scripts: A series of instructions to the shell,or command line interpreter, of a UNIX system

When used like this, scripting languages take

pre-existing components, maybe in different

languages, and combine them so they work

together.


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Example Shell Script

from Ousterhout select | grep scripting | wc

where

selectis a program to read and output the text

that is currently selected on the display grep is a program reads input and outputs the

lines containing "scripting";

wcis a program to count the lines in its input.

| is the pipe command that directs the outputfrom one program to the input of the nextprogram.


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Modern Scripting Languages

Perl, Python, Ruby,

More like a traditional programming

language than early scripting languages. Higher level in the sense that programmers

dont have to worry about many of the

details that are a part of C-like languages

No explicit memory allocation/deallocation, for

example


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Modern Scripting Languages

Its possible to write full-featured programs

in these languages

Used frequently today for quick-and-dirtyprogramming

Quick results

Small programs

See next definition


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What is a Scripting Language?

"scripting language." Encyclopdia Britannica. 2008.Encyclopdia Britannica Online. 30 Sep. 2008.

Scripting languages are sometimes called littlelanguages. They are intended to solve relatively smallprogramming problems that do not require the overheadof data declarations and other features needed to makelarge programs manageable. Scripting languages are

used for writing operating system utilities, for special-purpose file-manipulation programs, and, because theyare easy to learn.


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Scripts are Usually Interpreted

Using an interpreter instead of a compiler

makes sense when programs change

frequently and/or are very interactive.

Reason: no extra compile time

In the scripting language-as-glue-language

mode, performance is dominated by the

modules being connected by the scripting

commands


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Why Python?

No universal agreement on which scriptinglanguage is best

Perl, Ruby, others also have advocates

Perl: good for system administrationduties, traditional scripting

Awkward syntax, OO as an afterthought, less

readable than other languages Python has attracted many former Perl

users


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Python - Intro

Python is a general purpose language that

implements the imperative, object-oriented, and

functional paradigms.

Dynamic typing, automatic memory management,exceptions, large standard library, modular.

Extensions can be written in C and C++

Other language versions (Jython, IronPython) support

extensions written in Java and .Net languages)

Design philosophy: easy to read, easy to learn


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Versions

Production versions are 2.6.4 and 3.1.1

Both versions are stable and suitable for use

Python 2: compatible with more existing

software Python 3: a major redesign Not backward compatible.

Most features are the same or similar, but a few willcause older programs to break.

Part of the Python philosophy dont clutter up thelanguage with outdated features

Write your programs to run in the CS lab (v 3.1).


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Interesting Features

White space does indicate meaning

Instead of curly brackets or begin-end pairs,

whitespace is used for block delimiters.

No variable declarations

Dynamic typing

Associative arrays (dictionaries) Lists and slices


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Execution Modes

Calculator mode: type in an expression,Python will evaluate it:

>>> a = 5>>> b = 10

>>> a+b15

Dynamic change of type>>> a = 'horse>>> a = cart>>> a + b

horse cart


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Execution Modes - Program

#factorial.py# compute the factorial of a number

def main( ):

n = input(enter a whole number)

fact = 1

for factor in range (n, 1, -1)

fact = fact * factor

print thefactorial o

f, n, is,

fact

main( )


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Program Elements

Identifiers:

Must begin with letter or underscore, followed by any

number of letters, digits, underscores

Variables: Do not need to be declared

A variable is created when a value is assigned to it:Examples: num = 3

Cant be used in an expression unless it has a value

Error message: Name Error means no value is

associated with this name


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Variables

Variable names dont have types values

(or objects) do

A variable name has the type of the value itcurrently references

Variables actually contain references to

values (similar to pointers).

This makes it possible to assign different

object types to the same variable


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A 3.5A 3.5

7.0

A = A * 2

A = cat

cat

Python handles memory management automatically. It will

create new objects and store them in memory; it will also

execute garbage collection algorithms to reclaim anyinaccessible memory locations.

Python does not implement reference semantics; if A = 10 and

B = A, A = A + 1 does not change the value of B

Variables contain references to data values


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Expressions

An expression calculates a value

Arithmetic operators: +, -, *, /, **

(exponentiation) Add, subtract, multiply, divide work just as

they do in other C-style languages

Spaces in an expression are not significant Parentheses override normal precedence


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Output Statements

Statement syntax: (EBNF notation)print {expression,}

where the final comma in the list is optional

Examples:print # prints a blank lineprint 3, y, z + 10 # go to new lineprint the result is z,# no new line

Output items are automatically separated by asingle space.


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Assignment Statements

Syntax: Assignment variable = expression A variables data type is determined by the type of the value

assigned to it.

Multiple_assign var{, var} = expr{, expr)>>> x, y = 4, 7

>>> x

4

>>> y

7

>>> x, y = y, x

>>> x

7

>>> y

4

>>>


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Input

Syntax: input variable = input(string)The string is used as a prompt.

