python prog

8/19/2019 Python Prog

1/98

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P YTHON PROGRAMMING

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2/98

Python Programming

Permission is granted to copy, distribute and/or modify this document under the terms

of the GNU Free Documentation icense, !ersion "#$ or any later %ersion published by

the Free &oft'are Foundation( 'ith no )n%ariant &ections, no Front-*o%er +exts, and no

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Contents

)ntroduction########################################################################################################################################## $

%er%ie'##########################################################################################################################################$

Getting Python################################################################################################################################# 0

)nteracti%e mode############################################################################################################################## 1

earning to program in Python#############################################################################################################2

*reating Python programs############################################################################################################### 2

Using %ariables and math#################################################################################################################3

Python conepts####################################################################################################################################""

asic syntax################################################################################################################################### ""Data types###################################################################################################################################### "4

Numbers####################################################################################################################################"5

&trings####################################################################################################################################### "2

ists########################################################################################################################################### $1

+uples########################################################################################################################################$3

Dictionaries############################################################################################################################### 0$

&ets############################################################################################################################################ 04

perators########################################################################################################################################06Flo' control###################################################################################################################################47

Functions########################################################################################################################################44

&coping########################################################################################################################################## 42

8xceptions######################################################################################################################################42

)nput and output#############################################################################################################################1"

9odules######################################################################################################################################### 11

*lasses########################################################################################################################################### 159eta*lasses################################################################################################################################### 55

:ocking the Python ;9odules


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Python Programming

OVERVIEW

li%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

Python is a high-le%el, structured, open-source programming language that can be used for a

'ide %ariety of programming tasks# )t is good for simple Buick-and-dirty scripts, as 'ell as complex

and intricate applications#

)t is an interpreted programming language that is automatically compiled into bytecode before

execution ;the bytecode is then normally sa%ed to disk, Cust as automatically, so that compilation

need not happen again until and unless the source gets changed


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GETTING PYTHON


)n order to program in Python you need the Python soft'are#

Installing Python in Windows

Go to http?//'''#python#org/do'nload/ or the cti%e&tate 'ebsiteE" and get the proper

%ersion for your platform# Do'nload it, read the instructions and get it installed#

)n order to run Python from the command line, you 'ill need to ha%e the python directory in

your P+# lternati%ely, you could use an )ntegrated De%elopment 8n%ironment ;)D8< for

Python like DrPythonE$, ericE0, or Py&cripter E4#

Installing Python in Unix

Python is standard eBuipment in many Unix-like operating systems( Cust type which pythn

to check for it# )f present, it may not be the latest, but it should be enough to get you started#

)f its not installed, check your operating systems 'eb page for the proper package# Failing

that, you 'ill need to do'nload the appropriate file from http?//'''#python#org/do'nload or the

cti%e&tate 'ebsiteE1#

)f you decide to compile Python from source, make sure you compile in the tk extension if you

'ant to use )D8#

n Debian based inux systems, you can do'nload it by starting the command line, changing

to the superuser mode using su - and then by typing apt-get install python#

Installing Python PyDE Pl!g"in #o$ E%li&se IDEHou can use the 8clipse )D8 as your Python )D8# +he only reBuirement is 8clipse and the

8clipse PyD8! Plug-in#

Go to http?//do'nload#eclipse#org/do'nloads and get the proper 8clipse )D8 %ersion for your

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Python Programming

Python Mode #o$ E'a%s

+here is also a python mode for 8macs 'hich pro%ides features such as running pieces of code,

and changing the tab le%el for blocks# Hou can do'nload the mode athttp?//sourceforge#net/proCects/python-mode/


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INTERACTIVE MODE


Python has t'o basic modes? +he normal .mode. is the mode 'here the scripted and finished

.py files are run in the python interpreter# )nteracti%e mode is a command line shell 'hich gi%es

immediate feedback for each statement, 'hile running pre%iously fed statements in acti%e memory#

s ne' lines are fed into the interpreter, the fed program is e%aluated both in part and in 'hole#

+o get into interacti%e mode, simply type .python. 'ithout any arguments# +his is a good 'ay

to play around and try %ariations on syntax# Python should print something like this?

! python"ythn 2.3.4 (#2, $%& 29 2004, 02:04:10)' 3.3.4 (*ebian 1:3.3.49) n lin%-2ype /help/, /cpyri&ht/, /creit/ r /licene/ r re inratin.>>>

;)f Python 'ouldnt run, make sure your path is set correctly# &ee Getting Python#<

+he >>> is Pythons 'ay of telling you that you are in interacti%e mode# )n interacti%e mode

'hat you type is immediately run# +ry typing 1+1 in# Python 'ill respond 'ith 2# )nteracti%e mode

allo's you to test out and see 'hat Python 'ill do# )f you e%er feel the need to play 'ith ne'

Python statements, go into interacti%e mode and try them out#

sample interacti%e session?

>>> 55>>> print 5*735>>> "hello" * 4hellhellhellhell>>> "hello".__class__

o'e%er, you need to be careful in the interacti%e en%ironment# )f you arent confusion may

ensue# For example, the follo'ing is a %alid Python script?

if 1:print "True"

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Python Programming

you should ha%e entered 'as a blank line, to end the first ;i#e#, .if.< statement, before you started

'riting the next print statement# For example, you should ha%e entered the statements as though

they 'ere 'ritten?

if 1: print "True"

print "one"

>hich 'ould ha%e resulted in the follo'ing?

>>> if 1:... print "True"...r%e>>> print "one"*ne>>>


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CREATING PYTHON PROGRAMS


Python programs are nothing more than text files, and they may be edited 'ith standard text

editors#I

)n >indo's, notepad 'ill be sufficient for a little 'hile, but you 'ill soon find that a more

po'erful editor, such as %im, emacs, or pythons built-in )D8, )D8 makes editing much easier#

)n Unix, nano or pico are respectable beginners editors, 'hile %im and emacs are used 'hen

more po'er is needed#

dditional editors exist that are Python friendly ;e#g#, use Python syntax highlighting


10/98

Python Programming

Result

+he program should print ell, wrl# *ongratulationsK Houre 'ell on your 'ay to

becoming a Python programmer#

Inter!ti"e #ode

)nstead of python exiting 'hen the program is finished, you can use the -i flag to start an

interacti%e session# +his can be "er$ useful for debuging and prototyping#

pythn i hell.py

Exer!ises

• 9odify the hell.py program to say hello to a historical political leader ;or to da

o%elace


11/98


USING VARIA%&ES AND MATH


Using a (a$ia)le

variable is something 'ith a %alue that may change# )n Python, %ariables are strongly typed,

meaning that if a %ariable has a number , it cant be treated as a string, or %ice %ersa# ere is a

program that uses a %ariable?

