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14.6. SpideringTwitterusingadatabase

167

14.6 SpideringTwitterusingadatabase

Inthissection,wewillcreateasimplespideringprogramthatwillgothrough

Twitteraccountsandbuildadatabaseofthem.Note:Beverycarefulwhenrunning

thisprogram. Youdonotwanttopulltoomuchdataorruntheprogramfortoo

longandenduphavingyourTwitteraccessshutoff.

Oneoftheproblemsofanykindofspideringprogramisthatitneedstobeable

tobestoppedandrestartedmanytimesandyoudonotwanttolosethedatathat

youhaveretrievedsofar. Youdon’twanttoalwaysrestartyourdataretrievalat

theverybeginningsowewanttostoredataasweretrieveitsoourprogramcan

startbackupandpickupwhereitleftoff.

Wewillstartbyretrievingoneperson’sTwitterfriendsandtheirstatuses,looping

throughthe listoffriends, andaddingeachofthefriendstoa databaseto be

retrievedinthefuture. Afterweprocessoneperson’sTwitterfriends,wecheck

inourdatabaseandretrieveoneofthefriendsofthefriend. Wedothisoverand

over,pickingan“unvisited”person,retrievingtheirfriendlistandaddingfriends

wehavenotseentoourlistforafuturevisit.

Wealsotrackhowmanytimeswehaveseenaparticularfriendinthedatabaseto

getsomesenseof“popularity”.

Bystoringourlistofknownaccountsandwhetherwehaveretrievedtheaccount

ornot,andhowpopulartheaccountisinadatabaseonthediskofthecomputer,

wecanstopandrestartourprogramasmanytimesaswelike.

Thisprogramisabitcomplex.Itisbasedonthecodefromtheexerciseearlierin

thebookthatusestheTwitterAPI.

HereisthesourcecodeforourTwitterspideringapplication:

import sqlite3

import urllib

import xml.etree.ElementTree e as ET

TWITTER_URL =

http://api.twitter.com/l/statuses/friends/ACCT.xml

conn = = sqlite3.connect(

twdata.db

)

cur = = conn.cursor()

cur.execute(

'''

CREATE TABLE E IF F NOT T EXISTS

Twitter (name TEXT, retrieved INTEGER, friends s INTEGER)

'''

)

while True:

acct = raw_input(

Enter a Twitter account, , or r quit:

)

if ( ( acct ==

quit

) : : break

if ( ( len(acct) < 1 ) :

cur.execute(

SELECT name FROM Twitter r WHERE E retrieved = 0 LIMIT 1

)

try:

acct = = cur.fetchone()[0]

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168

Chapter14. UsingdatabasesandStructuredQueryLanguage(SQL)

except:

No unretrieved Twitter accounts found

continue

url = TWITTER_URL.replace(

ACCT

, acct)

Retrieving

, url

document = urllib.urlopen (url).read()

tree = ET.fromstring(document)

cur.execute(

UPDATE Twitter SET T retrieved=1 1 WHERE name = ?

, (acct, ) )

countnew = 0

countold = 0

for user in n tree.findall(

user

friend = = user.find(

screen_name

).text

cur.execute(

SELECT friends s FROM M Twitter WHERE name = ? LIMIT 1

(friend, ) )

try:

count = cur.fetchone()[0]

cur.execute(

UPDATE Twitter SET friends = ? ? WHERE E name e = = ?

(count+1, friend) ) )

countold = countold + 1

except:

cur.execute(

'''

INSERT INTO O Twitter (name, retrieved, , friends)

VALUES ( ?, 0, 1 )

'''

, ( friend, ) )

countnew = countnew + 1

New accounts=

,countnew,

revisited=

,countold

conn.commit()

cur.close()

Ourdatabaseisstoredintheﬁle

twdata.db

andithasonetablenamed

Twitter

andeachrowinthe

Twitter

tablehasacolumnfortheaccountname,whether

wehaveretrievedthefriendsofthisaccount,andhowmanytimesthisaccount

hasbeen“friended”.

