CS 683 Emerging Technologies

Fall Semester, 2005

Doc 1 Python Basics

Contents

 

Reading Assignment    

Running Python    

Python Built-in Types    

Python Numbers    

Strings    

Lists    

Tuples    

Sequences    

Dictionaries    

Files    

Equality    

Boolean Values    

 

Copyright ©, All rights reserved. 2005 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA. OpenContent ( http://www.opencontent.org/opl.shtml ) license defines the copyright on this document.

 

 

References

 

Python Tutorial, Guido van Rossum, http://www.python.org/doc/current/tut/tut.html

 

Python Reference Manual, Guido van Rossum, http://docs.python.org/ref/ref.html

 

Python Library Reference, Guido van Rossum, http://docs.python.org/lib/lib.html

 

Learning Python, Lutz & Ascher, O'Reilly, 1999

 

 

Reading Assignment

 

Sept 6. Chapters 1-4 of the Python Tutorial

Sept 8. Chapters 5-8 of the Python Tutorial

Sept 13. Chapters 9-11 of the Python Tutorial

 

 

Some Useful Python Web Sites

Main Python Site

http://www.python.org /

Beginner's Guide to Python

http://wiki.python.org/moin/BeginnersGuide

Beginner's Download Guide

http://wiki.python.org/moin/BeginnersGuide/Download

General Download Guide

http://www.python.org/download/

How to run a Python program under Windows

http://www.python.org/doc/faq/windows.html#how-do-i-run-a-python-program-under-windows

List of Python Introductions

http://www.python.org/doc/Intros.html

Python Tutorial, Guido van Rossum,

http://www.python.org/doc/current/tut/tut.html

Dive Into Python, Mark Pilgrim, online Book

http://diveintopython.org /

comp.lang.python, python news group

http://groups.google.com/group/comp.lang.python

 

 

 

Running Python

 

1. Interpreter

2. Script file

3. Compiled code

4. Application

 

Interpreter

Al 11-> python

Python 2.3.5 (#1, Mar 20 2005, 20:38:20)

[GCC 3.3 20030304 (Apple Computer, Inc. build 1809)] on darwin

Type "help", "copyright", "credits" or "license" for more information.

>>> 1 + 2

3

>>>

 

Instructions for Windows machines

 

 

Script File

Unix Example

File name: helloWorld.py

#! /usr/bin/env python

 

print 'Hello World'

 

Make the file executable:

Al 45->chmod u+x helloWorld.py

 

Run the file

Al 46->helloWorld.py

 

 

Note details may differ between Unix machines

 

Instructions for Windows machines

 

 

Python Built-in Types

Python Numbers

 

>>> a = 5

>>> b =2

>>> a * b

10

>>> a / 3

1

>>> a / 3.0

1.6666666666666667

>>> c = 3 + 2j

>>> a + c

(8+2j)

>>> abs(-5)

5

 

 

Numeric Operations

 

For more information see:

5. Operations on all numeric types

6. Math module

 

 

 

Variable Names

 

7. Case sen sitive

8. Must start with underscore or letter

9. May contain letters, digits or underscores cat_3

10. Can not use reserved words

 

Reserved Words

 

 

Operator Precedence

 

 

 

Strings

Immutable - once create can not change

aString = 'Cat in the hat'    #a Comment

bString = "Cat in the hat"

multiLineString = """Cat in

    the

    hat"""

 

>>> a = 'cat'

>>> b = 'dog'

>>> a + b

'catdog'

>>> a * 3

'catcatcat'

>>> a[0]

'c'

>>> a[1]

'a'

>>> len(a)

3

>>> a[-1]                #a[len(a) - 1] that is index from end

't'

>>> 'a' in a

True

>>> 'a' not in b

True

>>> min(a)                #min element in a

'a'

>>> max(b)

'o'

 

 

Slicing

 

>>> six = '012345'

>>> len(six)

6

>>> six[2:4]            #from index 2 up to index 4

'23'

>>> six[:4]                #from start up to index 4

'0123'

>>> six[3:]                #from index 3 to end

'345'

>>> six[:]                #Make a copy - from start to end

'012345'

 

>>> six[1:5:2]        #from index 1 up to 5 step 2

'13'

 

 

More on slicing

 

 

 

Formatting

 

like springf in C

 

>>> template = 'We have %d too many %s'

 

>>> template % (10, 'crashers')

'We have 10 too many crashers'

 

>>> template % (5, 'hurricanes')

'We have 5 too many hurricanes'

 

String formatting details

 

 

String methods

 

>>> string.capitalize()

'This is an example'

>>> string.split()

['this', 'is', 'an', 'example']

>>> string.split('i')

['th', 's ', 's an example']

>>> string.split('ia')

['this is an example']

>>> string.split('an')

['this is ', ' example']

 

List of String methods

 

 

 

Iterating Strings

 

for variableName in aString:

 

>>> a = 'c1t2'

>>> for c in a:

...     if c.isdigit(): print c

...