#;%r;bin;pythn

nae = $a elaceprint /bye, / + nae +

;opsK ) used single Buotes for das name, then double Buotes around Goodbye# +hats M,

ho'e%er, because these t'o Buotes do exactly the same thing in Python# +he only thing you cant do

is mix them and try to make a string like this? /will nt wr?#<

+his program isnt much use, of course# ut 'hat about %ariables that the program truly cant

guess aboutL

$aw*in&!t+,#;%r;bin;pythn

print "leae enter y%r nae.nae = raw@inp%t()print w are y%, + nae + A

;>hats raw@inp%t() doingL 8%idently, its getting input from you# &ee )nput and output#<

f course, 'ith the po'er of Python at hand, the urge to determine ones mass in stone is nearly

irresistible# concise program can make short 'ork of this task# &ince a stone is "4 pounds, andthere are about $#$ pounds in a kilogram, the follo'ing formula should do the trick?

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Python Programming

• >hen the pre%ious program asked for your name, you 'ere typing belo' the Buestion#

+his time, youre typing at the end of the line that asks, /Bhat i y%r a in

?il&raA/# >hats happening here is that, normally, the print statement 'ill add a

newline to the end 'hat youre printing# +hats 'hy the cursor 'ent to the next line in the pre%ious program# ut in this program, ) added a little comma to the end# +hat makes

print omit the ne'line#

• int() - this handy function takes a string, and returns a number# :emember 'hen you

read that Python is strongly typedL Python 'ont allo' us to do math on a string# >hate%er

you type is a string, e%en if it consists of digits# ut int() 'ill recognie a string made of

digits and return a number#

•

+he tr(mass@stone) in the print statement# )t turns out that you cant add togetherstrings and numbers( /D% wei&h / + a@tne Cust 'ouldnt 'ork# &o, 'e ha%e

to take the number and turn it into a string# )ncidentally, E 'ould do the same thing as the

str;< function, but that is deprecated#

.o$'atting o!t&!t

)n the pre%ious program, 'e used this line of code to print the result?

print /D% wei&h / + tr(a@tne) + / tne./

+here are a couple of problems 'ith this# First, it mixes up operators and Buotes, and can be a

little tough to read# &econd, the number 'ont be printed %ery nicely, as the follo'ing example

illustrates?

! .;?&2tne

Bhat i y%r a in ?il&raA F5D% wei&h 10.214285G14285G14 tne.

Not only is that much accuracy unCustified, it doesnt look nice# Pythons O operator comes to

the rescue# )t allo's print-like formatting, in the form?

STRING H (ar&1, ar&2, ...)

+he string contains one format code for each argument# +here are se%eral types of format

codes( see the strings section for a complete list#

+ i C t d th H f t d

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%ASIC SYNTA'


+here are four fundamental concepts in Python#

Case -ensiti(ity

ll %ariables are case-sensiti%e# Python treats number and Number as seperate, unrelated

entities#

-&a%es and ta)s don/t 'ix

ecause 'hitespace is significant, remember that spaces and tabs dont mix, so use only one or

the other 'hen indenting your programs# common error is to mix them# >hile they may look the

same in editor the interpreter 'ill read them differently and it 'ill result in either an error or

unexpected beha%ior# o'e%er, tab are read as eBual to eight spaces, so changing your tab 'idth to6 in your editor helps if you find yourself freBuently making this mistake#

O)0e%ts

)n Python, like all obCect oriented languages, there are aggregations of code and data called

bCects, 'hich typically represent the pieces in a conceptual model of a system#

bCects in Python are created ;i#e#, instantiated< from templates called *lasses ;'hich are

co%ered later, as much of the language can be used 'ithout understanding classes


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Python Programming

cannot be easily accessed# +here are t'o 'ays of delimiting regions in Python? 'ith functions or

'ith modules# +hey each ha%e different 'ays of accessing the useful data that 'as produced 'ithin

the scope from outside the scope# >ith functions, that 'ay is to return the data# +he 'ay to access

names from other modules lead us to another concept#

Na'es&a%es

)t 'ould be possible to teach Python 'ithout the concept of namespaces because they are so

similar to attributes, 'hich 'e ha%e already mentioned, but the concept of namespaces is one that

transcends any particular programming language, and so it is important to teach# +o begin 'ith,

there is a built-in function dir() that can be used to help one understand the concept ofnamespaces# >hen you first start the Python interpreter ;i#e#, in interacti%e mode


15/98


'@@c@@, @@nae@@, ac, ain, atan, atan2, ceil, c, ch,e&ree, e,e-p, ab, lr, , re-p, hypt, le-p, l&, l&10,, pi, pw,

raian, in, inh, Nrt, tan, tanh>>>

)f you look closely, you 'ill notice that both the default namespace, and the math module

namespace ha%e a @@name@@ obCect# +he fact that each layer can contain an obCect 'ith the same

name is 'hat scope is all about# +o access obCects inside a namespace, simply use the name of the

module, follo'ed by a dot, follo'ed by the name of the obCect# +his allo' us to differentiate

bet'een the @@nae@@ obCect 'ithin the current namespace, and that of the obCect 'ith the same

name 'ithin the ath module# For example?

>>> print @@nae@@@@ain@@>>> print ath.@@nae@@ath>>> print ath.@@c@@hi %le i alway aailable. Kt prie acce t theatheatical %nctin eine by the tanar.>>> ath.pi3.141592F53589G931


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Python Programming

DATA TYPES


Data types determine 'hether an obCect can do something, or 'hether it Cust 'ould not make

sense# ther programming languages often determine 'hether an operation makes sense for an

obCect by making sure the obCect can ne%er be stored some'here 'here the operation 'ill be

performed on the obCect ;this type system is called static typing


17/98


bar/ e nt wr?, an/baL e nt wr? either./N%%- i ri&ht %t.

+riple Buoted strings are like single Buoted strings, but can span more than one line# +heirstarting and ending delimiters must also match# +hey are entered 'ith three consecuti%e single or

double Buotes, so

wr?, an////// wr? a well, b%t/bar/ e nt wr?, an

///baL e nt wr? either./N%%-// i ri&ht %t.