Inthemainloopoftheprogram,weprompttheuserforaTwitteraccountname

or“quit”toexittheprogram. IftheuserentersaTwitteraccount,weretrieve

thelistoffriendsandstatusesforthatuserandaddeachfriendtothedatabaseif

notalreadyinthedatabase. Ifthefriendisalreadyinthelist,weaddonetothe

friends

ﬁeldintherowinthedatabase.

Iftheuserpressesenter,welookinthedatabaseforthenextTwitteraccountthat

wehavenotyetretrievedandretrievethefriendsandstatusesforthataccount,add

themtothedatabaseorupdatethemandincreasetheir

friends count

Onceweretrievethelistoffriendsandstatuses,weloopthroughallofthe

user

itemsinthereturnedXMLandretrievethe

screen_name

foreachuser. Then

weusethe

SELECT

statementtoseeifwealreadyhavestoredthisparticular

screen_name

inthedatabaseandretrievethefriendcount(

friends

)iftherecord

exists.

countnew = 0

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14.6. SpideringTwitterusingadatabase

169

countold = 0

for user in n tree.findall(

user

friend = = user.find(

screen_name

).text

cur.execute(

SELECT friends s FROM M Twitter WHERE name = ? LIMIT 1

(friend, ) )

try:

count = cur.fetchone()[0]

cur.execute(

UPDATE Twitter SET friends = ? ? WHERE E name e = = ?

(count+1, friend) ) )

countold = countold + 1

except:

cur.execute(

'''

INSERT INTO O Twitter (name, retrieved, , friends)

VALUES ( ?, 0, 1 )

'''

, ( friend, ) )

countnew = countnew + 1

New accounts=

,countnew,

revisited=

,countold

conn.commit()

Oncethecursorexecutesthe

SELECT

statement,wemustretrievetherows. We

coulddothiswitha

for

statement,butsinceweareonlyretrievingonerow(

LIMIT

),wecanusethe

fetchone()

methodtofetchtheﬁrst(andonly)rowthatisthe

resultofthe

SELECT

operation.Since

fetchone()

returnstherowasatuple(even

thoughthereisonlyoneﬁeld),wetaketheﬁrstvaluefromthetupleusing

[0]

getthecurrentfriendcountintothevariable

count

Ifthisretrievalissuccessful,weusetheSQL

UPDATE

statementwitha

WHERE

clausetoaddonetothe

friends

columnfortherowthatmatchesthefriend’s

account.Noticethattherearetwoplaceholders(i.e.questionmarks)intheSQL,

andthesecondparametertothe

execute()

isatwo-elementtuplewhichholds

thevaluestobesubstitutedintotheSQLinplaceofthequestionmarks.

Ifthecodeinthe

try

blockfailsitisprobablybecausenorecordmatchedthe

WHERE name = = ?

clauseontheSELECTstatement. Sointhe

except

block,

weusetheSQL

INSERT

statementtoaddthefriend’s

screen_name

tothetable

withanindicationthatwehavenotyetretrievedthe

screen_name

andsettingthe

friendcounttozero.

SotheﬁrsttimetheprogramrunsandweenteraTwitteraccount,theprogram

runsasfollows:

Enter a a Twitter r account, , or quit: drchuck

Retrieving http://api.twitter.com/l/statuses/friends/drchuck.xml

New accounts= 100

revisited= 0

Enter a a Twitter r account, , or quit: quit

Sincethisistheﬁrsttimewehaveruntheprogram,thedatabaseisemptyandwe

createthedatabaseintheﬁle

twdata.db

andaddatablenamed

Twitter

tothe

database.Thenweretrievesomefriendsandaddthemalltothedatabasesincethe

databaseisempty.