1

2

 

 

 

Lists

Mutable ordered collection

>>> aList = ['cat', 'dog', 3, 'mouse']

>>> aList[2]

3

>>> aList[1:3]

['dog', 3]

 

>>> aList.sort()

>>> aList

[3, 'cat', 'dog', 'mouse']

 

>>> aList[0] = 'tea'

>>> aList

['tea', 'cat', 'dog', 'mouse']

 

>>> aList.append('trap')

>>> aList

['tea', 'cat', 'dog', 'mouse', 'trap']

 

>>> del aList[1]

>>> aList

['tea', 'dog', 'mouse', 'trap']

 

>>> del aList[1:3]

>>> aList

['tea', 'trap']

 

 

 

Nesting Lists

 

>>> a = ['cat', 'dog']

>>> b = [1, a, 2]

>>> b

[1, ['cat', 'dog'], 2]

 

>>> len(b)

3

>>> b[1]

['cat', 'dog']

 

>>> b[1][0]

'cat'

 

>>> b[1][0] = 'new'

>>> b

[1, ['new', 'dog'], 2]

>>> a

['new', 'dog']

 

 

 

Assignments Create References

 

>>> a = ['cat', 'dog']

>>> b = [1, a, 2]

 

 

 

 

Tuples

Immutable ordered collection

Immutable list

>>> aTuple = ('cat', 'dog', 'mouse')

>>> aTuple[2]

'mouse'

>>> for item in aTuple:

...     print item

...

cat

dog

mouse

 

>>> aTuple[1:2]

('dog',)

 

>>> tupleAlso = 1, 2, 3

>>> tupleAlso

(1, 2, 3)

>>> emptyTuple = ()

 

>>> aTuple[1] = 5

Traceback (most recent call last):

  File "<stdin>", line 1, in ?

TypeError: object doesn't support item assignment

 

del and append are also illegal

 

 

Sequences

11. String

12. Unicode String

13. List

14. Tuple

15. Buffer

16. Xrange objects

 

Sequence operations

 

 

Dictionaries

Mutable map (or hashtable)

Keys must be immutable

>>> offices = { 'whitney': 'GMCS 561', 'beck':'GMCS-407B' }

>>> offices['whitney']

'GMCS 561'

 

>>> offices.keys()

['beck', 'whitney']

>>> offices.values()

['GMCS-407B', 'GMCS 561']

 

>>> offices.has_key(' lewis ')

False

 

>>> offices['lewis'] = 'GMCS-544'

 

>>> offices.has_key('lewis')

True

>>> offices.has_key('Lewis')

False

 

>>> del offices['whitney']

>>> offices.keys()

['beck', 'lewis']

 

More about Dictionaries

 

 

Long Lines

 

Use a "\" to continue code on the next line

 

if 1900 < year < 2100 and 1 <= month <= 12 \

   and 1 <= day <= 31 and 0 <= hour < 24 \

   and 0 <= minute < 60 and 0 <= second < 60:  #Valid date

        return 1

 

Inside [], () and {} don't need the backslashes

 

month_names = ['Januari', 'Februari', 'Maart',    

               'April',   'Mei',      'Juni',                   #Dutch names

               'Juli',    'Augustus', 'September',  

               'Oktober', 'November', 'December']  

 

 

 

Files

>>> sampleFile = open('test', 'w')

>>> sampleFile.write('This is a test\n')

>>> shopingList = 'bread' , 'cheese', 'lentils'

>>> shopingList

('bread', 'cheese', 'lentils')

>>> sampleFile.writelines(shopingList)

>>> sampleFile.writelines(shopingList)

>>> sampleFile.close()

 

>>> sampleFile = open('test', 'r')

>>> sampleFile.readline()

'This is a test\n'

>>> sampleFile.readline()

'breadcheeselentilsbreadcheeselentils'

 

>>> sampleFile = open('test', 'r')

>>> sampleFile.readline()

'This is a test\n'

>>> sampleFile.readlines()

['breadcheeselentilsbreadcheeselentils']

 

File modes: 'r', 'w', 'a' (append)

 

More file operations

 

 

 

Equality

 

a is b

Do a & b point to same object

a == b

    Do a & b have point to objects that have same structure and value

 

>>> a = [1, ('cat', 3)]

>>> b = [1, ('cat', 3)]

>>> c = a

>>> a == b, a is b, a == c, a is c

(True, False, True, True)

 

 

 

Comparing

 

>>> a = [1, ('cat', 3)]

>>> d = [1, ('cat', 4)]

>>> a < d, a == d, a > d

(True, False, False)

 

>>> {1:1, 2:2} < {1:2, 2:1}, {1:1, 2:2} > {1:2, 2:1}        

(True, False)

 

 23 < 'cat' , 23 == 'cat', 23 > 'cat'

(True, False, False)

 

>>> 23 < [1, 'cat']

True

 

>>> ['cat'] < ('cat'), ['cat'] == ('cat'), ['cat'] > ('cat')

(True, False, False)

 

Numbers are compared as number

String are compared lexicographically

Lists & tuples are compared by comparing each component, from left to right

Dictionaries are compared as sorted (key, value) lists

Can comparing different types, but results can be meaningless

 

 

Boolean Values

 

Language defines boolean types

17. True

18. False

 

However other values can be evaluated as boolean

>>> if 'cat': print 'hi'

...

hi

 

Values equivalent to False

19. None

20. Numeric zero of all types

21. Empty sequences: '' [] ()

22. Empty mappings: {}

 

All other values are interpreted as True