+uples are entered in parenthesis, 'ith commas bet'een the entries?

(10, Jary ha a little lab)

lso, the parenthesis can be left out 'hen its not ambigouous to do so?

10, whe leece wa a white a nw

Note that one-element tuples can be entered by surrounding the entry 'ith parentheses and

adding a comma like so?

(thi i a t%pi t%ple,)

ists are similar, but 'ith brackets?

'abc, 1,2,3

Dicts are created by surrounding 'ith curly braces a list of key,%alue pairs separated from each

other by a colon and from the other entries 'ith commas?

O hell: wrl, wei&ht: $rican r %rpeanA P

ny of these composite types can contain any other, to any depth?

(((((((((bb,),'Jary, ha, a, little, lab), O hell : wrl P ),),),),),),)



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Python Programming

NUM%ERSli%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

Python supports 4 types of Numbers, the int, the long, the float and the complex# Hou dont

ha%e to specify 'hat type of %ariable you 'ant( Python does that automatically#

• Int: +his is the basic integer type in python, it is eBui%ilant to the hard'are c long for

the platform you are using#

• Long: +his is a integer number thats length is non-limited# )n python $#$ and later, )nts

are automatically turned into long ints 'hen they o%erflo'#• Float: +his is a binary floating point number# ongs and )nts are automatically

con%erted to floats 'hen a float is used in an expression, and 'ith the true-di%ision /

operator#

• Complex: +his is a complex number consisting of t'o floats# )t is in engineering style

notation#

)n general, the number types are automatically up cast in this order?

)nt --J ong --J Float --J *omplex# the farther to the right you go, the higher the precedence#

>>> - = 5 >>> type(-) >>> - = 18GF8GF545F4F589G09G89098F95GF453 >>> type(-)

>>> - = 1.34GF3 >>> type(-) >>> - = 5 + 2Q >>> type(-)

o'e%er, some expressions may be confusing since in the current %ersion of python, using the

/ operator on t'o integers 'ill return another integer, using floor di%ision# For example, 1/$ 'illgi%e you $# Hou ha%e to specify one of the operands as a float to get true di%ision, e#g# 1/$# or 1#/$

;the dot specifies you 'ant to 'ork 'ith float< to ha%e $#1# +his beha%ior is deprecated and 'ill

disappear in a future python release as sho'n from the from @@future@@ import#

;2

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STRINGSli%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

-t$ing 'ani&!lation

Strin( o)ertions

E*ulit$

+'o strings are eBual if and only if they ha%e exactly the same contents, meaning that they are

both the same length and each character has a one-to-one positional correspondence# 9any other

languages test strings only for identity( that is, they only test 'hether t'o strings occupy the same

space in memory# +his latter operation is possible in Python using the operator i#

8xample?

>>> a = hellR b = hell # $i&n hell t a an b.>>> print a == b # r%er%e>>> print a == hell #r%e>>> print a == /hell/ # (chice eliiter i %niprtant)r%e>>> print a == hell # (e-tra pace)ale

>>> print a == ell # (wrn& cae)ale

Nu#eri!l

+here are t'o Buasi-numerical operations 'hich can be done on strings -- addition and

multiplication# &tring addition is Cust another name for concatenation# &tring multiplication is

repetiti%e addition, or concatenation# &o?

>>> c = a>>> c + bab>>> c C 5aaaaa

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Python Programming

Indexin( nd Sli!in(

9uch like arrays in other languages, the indi%idual characters in a string can be accessed by an

integer representing its position in the string# +he first character in string s 'ould be sE7 and the nth

character 'ould be at sEn-"#

>>> = STana%U>>> '1VaW

Unlike arrays in other languages, Python also indexes the arrays back'ards, using negati%e

numbers# +he last character has index -", the second to last character has index -$, and so on#

>>> '4VnW

>e can also use QslicesR to access a substring of s# sEa?b 'ill gi%e us a string starting 'ith sEa

and ending 'ith sEb-"#

>>> '1:4

VanaW

Neither of these is assignable#

>>> print >>> '0 = Xracebac? (t recent call lat): ile SU, line 1, in Ayperrr: bQect e nt %pprt ite ai&nent>>> '1:3 = S%pUracebac? (t recent call lat): ile SU, line 1, in Ayperrr: bQect e nt %pprt lice ai&nent>>> print

utputs ;assuming the errors 'ere suppressed


21/98


leent: 1 2 3 4Kne-: 0 1 2 3 4 4 3 2 1

&o, 'hen 'e ask for the E"?0 slice, that means 'e start at index ", and end at index 0, and takee%erything in bet'een them# )f you are used to indexes in * or Sa%a, this can be a bit disconcerting

until you get used to it#

Strin( !onstnts

&tring constants can be found in the standard string module# 8ither single or double Buotes may

be used to delimit string constants#

Strin( #et+ods

+here are a number of methods of built-in string functions?

• !)itli,e

• !enter

• !ount

• decode

• encode

• ends'ith

• ex)ndt-s

• .ind

• index

• islnu# • isl)+

• isdi(it

• islower

• iss)!e

• istitle

• isu))er

•

/oin • l/ust

• lower

• lstri)

• re)l!e

• r.ind

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22/98

Python Programming

nly emphasied items 'ill be co%ered#

is0

isalnum;


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stri)1 rstri)1 lstri)

:eturns a copy of the string 'ith the leading ;lstrip< and trailing ;rstrip< 'hitespace remo%ed#

strip remo%es both#

>>> = t ell, wrlnt >>> print ell, wrl >>> print .trip()ell, wrl>>> print .ltrip()ell, wrl

# en here>>> print .rtrip() ell, wrl

Note the leading and trailing tabs and ne'lines#

&trip methods can also be used to remo%e other types of characters#

iprt trin& = www.wi?ib?.r&print print .trip(w) # Yee all w r %tieprint .trip(trin&.lwercae) # Yee all lwercae letter r %tieprint .trip(trin&.printable) # Yee all printable character

utputs?

www.wi?ib?.r&.wi?ib?.r&.wi?ib?.