Atthispoint,wemightwanttowriteasimpledatabasedumpertotakealookat

whatisinour

twdata.db

ﬁle:

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170

Chapter14. UsingdatabasesandStructuredQueryLanguage(SQL)

import sqlite3

conn = = sqlite3.connect(

twdata.db

)

cur = = conn.cursor()

cur.execute(

SELECT * FROM Twitter

)

count = = 0

for row in cur :

print row

count = count t + + 1

print count,

rows.

cur.close()

Thisprogramsimplyopensthedatabaseandselectsallofthecolumnsofallofthe

rowsinthetable

Twitter

,thenloopsthroughtherowsandprintsouteachrow.

IfwerunthisprogramaftertheﬁrstexecutionofourTwitterspiderabove, its

outputwillbeasfollows:

opencontent

, 0, 1)

lhawthorn

, 0, , 1)

steve_coppin

, 0, 1)

davidkocher

, 0, 1)

hrheingold

, 0, 1)

...

100 rows.

Weseeonerowforeach

screen_name

,thatwehavenotretrievedthedataforthat

screen_name

andeveryoneinthedatabasehasonefriend.

NowourdatabasereﬂectstheretrievalofthefriendsofourﬁrstTwitteraccount

(drchuck).Wecanruntheprogramagainandtellittoretrievethefriendsofthe

next“unprocessed”accountbysimplypressingenterinsteadofaTwitteraccount

asfollows:

Enter a a Twitter r account, , or quit:

Retrieving http://api.twitter.com/l/statuses/friends/opencontent.xml

New accounts= 98

revisited= 2

Enter a a Twitter r account, , or quit:

Retrieving http://api.twitter.com/l/statuses/friends/lhawthorn.xml

New accounts= 97

revisited= 3

Enter a a Twitter r account, , or quit: quit

Sincewepressedenter(i.e. wedidnotspecifyaTwitteraccount),thefollowing

codeisexecuted:

if ( ( len(acct) < 1 ) :

cur.execute(

SELECT name FROM Twitter r WHERE E retrieved = 0 LIMIT 1

)

try:

acct = = cur.fetchone()[0]

except:

No unretrieved twitter accounts found

continue

WeusetheSQL

SELECT

statementtoretrievethenameoftheﬁrst(

LIMIT 1

)user

whostillhastheir“haveweretrievedthisuser”valuesettozero.Wealsousethe

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14.6. SpideringTwitterusingadatabase

171

fetchone()[0]

patternwithinatry/exceptblocktoeitherextracta

screen_name

fromtheretrieveddataorputoutanerrormessageandloopbackup.

Ifwesuccessfullyretrievedanunprocessed

screen_name

,weretrievetheirdata

asfollows:

url = TWITTER_URL.replace(

ACCT

, acct)

Retrieving

, url

document = urllib.urlopen (url).read()

tree = ET.fromstring(document)

cur.execute(

UPDATE Twitter SET T retrieved=1 1 WHERE name = ?

, (acct, ) )

Onceweretrievethedatasuccessfully, weusethe

UPDATE

statementtosetthe

retrieved

columntoonetoindicatethatwehavecompletedtheretrievalofthe

friendsofthisaccount. Thiskeepsusfromre-retrievingthesamedataoverand

overandkeepsusprogressingforwardthroughthenetworkofTwitterfriends.

Ifwerunthefriendprogramandpressentertwicetoretrievethenextunvisited

friend’sfriends,thenrunthedumpingprogram,itwillgiveusthefollowingout-

put:

opencontent

, 1, 1)

lhawthorn

, 1, , 1)

steve_coppin

, 0, 1)

davidkocher

, 0, 1)

hrheingold

, 0, 1)

...

cnxorg

, 0, , 2)

knoop

, 0, , 1)

kthanos

, 0, 2)

LectureTools

, 0, 1)

...

295 rows.

We cansee that we have properly recorded that we e have e visited

lhawthorn

and

opencontent

. Alsotheaccounts

cnxorg

and

kthanos

alreadyhave two

followers. Sincewenowhaveretrievedthefriendsofthreepeople(

drchuck

opencontent

and

lhawthorn

)ourtablehas295rowsoffriendstoretrieve.