>>> y = input("enter a name ")

enter a name "max"

>>> y'max'

>>> number = input("Enter an integer ")

Enter an integer 32

>>> number

32>>> thing = input("enter an expression ")

enter an expression 3 * 8 / 4

>>> thing

6


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Input

The input statement can be part of a multipleassignment statement:

>>> x, y = input("enter two comma-separated

values: ")

enter two comma-separated values: 13, 26

>>> x

13

>>> y

26


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Input

The raw_input function is another way to

read string data

No need to quote the input strings

>>> s = raw_input("Enter a sentence ")

Enter a sentence Python is easy!

>>> s

'Python is easy!'

>>>


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Raw_input versus input

raw_inputreads everything as a string

Using raw_inputis a safety measure for

reading data use a cast to get correcttype, if necessaryr = float(raw_input(enter radius))

Now, if user enters something inappropriate,Python will throw an exception.


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Python Control Structures

Python loop types:

while

for

Decision statements: if

Related features:

range( )#

af

unction break # statements similar to

continue # those in C/C++


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General Information

The first statement in the body of a loop

must be indented.

All other statements must be indented bythe same amount

To terminate a loop body, enter a blank

line.


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While Loop

>>> x = 0>>> while (x < 10): #remember semicolon!

print x, #prints on one linex = x + 1

0 1 2 3 4 5 6 7 8 9

Conditions are evaluated just as in C/C++: 0 is

false, non-zero is trueThe conditional operators are also the same

Note indentation provided by the IDLE GUI


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For Loops

Syntax:

for in :

can be a list of values or itcan be defined by the range( ) function

range(n) produces a list of values: 0, 1, ,n-1

range(start, n): begins at start instead of 0 range(start, n, step): uses step as the

increment


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>>> for i in range(3):print i,

0 1 2>>> for i in range(5,10):

print i,

5 6 7 8 9>>> for i in range(6,12,2):

print i

68

10


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Ranges can also be specified usingexpressions:

>>> n = 5

>>> for i in range(2*n +3):print i,

0 1 2 3 4 5 6 7 8 9 10 11 12


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Using a List in a for loop

Lists are enclosed in square brackets

>>> for i in [3, 2, 1]:

print i,

3 2 1>>> for i in ['cat', 'dog', 'elephant']:

print i,

cat dog elephant


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If Statement

Python if statement has three versions: The if (only one option)

The if-else (two options)

The if-elif-else (three or more options)

The if-elif-else substitutes for theswitch statement in other languages.

Each part must be followed with asemicolon and whitespace is used as thedelimiters.


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If-elif-else Statement

x = input("enter an integer: )

if x < 0:

print Negative

elif x == 0:print 'Zero'

elif x == 1:

print 'Single'else:

print 'More'


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Strings

String literals consist of characters enclosedin quotation marks (single or double)

Use raw_input instead ofinput

Individual characters are referenced by theirindex (indexes start at 0)

Negative indexes work from right to left:

>>> myName = "Joe Smith>>> myName[-3]

'i'


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String Operations

Slicing: selects a slice or segment of a string[:]>>> myName[2:7]'e Smi

If eitheroris omitted, the startand end of the string are assumed>>> myName[:5]

'Joe S'

>>> myName[4:]'Smith'

>>> myName[:]

'Joe Smith'


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String Operations

Concatenation (+)>>> "happy" + "birthday"

'happybirthday'

>>> 'happy' + ' birthday'

'happy birthday

Repetition (*)>>> word = 'ha'

>>> 3 * word

'hahaha'

>>> word + 3 * '!'

'ha!!!'


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String Operations

Other examples:>>> len(word) # length function2

>>> len("ham and eggs")12>>> for ch in myName:

print ch,

J o e S m i t h


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Lists

A list is a comma-separated sequence of

items, enclosed in square brackets

Lists can be heterogeneous items donthave to be from the same data type

Like strings, lists can be sliced, indexed,

concatenated, and repeated.

The len() function will return the number

of elements in the list


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>>> myList = ['milk','eggs','bread',10]

>>> myList[0:3]

['milk', 'eggs', 'bread']

>>> x, y, z = 3, 5, 9

>>> yourList = [x, y, z, "018"]

>>> yourList[0:2][3, 5]

>>> 2 * myList[1:2]

['eggs', 'eggs']

>>> len(myList)4>>> yourList[:8] #try to exceed length[3, 5, 9, '018']


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Lists

Unlike strings, lists are mutable (can be

changed)

Make an assignment, using the index

>>> myList[1] = 'butter'

>>> myList

['milk', 'butter', 'bread', 100]

You can also assign to slices, and even

change the length of the list


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List Slice Operations#create a list>>> data = ['bob', 32, 'sue', 44]>>> data['bob', 32, 'sue', 44]#assign to a list slice

>>> data[1:3] = ['dave', 14]>>> data['bob', 'dave', 14, 44]#insert an element>>> data[1:1] = [19]

>>> data['bob', 19, 'dave', 14, 44]#delete an element>>> data[3:4] = []>>> data

['bob', 19, 'dave', 44]


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Sequences

Strings and lists are both sequences.