Note that string#lo'ercase and string#printable reBuire an import string statement

l/ust1 r/ust1 !enterleft, right or center Custifies a string into a gi%en field sie ;the rest is padded 'ith spaces>> = >>>

P h P i

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Python Programming

>>> +.Qin(eN)1+2+3+4+5

map may be helpful here? ;it con%erts numbers in seB into strings<

>>> eN = '1,2,3,4,5>>> .Qin(ap(tr, eN))1 2 3 4 5

no' arbitrary obCects may be in seB instead of Cust strings#

.ind1 index1 r.ind1 rindex+he find and index functions returns the index of the first found occurrence of the gi%en

subseBuence# )f it is not found, find returns -" but index raises a !alue8rror# rfind and rindex are the

same as find and index except that they search through the string from right to left ;i#e# they find the

last occurance<

>>> = ell, wrl>>> .in(l)

2>>> '.ine-(l):ll, wrl>>> .rin(l)10>>> ':.rine-(l)ell, wr>>> '.ine-(l):.rine-(l)ll, wr

ecause Python strings accept negati%e subscripts, index is probably better used in situations

like the one sho'n because using find instead 'ould yield an incorrect %alue#

re)l!e

:eplace 'orks Cust like it sounds# )t returns a copy of the string 'ith all occurrences of the first

parameter replaced 'ith the second parameter#

>>> ell, wrl.replace(, T)ellT, wTrl

r, using %ariable assignment?

F Wikib k the e te t te tb k lle ti

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ex)ndt-s

:eplaces tabs 'ith the apropriate number of spaces# ;default number of spaces per tab 6( this

can be changed by passing the tab sie as an argument<

= abce&tabctaprint print len()t = .e-pantab()print tprint len(t)

abce& abc a

13abce& abc a1G

Notice ho' ;although these both look the same< the second string ;t< has a different length

because each tab is represented by spaces not tab characters#

+o use a a tab sie of 4 instead of 6?

= .e-pantab(4)print print len()

utputs?

abce& abc a

13

s)lit1 s)litlines

+he s)lit method returns a list of the 'ords in the string# )t can take a separator argument to use

instead of 'hitespace#

>>> = ell, wrl

>>> .plit() 'ell, , wrl >>> .plit(l) 'e, , , wr,

Note that in neither case is the separator included in the split strings, but empty strings are

Python Programming

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Python Programming

', Ine line, w line, Ye line, Ml%e line, reen line, >>> .plitline()', Ine line, w line, Ye line, Ml%e line, reen line


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From Wikibooks, the open content textbooks collection

&ISTSli%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

A-out lists in P$t+on

list in Python is an ordered group of items# )t is a %ery general structure# Hou can ha%e any

kind of things in a lists, and they dont ha%e to be all of the same type# For instance, you could put

numbers, letters, strings and donkeys all on the same list#

)f you are using a modern %ersion of Python ;and you should beriting lists this 'ay is %ery Buick ;and ob%ious


28/98

Python Programming

'p, , t, lp, l, lt, p, , t, wp, w, wt, ep,e, et, rp, r, rt

Pythons list comprehension does not define a scope# ny %ariables that are bound in an

e%aluation remain bound to 'hate%er they 'ere last bound to 'hen the e%aluation 'as completed?

>>> print -, yr t

+his is exactly the same as if the comprehension had been expanded into an explicitly-nested

group of one or more for statements and 7 or more if statements#

&ist !retion s+ort!uts

Python pro%ides a shortcut to initialie a list to a particular sie and 'ith an initial %alue for

each element?

>>> Ler='0C5>>> print Ler'0, 0, 0, 0, 0

+his 'orks for any data type?

>>> ='C8>>> print ', , , , , , ,

'ith a ca%eat# >hen building a ne' list by multiplying, Python copies each item by reference#

+his poses a problem for mutable items, for instance in a multidimensional array 'here eachelement is itself a list# Houd guess that the easy 'ay to generate a t'o dimensional array 'ould be?

litlit=' '0C4 C5

and this 'orks, but probably doesnt do 'hat you expect?

>>> litlit=' '0C4 C5

>>> print litlit''0, 0, 0, 0, '0, 0, 0, 0, '0, 0, 0, 0, '0, 0, 0, 0, '0, 0, 0, 0>>> litlit'0'2=1>>> print litlit''0, 0, 1, 0, '0, 0, 1, 0, '0, 0, 1, 0, '0, 0, 1, 0, '0, 0, 1, 0


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, p

>>> litlit=''0C4 r i in ran&e(5)>>> print litlit''0, 0, 0, 0, '0, 0, 0, 0, '0, 0, 0, 0, '0, 0, 0, 0, '0, 0, 0, 0>>> litlit'0'2=1

>>> print litlit''0, 0, 1, 0, '0, 0, 0, 0, '0, 0, 0, 0, '0, 0, 0, 0, '0, 0, 0, 0

O)ertions on lists

&ist Attri-utes

ength?

in the len&th a lit %e the b%ilt in len() eth. >>> len('1,2,3) 3 >>> a = '1,2,3,4 >>> len( a ) 4

Co#-inin( listsists can be combined in se%eral 'ays# +he easiest is Cust to add them# For instance?

>>> '1,2 + '3,4'1, 2, 3, 4

nother 'ay to combine lists is 'ith extend# )f you need to combine lists inside of a lamda,

extend is the 'ay to go#

>>> a = '1,2,3>>> b = '4,5,F>>> a.e-ten(b)>>> print a'1, 2, 3, 4, 5, F

+he other 'ay to append a %alue to a list is to use ))end# For example?

>>> p='1,2>>> p.appen('3,4)>>> p'1, 2, '3, 4

Python Programming

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9uch like the slice of a string is a substring, the slice of a list is a list# o'e%er, lists differ

from strings in that 'e can assign ne' %alues to the items in a list#

>>> lit'1 = 1G

>>> lit'2, 1G, V%%rpW, 9.0,WnW

>e can e%en assign ne' %alues to slices of the lists, 'hich dont e%en ha%e to be the same

length

>>> lit'1:4 = 'Spprt%niticU, Sel?U>>> lit

'2, Vpprt%niticW, Vel?W, VnW

)ts e%en possible to append things onto the end of lists by assigning to an empty slice?

>>> lit':0 = '3.14,2.G1>>> lit'3.14, 2.G1, 2, Vpprt%niticW, Vel?W, VnW

Co#)rin( lists

ists can be compared for eBuality#

>>> '1,2 == '1,2r%e>>> '1,2 == '3,4ale

Sortin( lists

&orting lists is easy 'ith a sort method#

>>> lit = '2, 3, 1, a, b>>> lit.rt()>>> lit'1, 2, 3, a, b

Note that the list is sorted in place, and the sort;< method returns None to emphasie this side

effect#

)f you use Python $#4 or higher there are some more sort parameters?