Eachtime weruntheprogramandpress enter, itwillpickthenextunvisited

account(e.g.thenextaccountwillbe

steve_coppin

),retrievetheirfriends,mark

themasretrievedandforeachofthefriendsof

steve_coppin

,eitheraddthem

totheendofthedatabase,orupdatetheirfriendcountiftheyarealreadyinthe

database.

Sincetheprogram’sdataisallstoredondiskinadatabase,thespideringactivity

canbesuspendedandresumedasmanytimesasyoulikewithnolossofdata.

Note: Onemoretimebeforeweleavethistopic,beverycarefulwhenrunning

thisTwitterspideringprogram.Youdonotwanttopulltoomuchdataorrunthe

programfortoolongandenduphavingyourTwitteraccessshutoff.

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172

Chapter14. UsingdatabasesandStructuredQueryLanguage(SQL)

14.7 Basicdatamodeling

Therealpowerofarelationaldatabaseiswhenwemakemultipletablesandmake

linksbetweenthosetables. Theactofdecidinghowtobreakupyourapplication

dataintomultipletablesandestablishingtherelationshipsbetweenthetwotables

iscalleddatamodeling. Thedesigndocumentthatshowsthetablesandtheir

relationshipsiscalledadatamodel.

Datamodelingisarelativelysophisticatedskillandwewillonlyintroducethe

mostbasicconceptsofrelationaldatamodelinginthissection.Formoredetailon

datamodelingyoucanstartwith:

http://en.wikipedia.org/wiki/Relational_model

Let’ssayforourTwitterspiderapplication, insteadofjustcountingaperson’s

friends,wewantedtokeepalistofalloftheincomingrelationshipssowecould

ﬁndalistofeveryonewhoisfollowingaparticularaccount.

Sinceeveryonewillpotentiallyhavemanyaccountsthatfollowthem,wecannot

simplyaddasinglecolumntoour

Twitter

table. Sowecreateanewtablethat

keepstrackofpairsoffriends. Thefollowingisasimplewayofmakingsucha

table:

CREATE TABLE E Pals (from_friend TEXT, , to_friend TEXT)

Eachtimeweencounterapersonwho

drchuck

isfollowing,wewouldinserta

rowoftheform:

INSERT INTO O Pals s (from_friend,to_friend) ) VALUES S (

drchuck

lhawthorn

)

Asweareprocessingthe100friendsfromthe

drchuck

Twitterfeed,wewillinsert

100recordswith“drchuck”astheﬁrstparametersowewillendupduplicating

thestringmanytimesinthedatabase.

Thisduplicationofstringdataviolatesthebestpracticesfordatabasenormal-

izationwhichbasicallystatesthatweshouldneverputthesamestringdatainthe

databasemorethanonce.Ifweneedthedatamorethanonce,wecreateanumeric

keyforthedataandreferencetheactualdatausingthiskey.

Inpracticalterms,astringtakesupalotmorespacethananintegeronthedisk

andinthememoryofourcomputerandtakesmoreprocessortimetocompareand

sort.Ifweonlyhaveafewhundredentriesthestorageandprocessortimehardly

matters.Butifwehaveamillionpeopleinourdatabaseandapossibilityof100

millionfriendlinks,itisimportanttobeabletoscandataasquicklyaspossible.

WewillstoreourTwitteraccountsinatablenamed

People

insteadofthe

Twitter

tableusedinthepreviousexample. The

People

tablehasanadditionalcolumn

tostorethenumerickeyassociatedwiththerowforthisTwitteruser.SQLitehas

afeaturethatautomaticallyaddsthekeyvalueforanyrowweinsertintoatable

usingaspecialtypeofdatacolumn(

INTEGER PRIMARY Y KEY

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14.8. Programmingwithmultipletables

173

Wecancreatethe

People

tablewiththisadditional

columnasfollows:

CREATE TABLE E People

(id INTEGER R PRIMARY Y KEY, name TEXT UNIQUE, , retrieved INTEGER)