Sequence operations include

+ concatenation * (int-exp> repetition

[ ] indexing

[ : ] slicing

len() length

for in iteration


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Nested Lists

>>> grades = [100, 97, 85]

>>> stRec = ['A000','jack',grades]

>>> stRec['A000', 'jack', [100, 97, 85]]

>>> len(stRec)

3


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More About Lists

See Section 5.1 in the Python tutorial to

get a whole set of list functions:

append(x)

insert(i, x)

etc.

Since lists are objects, dot notation is used

to call the functions


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More Examples

>>> stRec.remove(grades)>>> stRec['A000', 'jack']

>>> stRec.append([100, 97, 85])>>> stRec['A000', 'jack', [100, 97, 85]]>>> stRec.pop(2)

[100, 97, 85]>>> stRec['A000', 'jack']


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Dictionaries

Also called associative arrays

Resemble hash tables

Unlike sequences (lists, strings) which areindexed sequentially, dictionaries are an

unordered collection of key-value pairs.

Items are retrieved according to their keys.

See Section 5.5 in the tutorial for more

examples


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>>> roll = {1023: 'max', 0404: 'sue'}>>> roll[1023]

'max'>>> roll[9450] = 'alice'>>> roll{9450:'alice', 260:'sue', 1023:'max'}

>>> roll.keys()[9450, 260, 1023]>>> roll.has_key(2600)False

>>>


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Python Functions

A Python function can be either void orvalue returning, although the definition isntidentified as such.

Technically, they are all value returning, sincea void function returns the value none

Each function defines a scope. Parametersand local variables are accessible in thescope; values in other functions areavailable only as parameters.


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Function Syntax

Function definition

def (formal-parameters>):

Function call syntax:

.()


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>>> def square(x):return x * x

#the blank line terminates the function#definition>>> square(10)100>>> z = 23>>> square(z * 4)8464>>> square(z)

529

This function was typed in at the command line.

Later we will see how to use files and build

traditional programs.


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A main program file

#File: chaos.py

#A simple program illustrating chaotic behavior

def main( ):

print "a chaotic function"x = input("enter a number between 0 and 1: ")

for i in range (10):

x = 3.9 * (1 - x)

print x

y = input("Press any key to exit ")

main()


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Functions That Return Values

>>> def sum(x, y):

sum = x + y

return sum

>>> num1,num2 = 3,79

>>> sum(num1, num2)

82

>>> def SumDif(x,y):

sum = x + y

dif = x - y

return sum, dif

>>> a,b = SumDif(x,y)

>>> a

17

>>> print a, b

17 -3

>>>


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Parameters

Parameters are passed by value, so ingeneral the actual parameters cant bechanged by the function only their local

copies Mutable objects (e.g., lists) act like

reference parameters

the value passed is a pointer to a list ofpointers, and these pointers remain the same;only the things pointed to change!

(dont worry about the subtleties here)


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Lists and Strings Review

Strings are sequences of characters, lists

are sequences of individual items

aStr: 47^abc

aList: [sue, 479, 3.7, tree]

String elements and list elements can be

accessed by an index

String elements cant be changed in place;

list items can be.


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Common Operations

Slicing Concatenation & repetition

Indexing

Applying the length function : len(aList) or len(aStr)

Iteration through elements >>> for i in range(len(myStr)):

print myStr[i] >>> for i in range(len(myStr)):

print myStr[i]


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Additional String Operations

split: divides a list into a list of substrings >>> myStr = 'The fat black cat>>> myStr.split()

['The', 'fat', 'black', 'cat'] split defaults to blank as the delimiter,

but you can specify a different character:>>> myStr = '12/10/2008'

>>> myStr.split('/')

['12', '10', '2008']


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Strings Continued

When using string functions begin byimporting the string library: import string

After you have split a string into a list ofsubstrings, you may want to convert someof the substrings to specific data types. Thegeneric function is eval() and thereare specific casts: int( ), float( ), long( ), andstr( ).


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Example

>>> x = myStr.split('/')

>>> x

['12', '10', '2008']

>>> print eval(x[0]),int(x[1]),float(x[2])

12 10 2008.0


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File I/O

= open(, )

where name is the name of the file on disk, andmode is either r or w

infile = open(data.in, r)

outfile = open(data.out, w)

Files must also be closed:

infile.close( )

outfile.close( )


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Input Operations

.read: returns the remaining

contents of the file as a multi-line string (lines in

the file separated by \n)

.readline( ): returns next lineas a string; includes the newline character (you

can slice it off, if needed)

.readlines( ): returns the

remaining lines in the file, as a list of lines (eachlist item includes the newline character)


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Input/Output

Output operations are less flexible, the

only parameter is a string.

If you want to put a newline character in the

file you must do so specifically

Output formatting: % character

% ()

Possible formats are d (decimal), f (float)

and s (string)


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Example

>>> outfile=open("C:\Temp\exmp.out","w")

>>> outfile.write("this is the first

line\n")>>> outfile.write('and this is thesecond\n')

>>> outfile.write("the value of x is%d and y is %f" % (x, y))

>>> outfile.close()

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