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TUP&ESli%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

A)o!t t!&les in Python

tuple in Python is much like a list except that it is immutable ;unchangable< once created#

+hey are generally used for data 'hich should not be edited#

Tu)le nottion

+uples may be created directly or con%erted from lists# Generally, tuples are enclosed in

parenthesis#

>>> l = '1, a, 'F, 3.14>>> t = (1, a, 'F, 3.14)>>> t(1, a, 'F, 3.1400000000000001)

>>> t%ple(l)(1, a, 'F, 3.1400000000000001)>>> t == t%ple(l)r%e>>> t == lale

lso, tuples 'ill be created from items separated by commas#

>>> t = $, t%ple, nee, n, paren >>> t ($, t%ple, nee, n, paren)

one item tuple is created by a item in parens follo'ed by a comma?

>>> t = ($ in&le ite t%ple,) >>> t

($ in&le ite t%ple,)

P!4in( nd Un)!4in(

Hou can also perform multiple assignment using tuples#

Python Programming

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ssigning a tuple to a se%eral different %ariables is called Qtuple unpacking,R 'hile assigning

multiple %alues to a tuple in one %ariable is called Qtuple packing#R >hen unpacking a tuple, or

performing multiple assignment, you must ha%e the same number of %ariables being assigned to as

%alues being assigned#

O)ertions on tu)les

+hese are the same as for lists except that 'e may not assign to indices or slices, and there is no

.append. operator#

>>> a = (1, 2)

>>> b = (3, 4)>>> a + b(1, 2, 3, 4)>>> a(1, 2)>>> b(3, 4)>>> print a.appen(3)racebac? (t recent call lat):

ile //, line 1, in A$ttrib%terrr: t%ple bQect ha n attrib%te appen>>> a(1, 2)>>> a'0 = 0racebac? (t recent call lat): ile //, line 1, in Ayperrr: bQect e nt %pprt ite ai&nent>>> a(1, 2)

For lists 'e 'ould ha%e had?

>>> a = '1, 2>>> b = '3, 4>>> a + b'1, 2, 3, 4>>> a

'1, 2>>> b'3, 4>>> a.appen(3)>>> a'1, 2, 3>>> '0 0




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Python Programming

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DICTIONARIESli%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

A)o!t di%tiona$ies in Python

dictionary in python is a collection of unordered %alues 'hich are accessed by key#

Di!tionr$ nottion

Dictionaries may be created directly or con%erted from seBuences# Dictionaries are enclosed in

curly braces, OP

>>> = Ocity:"ari, a&e:38, (102,1F50,1F01):$ atri- crinateP>>> eN = '(city,"ari), (a&e, 38), ((102,1F50,1F01),$ atri-crinate)>>> Ocity: "ari, a&e: 38, (102, 1F50, 1F01): $ atri- crinateP

>>> ict(eN)Ocity: "ari, a&e: 38, (102, 1F50, 1F01): $ atri- crinateP>>> == ict(eN)r%e

lso, dictionaries can be easily created by ipping t'o seBuences#

>>> eN1 = (a,b,c,)

>>> eN2 = '1,2,3,4>>> = ict(Lip(eN1,eN2))>>> Oa: 1, c: 3, b: 2, : 4P

O)ertions on Di!tionries

+he operations on dictionaries are some'hat uniBue# &licing is not supported, since the items

ha%e no intrinsic order#

>>> = Oa:1,b:2, cat:l%erP>>> .?ey()'a, b, cat>>> .al%e()'1 2 l


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Co#-inin( two Di!tionries

Hou can combine t'o dictionaries by using the update method of the primary dictionary# Note

that the update method 'ill merge existing elements if they conflict#

>>> = Oapple: 1, ran&e: 3, pear: 2P>>> % = Opear: 4, &rape: 5, len: FP>>> .%pate(%)>>> O&rape: 5, pear: 4, len: F, apple: 1, ran&e: 3P>>>

Deletin( .ro# di!tionr$el ictinaryZae'ebernae


Python Programming

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SETSli%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

Python also has a implementation of the mathematical set# Unlike seBuence obCects such as lists

and tuples, in 'hich each element is indexed, a set is an unordered collection of obCects# &ets also

cannot ha%e duplicate members - a gi%en obCect appears in a set 7 or " times# For more information

on sets, see the &et +heory 'ikibook#

Constru!tin( Sets

ne 'ay to construct sets is by passing any seBuential obCect to the .set. constructor#

>>> et('0, 1, 2, 3)et('0, 1, 2, 3)>>> et(/bt%e/)et('b, e, , , %, t)

>e can also add elements to sets one by one, using the .add. function#

>>> = et('12, 2F, 54)>>> .a(32)>>> et('32, 2F, 12, 54)

Note that since a set does not contain duplicate elements, if 'e add one of the members of s to s

again, the add function 'ill ha%e no effect# +his same beha%ior occurs in the .update. function,

'hich adds a group of elements to a set#

>>> .%pate('2F, 12, 9, 14)>>> et('32, 9, 12, 14, 54, 2F)

Note that you can gi%e any type of seBuential structure, or e%en another set, to the update

function, regardless of 'hat structure 'as used to initialie the set#

+he set function also pro%ides a copy constructor# o'e%er, remember that the copy

constructor 'ill copy the set, but not the indi%idual elements#

>>> 2 = .cpy()


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>e can also test the membership of entire sets# Gi%en t'o sets S " and S

$, 'e check if S

" is a

subset or a superset of S $#

>>> .i%bet(et('32, 8, 9, 12, 14, 4, 54, 2F, 19))r%e>>> .i%peret(et('9, 12))r%e

Note that .issubset. and .issuperset. can also accept seBuential data types as arguments

>>> .i%peret('32, 9)r%e

Note that the X and J operators also express the issubset and issuperset functions

respecti%ely#

>>> et('4, 5, G) = et('9, 12)r%e

ike lists, tuples, and string, 'e can use the .len. function to find the number of items in a set#

Re#o"in( Ite#s

+here are three functions 'hich remo%e indi%idual items from a set, called pop, remo%e, and

discard# +he first, pop, simply remo%es an item from the set# Note that there is no defined beha%ior

as to 'hich element it chooses to remo%e#

>>> = et('1,2,3,4,5,F)>>> .pp()1>>> et('2,3,4,5,F)