Noticethatwearenolongermaintainingafriendcountineachrowofthe

People

table. Whenweselect

INTEGER PRIMARY Y KEY

asthe typeofour

column,

weareindicatingthatwewouldlikeSQLitetomanagethiscolumnandassigna

uniquenumerickeytoeachrowweinsertautomatically.Wealsoaddthekeyword

UNIQUE

toindicatethatwewillnotallowSQLitetoinserttworowswiththesame

valuefor

name

Nowinsteadofcreatingthetable

Pals

above,wecreateatablecalled

Follows

withtwointegercolumns

from_id

and

to_id

andaconstraintonthetablethatthe

combinationof

from_id

and

to_id

mustbeuniqueinthistable(i.e. wecannot

insertduplicaterows)inourdatabase.

CREATE TABLE E Follows

(from_id INTEGER, to_id INTEGER, , UNIQUE(from_id, , to_id) )

Whenweadd

UNIQUE

clausestoourtables,wearecommunicatingasetofrules

thatweareasking thedatabasetoenforcewhenweattempttoinsertrecords.

Wearecreatingtheserulesasaconvenienceinourprogramsaswewillseeina

moment. Therulesbothkeepusfrommakingmistakesandmakeitsimplerto

writesomeofourcode.

Inessence,increatingthis

Follows

table,wearemodellinga”relationship”where

oneperson”follows”someoneelseandrepresentingitwithapairofnumbersin-

dicatingthat(a)thepeopleareconnectedand(b)thedirectionoftherelationship.

People

name

drchuck

opencontent

retrieved

Follows

from_id

to_id

lhawthorn

steve_coppin

...

14.8 Programming with multiple tables

We will now re-do the Twitter spider program using two tables, the primary keys,

and the key references as described above. Here is the code for the new version of

174

Chapter 14. Using databases and Structured Query Language (SQL)

the program:

import sqlite3

import urllib

import xml.etree.ElementTree as ET

TWITTER_URL =

http://api.twitter.com/l/statuses/friends/ACCT.xml

conn = sqlite3.connect(

twdata.db

)

cur = conn.cursor()

cur.execute(

'''

CREATE TABLE IF NOT EXISTS People

(id INTEGER PRIMARY KEY, name TEXT UNIQUE, retrieved INTEGER)

'''

)

cur.execute(

'''

CREATE TABLE IF NOT EXISTS Follows

(from_id INTEGER, to_id INTEGER, UNIQUE(from_id, to_id))

'''

)

while True:

acct = raw_input(

Enter a Twitter account, or quit:

)

if ( acct ==

quit

) : break

if ( len(acct) < 1 ) :

cur.execute(

'''

SELECT id,name FROM People

WHERE retrieved = 0 LIMIT 1

'''

)

try:

(id, acct) = cur.fetchone()

except:

No unretrieved Twitter accounts found

continue

else:

cur.execute(

SELECT id FROM People WHERE name = ? LIMIT 1

(acct, ) )

try:

id = cur.fetchone()[0]

except:

cur.execute(

'''

INSERT OR IGNORE INTO People

(name, retrieved) VALUES ( ?, 0)

'''

, ( acct, ) )

conn.commit()

if cur.rowcount != 1 :

Error inserting account:

,acct

continue

id = cur.lastrowid

url = TWITTER_URL.replace(

ACCT

, acct)

Retrieving

, url

document = urllib.urlopen (url).read()

tree = ET.fromstring(document)

cur.execute(

UPDATE People SET retrieved=1 WHERE name = ?

, (acct, ) )

countnew = 0

countold = 0

for user in tree.findall(

user

friend = user.find(

screen_name

).text

cur.execute(

SELECT id FROM People WHERE name = ? LIMIT 1

(friend, ) )

try:

14.8. Programming with multiple tables

175

friend_id = cur.fetchone()[0]

countold = countold + 1

except:

cur.execute(

'''

INSERT OR IGNORE INTO People (name, retrieved)

VALUES ( ?, 0)

'''

, ( friend, ) )

conn.commit()

if cur.rowcount != 1 :

Error inserting account:

,friend

continue

friend_id = cur.lastrowid

countnew = countnew + 1

cur.execute(

'''

INSERT OR IGNORE INTO Follows

(from_id, to_id) VALUES (?, ?)