>e also ha%e the .remo%e. function to remo%e a specified element#

>>> .ree(3)>>> et('2,4,5,F)

Python Programming

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>>> et(')

Itertion O"er Sets>e can also ha%e a loop mo%e o%er each of the items in a set# o'e%er, since sets are

unordered, it is undefined 'hich order the iteration 'ill follo'#

>>> = et(/bler&/)>>> r n in :... print n,...

r b e l &

Set O)ertions

Python allo's us to perform all the standard mathematical set operations, using members of

set# Note that each of these set operations has se%eral forms# ne of these forms, s"#function;s$<

'ill return another set 'hich is created by .function. applied to S " and S

$# +he other form,

s"#function@update;s$


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>>> 1 = et('4, F, 9)>>> 2 = et('1, F, 8)>>> 1.yetric@ierence(2)et('8, 1, 4, 9)>>> 1 6 2et('8, 1, 4, 9)>>> 1.yetric@ierence@%pate(2)>>> 1et('8, 1, 4, 9)

Set Di..eren!e

Python can also find the set difference of S " and S

$, 'hich is the elements that are in S

" but not

in S $#

>>> 1 = et('4, F, 9)>>> 2 = et('1, F, 8)>>> 1.ierence(2)et('9, 4)>>> 1 2et('9, 4)

>>> 1.ierence@%pate(2)>>> 1et('9, 4)

Re.eren!e

Python ibrary :eference on &et +ypes


Python Programming

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OPERATORSli%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

1asi%s

Python math 'orks like you 'ould expect#

>>> - = 2>>> y = 3

>>> L = 5>>> - C yF>>> - + y5>>> - C y + L11>>> (- + y) C L25

Powe$s

+here is a builtin exponentiation operator II, 'hich can take either integers, floating point or

complex numbers# +his occupies its proper place in the order of operations#

Di(ision and Ty&e Con(e$sionDi%iding t'o integers uses integer di%ision, also kno'n as floor di%ision# Using di%ision this

'ay is deprecated because it is intended to change in the future# )nstead, if you 'ant floor di%ision,

use //#

Di%iding by or into a floating point number ;there are no fractional types in Python< 'ill cause

Python to use true di%ision# +o coerce an integer to become a float, float;


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>>> - = 5>>> -5

A!g'ented Assign'ent

+here is shorthand for assigning the output of an operation to one of the inputs?

>>> - = 2>>> - # 22>>> - C= 3

>>> - # 2 C 3F>>> - += 4>>> - # 2 C 3 + 410>>> - ;= 5>>> - # (2 C 3 + 4) ; 52>>> - CC= 2

>>> - # ((2 C 3 + 4) ; 5) CC 24>>> - H= 3>>> - # ((2 C 3 + 4) ; 5) CC 2 H 31

>>> - = repeat thi >>> - # repeat thirepeat thi

>>> - C= 3 # ill with - repeate three tie>>> -repeat thi repeat thi repeat thi

1oolean

or? if a or b?

@thi

else?

thi

Python Programming



42/98

5&OW CONTRO&li%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

s 'ith most imperati%e languages, there are three main categories of program flo' control?

• loops

• branches

• function calls

Function calls are co%ered in a later section#

Generators might arguably be considered an ad%anced form of program flo' control, but they

are not co%ered here#

&oo)s

)n Python, there are t'o kinds of loops, for loops and 'hile loops#

5or loo)s

for loop iterates o%er elements of a seBuence ;tuple or list


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1098GF

54321

or

r i in ran&e(10,0,2): print i

+his 'ill output

108F4

2

or

r i in ran&e(10,0,1): print i,

+his 'ill output

10 9 8 G F 5 4 3 2 1

or

r i in ran&e(1,F): print i

for loops can ha%e names for each element of a tuple, if it loops o%er a seBuence of tuples# Forinstance

l = '(1,1), (2,4), (3,9), (4,1F), (5,25)r -,-N%are in l:

Python Programming

- = 5



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- 5while - > 0: print - - = - 1

'ill output

54321

Pythons 'hile loops can also ha%e an else clause, 'hich is a block of statements that isexecuted ;once< 'hen the statement starts out false# For example?

- = 5y = -while y > 0: print y y = y 1ele: print -

this 'ill output

5432

15

Unlike some languages, there is no postcondition loop#

%re4in(1 !ontinuin( nd t+e else !luse o. loo)s

Python includes statements to exit a loop ;either a for loop or a 'hile loop< prematurely# +o

exit a loop, use the break statement

- = 5while - > 0: print -

brea?


print -



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p

+his 'ill not produce any output#

+he else clause of loops 'ill be executed if no break statements are met in the loop#

l = ran&e(1,100)r - in l: i - == 100:

print - brea?ele: print /100 nt %n in ran&e/

%rn!+es

+here is basically only one kind of branch in Python, the if statement# +he simplest form of

the if statement simple executes a block of code only if a gi%en predicate is true, and skips o%er it if

the predicate is false

For instance,

>>> - = 10>>> i - > 0:... print /"itie/..."itie>>> i - < 0:... print /Ze&atie/

...Hou can also add .elif. ;short for .else if.< branches onto the if statement# )f the predicate on

the first QifR is false, it 'ill test the predicate on the first elif, and run that branch if its true# )f the

first elif is false, it tries the second one, and so on# Note, ho'e%er, that it 'ill stop checking

branches as soon as it finds a true predicate, and skip the rest of the if statement# Hou can also end

your if statements 'ith an .else. branch# )f none of the other branches are executed, then python

'ill run this branch#

>>> - = F>>> i - > 0:... print /"itie/... eli - == 0:... print /^er/

ele:

Python Programming

5



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5UNCTIONSli%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

.!n%tion %alls

callable object is an obCect that can accept some arguments ;also called parameters< and

possibly return an obCect ;often a tuple containing multiple obCects


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, y p p y g g g

function# )f you do not specify a %alue, then that parameter 'ill ha%e the default %alue gi%en 'hen

the function executes#

>>> e iplay@ea&e(ea&e, tr%ncate@ater = 4):... print ea&e':tr%ncate@ater...>>> iplay@ea&e(/ea&e/)e>>> iplay@ea&e(/ea&e/, F)ea&

Vri-le6&en(t+ Ar(u#ent &ists

Python allo's you to declare t'o special arguments 'hich allo' you to create arbitrary-lengthargument lists# +his means that each time you call the function, you can specify any number of

arguments abo%e a certain number#

e %nctin(irt,ecn,Creainin&): tateent1 tateent2 ...