'''

, (id, friend_id) )

New accounts=

,countnew,

revisited=

,countold

conn.commit()

cur.close()

This program is starting to get a bit complicated, but it illustrates the patterns that

we need to use when we are using integer keys to link tables. The basic patterns

are:

1. Creating tables with primary keys and constraints.

2. When we have a logical key for a person (i.e. account name) and we need

the

value for the person. Depending on whether or not the person is

already in the

People

table, we either need to: (1) look up the person in the

People

table and retrieve the

value for the person or (2) add the person

the the

People

table and get the

value for the newly added row.

3. Insert the row that captures the “follows” relationship.

We will cover each of these in turn.

14.8.1 Constraints in database tables

As we design our table structures, we can tell the database system that we would

like it to enforce a few rules on us. These rules help us from making mistakes and

introducing incorrect data into out tables. When we create our tables:

cur.execute(

'''

CREATE TABLE IF NOT EXISTS People

(id INTEGER PRIMARY KEY, name TEXT UNIQUE, retrieved INTEGER)

'''

)

cur.execute(

'''

CREATE TABLE IF NOT EXISTS Follows

(from_id INTEGER, to_id INTEGER, UNIQUE(from_id, to_id))

'''

)

We indicate that the

name

column in the

People

table must be

UNIQUE

.We also

indicate that the combination of the two numbers in each row of the

Follows

table

must be unique. These constraints keep us from making mistakes such as adding

the same relationship more than once.

We can take advantage of these constraints in the following code:

176

Chapter 14. Using databases and Structured Query Language (SQL)

cur.execute(

'''

INSERT OR IGNORE INTO People (name, retrieved)

VALUES ( ?, 0)

'''

, ( friend, ) )

We add the

OR IGNORE

clause to our

INSERT

statement to indicate that if this par-

ticular

INSERT

would cause a violation of the “

name

must be unique” rule, the

database system is allowed to ignore the

INSERT

.We are using the database con-

straint as a safety net to make sure we don’t inadvertently do something incorrect.

Similarly, the following code ensures that we don’t add the exact same

Follows

relationship twice.

cur.execute(

'''

INSERT OR IGNORE INTO Follows

(from_id, to_id) VALUES (?, ?)

'''

, (id, friend_id) )

Again we simply tell the database to ignore our attempted

INSERT

if it would

violate the uniqueness constraint that we speciﬁed for the

Follows

rows.

14.8.2 Retrieve and/or insert a record

When we prompt the user for a Twitter account, if the account exists, we must

look up its

value. If the account does not yet exist in the

People

table, we must

insert the record and get the

value from the inserted row.

This is a very common pattern and is done twice in the program above. This code

shows how we look up the

for a friend’s account when we have extracted a

screen_name

from a

user

node in the retrieved Twitter XML.

Since over time it will be increasingly likely that the account will already be in

the database, we ﬁrst check to see if the

People

record exists using a

SELECT

statement.

If all goes well

inside the

try

section, we retrieve the record using

fetchone()

and then retrieve the ﬁrst (and only) element of the returned tuple and store it in

friend_id

If the

SELECT

fails, the

fetchone()[0]

code will fail and control will transfer

into the

except

section.

friend = user.find(

screen_name

).text

cur.execute(

SELECT id FROM People WHERE name = ? LIMIT 1

(friend, ) )

try:

friend_id = cur.fetchone()[0]

countold = countold + 1

except:

cur.execute(

'''

INSERT OR IGNORE INTO People (name, retrieved)

VALUES ( ?, 0)

'''

, ( friend, ) )

conn.commit()

In general, when a sentence starts with “if all goes well” you will ﬁnd that the code needs to

use try/except.

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