>hen calling the abo%e function, you must pro%ide %alue for each of the first t'o arguments#

o'e%er, since the third parameter is marked 'ith an asterisk, any actual parameters after the first

t'o 'ill be packed into a tuple and bound to .remaining#.

>>> e print@tail(irt,Ctail):... print tail...

>>> print@tail(1, 5, 2, /e&a/)(5, 2, e&a)

)f 'e declare a formal parameter prefixed 'ith two asterisks, then it 'ill be bound to a

dictionary containing any key'ord arguments in the actual parameters 'hich do not correspond to

any formal parameters# For example, consider the function?

e a?e@ictinary(a-@len&th = 10, CCentrie):

ret%rn ict('(?ey, entrie'?ey) r i, ?ey in en%erate(entrie.?ey()) ii < a-@len&th)

)f 'e call this function 'ith any key'ord arguments other than max@length, they 'ill be placed

in the dictionary .entries#. )f 'e include the key'ord argument of max@length, it 'ill be bound to

the formal parameter max length as usual

Python Programming

functions return %alue can be used by assigning it to a %ariable, like so?



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y g g

- = ()y = bar(5,-)

2a')da .o$'s

esides assigning the return %alue of a function to a %ariable, 'e can also create %ariables that

contain functions# Python pro%ides the QlambdaR key'ord for defining unnamed functions 'hich

can be assigned to %ariables# Hou place the arguments before the colon, and the return %alue of the

lambda after# )f this is assigned to a %ariable, you can then use that %ariable as if it 'ere a function

'ith the same parameters and return %alue as the lambda#

>>> N%are = laba -: -C->>> N%are(3)9

Hou can also use %ariables other than the parameters in the lambda# o'e%er, note that the

lambda function 'ill also use the %alues of %ariables from the scope in 'hich it 'as created, rather

than the scope in 'hich it is run

>>> prei- = /Zte: />>> e ret%rn@laba(prei-):... ret%rn laba nte: prei- + nte...>>> prei- = /re: />>> = ret%rn@laba(/$ttn: /)>>> (/arnir% ctpi/)

$ttn: arnir% ctpi

Note that all functions in python can be stored to %ariables, and are in fact simply %ariables

themsel%es#

>>> a?e@nte = ret%rn@laba>>> a?e@nte(/7ee: /)(/laba calc%l%/)7ee: laba calc%l%



SCOPING

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SCOPINGli%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

a$ia)les

!ariables in Python are automatically declared by assignment# !ariables are al'ays references

to obCects, and are ne%er typed# ike all lgol-deri%ati%e languages, %ariables exist only in the

current scope or global scope# >hen they go out of scope, the %ariables are destroyed, but the

obCects to 'hich they refer are not ;unless the number of references to the obCect drops to ero>> ()15

>>> bar()

racebac? (t recent call lat): ile //, line 1, in tpleel bar()Zaerrr: nae bar i nt eine

+he name bar is not found because a higher scope does not ha%e access to the names lo'er in

the hierarchy#

)t is a common pitfall to fail to lookup an attribute ;such as a method< of an obCect ;such as acontainer< referenced by a %ariable before the %ariable is assigned the obCect# )n its most common

form?

>>> r - in ran&e(10):

Python Programming

E'CEPTIONS

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E'CEPTIONSli%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

Python handles all errors 'ith exceptions#

n exception is a signal that an error or other unusual condition has occurred# +here are a

number of built-in exceptions, 'hich indicate conditions like reading past the end of a file, or

di%iding by ero# Hou can also define your o'n exceptions#

Risin( ex!e)tions

>hene%er your program attempts to do something erroneous or meaningless, Python raises

exception to such conduct?

>>> 1 ; 0racebac? (t recent call lat): ile //, line 1, in Aêr*iiinrrr: inte&er iiin r %l by Ler

+his traceback indicates that the êr*iiinrrr exception is being raised# +his is a

built-in exception -- see belo' for a list of all the other ones#

Ct!+in( ex!e)tions

)n order to handle errors, you can set up exception handling blocks in your code# +he key'ords

try and e-cept are used to catch exceptions# >hen an error occurs 'ithin the try block, Pythonlooks for a matching e-cept block to handle it# )f there is one, execution Cumps there#

)f you execute this code?

try: print 1;0e-cept êr*iiinrrr:

print /D% cant iie by Ler, y% illy./

+hen Python 'ill print this?

D% cant iie by Ler, y% illy.


try:

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try: print (F)e-cept _al%errr: print /hat al%e wa inali./

)n this code, the print statement calls the function # +hat function calls the function &,

'hich 'ill raise an exception of type !alue8rror# Neither nor & has a try/e-cept block to

handle !alue8rror# &o the exception raised propagates out to the main code, 'here there is an

exception-handling block 'aiting for it# +his code prints?

hat al%e wa inali.

&ometimes it is useful to find out exactly 'hat 'ent 'rong, or to print the python error text

yourself# For example?

try: theile = pen(/the@parrt/)e-cept KIrrr, (rrrZ%ber, rrrJea&e): i rrrZ%ber == 2: # ile nt %n print /7rry, the@parrt ha apparently Qine the chir iniible./

ele: print /n&rat%latin D% hae ana&e t trip a #/ + tr(rrrZ%ber)+ / errr:/ print rrrJea&e

>hich of course 'ill print?

7rry, the@parrt ha apparently Qine the chir iniible.

Custo# Ex!e)tions

*ode similar to that seen abo%e can be used to create custom exceptions and pass information

along 'ith them# +his can be extremely useful 'hen trying to debug complicated proCects# ere is

ho' that code 'ould look( first creating the custom exception class?

cla %t-ceptin(-ceptin):

e @@init@@(el,al%e): el.paraeter=al%e e @@tr@@(el): ret%rn repr(el.paraeter)

nd then using that exception?

Python Programming

Exoti! uses o. ex!e)tions



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8xceptions are good for more than Cust error handling# )f you ha%e a complicated piece of code

to choose 'hich of se%eral courses of action to take, it can be useful to use exceptions to Cump out

of the code as soon as the decision can be made# +he Python-based mailing list soft'are 9ailmandoes this in deciding ho' a message should be handled# Using exceptions like this may seem like

its a sort of G+ -- and indeed it is, but a limited one called an escape continuation#

*ontinuations are a po'erful functional-programming tool and it can be useful to learn them#

Sust as a simple example of ho' exceptions make programming easier, say you 'ant to add

items to a list but you dont 'ant to 'rite clanky if statements to initialie the list( you can do?

r newKte in newKte: try: el.ite.appen(newKte) e-cept $ttrib%terrr: el.ite = 'newKte

+his is also much more efficient then an if statement because it assumes the code 'ill succeed#

)n fact it 'ill 'ork 33O of the time ?< n if statement 'ould continue to get executed e%en after the

array has been initialied#



INPUT AND OUTPUT

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INPUT AND OUTPUTli%e %ersion A discussion A edit lesson A comment A report an error A ask a Buestion

In&!t

Python has t'o functions designed for accepting data directly from the user?

• inp%t()

• raw@inp%t()

+here are also %ery simple 'ays of reading a file, and for stricter control o%er input, reading

from stdin is necessary#

rw7in)ut23

ra'@input;< asks the user for a string of data ;ended 'ith a ne' line


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try: - = int(raw@inp%t()) e-cept _al%errr:

print Knali Z%ber

is preferable, as input;< uses e%al;< to turn a literal into a python type# +his 'ill allo' a

malicious person to run arbitary code from inside your program tri%ially#

5ile In)ut

5ile O-/e!ts

Python includes a built-in file type# Files can be opened by using the file types constructor?

= ile(tet.t-t, r)

+his means f is open for reading# +he first argument is the filename and the second parameter

is the mode, 'hich can be r, ', or r', among some others#

+he most common 'ay to read from a file is simply to iterate o%er the lines of the file?

= pen(tet.t-t, r)r line in : print line'0.cle()

+his 'ill print the first character of each line# Note that a ne'line is attached to the end of eachline read this 'ay#

)t is also possible to read limited numbers of characters at a time, like so?

c = .rea(1)while len(c) > 0: i len(c.trip()) > 0: print c, c = .rea(1)

+his 'ill read the characters from f one at a time, and then print them if theyre not 'hitespace#

file obCect implicitly contains a marker to represent the current position# )f the file marker

h ld b d b k t th b i i ith l th fil bC t d it C t


iprt y



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For finer control o%er input, use sys#stdin#read;


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r i in ran&e(10): print i,printr i in ran&e(10,20): print i,

'ill output?

0 1 2 3 4 5 F G 8 910 11 12 13 14 15 1F 1G 18 19

)f the bare print statement 'ere not present, the abo%e output 'ould look like?

0 1 2 3 4 5 F G 8 9 10 11 12 13 14 15 1F 1G 18 19

)f it is not desirable to add spaces bet'een obCects, it is necessary to output only one string, by

concatenating the string representations of each obCect?

print tr(1)+tr(2)+tr(0-)+tr(0GGG)+tr(10+5Q)+tr(0.999)+tr(ap)+tr(y)

'ill output?

12255511(10+5Q)0.999

)f you 'ant to a%oid printing the trailing ne'line or space ;space 'hen you use comma at the

end


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9odules are a simple 'ay to structure a program# 9ostly, there are modules in the standard

library and there are other Python files, or directories containing python files, in the current

directory ;each of 'hich constitute a module


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*lasses are a 'ay of aggregating similar data and functions# class is basically a scope inside

'hich %arious code ;especially function definitions< is executed, and the locals to this scope become

attributes of the class, and of any obCects constructed by this class# n obCect constructed by a class

is called an instance of that class#

De#ining a Class

+o define a class, use the follo'ing format?

cla laZae: ... ...

+he capitaliation in this class definition is the con%ention, but is not reBuired by the language#

Instan%e Const$!%tion

+he class is a callable obCect that constructs an instance of the class 'hen called# +o construct

an instance of a class, .call. the class obCect?

= ()

+his constructs an instance of class Foo and creates a reference to it in f#

Class Me')e$s

)n order to access the member of an instance of a class, use the syntax Xclass

instanceJ#XmemberJ# )t is also possible to access the members of the class definition 'ith Xclass

nameJ#XmemberJ#

Met+ods

method is a function 'ithin a class +he first argument ;methods must al'ays take at least


In"o4in( Met+ods

*alling a method is much like calling a function but instead of passing the instance as the first

http://en.wikibooks.org/wiki/Python_Programming/Classeshttp://en.wikibooks.org/wiki/Talk:Python_Programming/Classeshttp://en.wikibooks.org/w/index.php?title=Python_Programming/Classes&action=edithttp://en.wikibooks.org/w/index.php?title=Talk:Python_Programming/Classes&action=edit&section=newhttp://en.wikibooks.org/w/index.php?title=Talk:Python_Programming/Classes&action=edit&section=newhttp://en.wikibooks.org/w/index.php?title=Python_Programming/Q%26A&action=edit&section=newhttp://en.wikibooks.org/wiki/Python_Programming/Classeshttp://en.wikibooks.org/wiki/Talk:Python_Programming/Classeshttp://en.wikibooks.org/w/index.php?title=Python_Programming/Classes&action=edithttp://en.wikibooks.org/w/index.php?title=Talk:Python_Programming/Classes&action=edit&section=newhttp://en.wikibooks.org/w/index.php?title=Talk:Python_Programming/Classes&action=edit&section=newhttp://en.wikibooks.org/w/index.php?title=Python_Programming/Q%26A&action=edit&section=newhttp://wikibooks.org/http://wikibooks.org/


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*alling a method is much like calling a function, but instead of passing the instance as the first

parameter like the list of formal parameters suggests, use the function as an attribute of the instance#

>>> .et-(5)>>> .bar()

+his 'ill output

5

)t is possible to call the method on an arbitrary obCect, by using it as an attribute of the definingclass instead of an instance of that class, like so?

>>> .et-(,5)>>> .bar()

+his 'ill ha%e the same output#

D$n#i! Clss Stru!ture

s sho'n by the method setx abo%e, the members of a Python class can change during runtime,

not Cust their %alues, unlike classes in languages like * or Sa%a# >e can e%en delete f#x after running

the code abo%e#

>>> el .->>> .bar()

racebac? (t recent call lat): ile //, line 1, in A ile //, line 5, in bar$ttrib%terrr: intance ha n attrib%te -

nother effect of this is that 'e can change the definition of the Foo class during

python prog

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