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Ifthisisn’tenoughforyou,the

pickle

moduleisalsoextensible.Ifyou’reinterested inextensibility,check

outthelinksinthe

FurtherReadingsectionattheendofthechapter.

13.1.1.AQ

UICK

OTE

BOUT

XAMPLES IN

HIS

HAPTER

ThischaptertellsatalewithtwoPython Shells.Alloftheexamplesinthis chapterarepartofasinglestory

arc.Youwillbeasked toswitchbackandforthbetweenthetwoPythonShells as Idemonstratethe

pickle

and

json

modules.

Tohelpkeepthings straight,openthePythonShelland definethefollowingvariable:

>>> shell = 1

Keepthatwindowopen.NowopenanotherPythonShellanddefinethefollowingvariable:

>>> shell = 2

Throughoutthischapter,Iwillusethe

shell

variabletoindicatewhichPythonShellis beingused ineach

example.

⁂

13.2.S

AVING

ATATOA

ICKLE

ILE

The

pickle

moduleworkswithdatastructures.Let’sbuild one.

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>>> shell

>>> entry = {}

>>> entry['title'] = 'Dive into history, 2009 edition'

>>> entry['article_link'] = 'http://diveintomark.org/archives/2009/03/27/dive-into-history-2009-edition'

>>> entry['comments_link'] = None

>>> entry['internal_id'] = b'\xDE\xD5\xB4\xF8'

>>> entry['tags'] = ('diveintopython', 'docbook', 'html')

>>> entry['published'] = True

>>> import time

>>> entry['published_date'] = time.strptime('Fri Mar 27 22:20:42 2009')

>>> entry['published_date']

time.struct_time(tm_year=2009, tm_mon=3, tm_mday=27, tm_hour=22, tm_min=20, tm_sec=42, tm_wday=4, tm_yday=86, tm_isdst=-1)

1. FollowalonginPythonShell#1.

2. TheideahereistobuildaPythondictionarythatcouldrepresentsomethinguseful,likean

entryinanAtom

feed.ButIalsowanttoensurethatitcontains severaldifferenttypesofdata,toshowoffthe

pickle

module.Don’treadtoomuchintothesevalues.

3. The

time

modulecontainsadatastructure(

time_struct

)torepresentapointintime(accuratetoone

millisecond)andfunctionstomanipulatetimestructs.The

strptime()

functiontakesaformattedstringan

convertsittoa

time_struct

.Thisstringisinthedefaultformat,butyoucancontrolthatwithformatcodes.

Seethe

time

moduleformoredetails.

That’s ahandsome-lookingPythondictionary.Let’ssaveittoafile.

>>> shell

①

>>> import pickle

>>> with open('entry.pickle', 'wb') as f:

②

...

pickle.dump(entry, f)

③

...

1. ThisisstillinPythonShell#1.

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2. Usethe

open()

functiontoopenafile.Setthefilemodeto

'wb'

toopenthefileforwriting

inbinarymode.

Wrapitina

with

statementtoensurethefileisclosedautomaticallywhenyou’redonewithit.

3. The

dump()

functioninthe

pickle

moduletakesaserializablePythondatastructure,serializesitintoa

binary,Python-specific formatusingthelatestversionofthepickleprotocol,andsaves ittoanopenfile.

Thatlastsentencewaspretty important.

• The

pickle

moduletakes aPythondatastructureandsavesittoafile.

• Todothis,itserializesthedatastructureusingadataformatcalled d “thepickleprotocol.”

• ThepickleprotocolisPython-specific;thereisnoguaranteeofcross-languagecompatibility.Youprobably

couldn’ttakethe

entry.pickle

fileyou justcreated and doanythingusefulwithitinPerl,

PHP

,Java,orany

otherlanguage.

• NoteveryPythondatastructurecanbeserializedbythe

pickle

module.Thepickleprotocolhaschanged

severaltimesasnewdatatypeshavebeenaddedtothePythonlanguage,buttherearestilllimitations.

• Asaresultofthesechanges,thereisnoguaranteeofcompatibilitybetweendifferentversions s ofPython

itself.NewerversionsofPythonsupporttheolderserializationformats,butolderversionsofPython donot

supportnewerformats(sincetheydon’tsupportthenewerdatatypes).

• Unlessyouspecifyotherwise,thefunctions s inthe

pickle

modulewillusethelatestversionofthepickle

protocol.This ensures thatyouhavemaximumflexibility inthetypesofdatayoucanserialize,butitalso

meansthattheresultingfilewillnotbereadablebyolderversionsofPythonthatdonotsupportthelatest

version ofthepickleprotocol.

• Thelatestversionofthepickleprotocolis s abinaryformat.Besuretoopenyourpicklefiles

inbinarymode,

orthedatawillgetcorruptedduringwriting.

⁂

13.3.L

OADING

ATAFROMA

ICKLE

ILE

Nowswitchtoyoursecond PythonShell—i.e.nottheonewhereyoucreatedthe

entry

dictionary.

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>>> shell

①

>>> entry

②

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

NameError: name 'entry' is not defined

>>> import pickle

>>> with open('entry.pickle', 'rb') as f:

③

...

entry = pickle.load(f)

④

...

>>> entry

⑤

{'comments_link': None,

'internal_id': b'\xDE\xD5\xB4\xF8',

'title': 'Dive into history, 2009 edition',

'tags': ('diveintopython', 'docbook', 'html'),

'article_link':

'http://diveintomark.org/archives/2009/03/27/dive-into-history-2009-edition',

'published_date': time.struct_time(tm_year=2009, tm_mon=3, tm_mday=27, tm_hour=22, tm_min=20, tm_sec=42, tm_wday=4, tm_yday=86, tm_isdst=-1),

'published': True}

1. ThisisPythonShell#2.

2. Thereisno

entry

variabledefined here.Youdefined an

entry

variableinPythonShell#1,butthat’sa

completelydifferentenvironmentwithitsownstate.

3. Openthe

entry.pickle

fileyou created inPythonShell#1.The

pickle

moduleusesabinarydataformat,so

youshould alwaysopenpicklefilesinbinary mode.

4. The

pickle.load()

functiontakesa

streamobject,readstheserializeddatafromthestream,createsanew

Pythonobject,recreatestheserializeddatainthenewPythonobject,and returns thenewPythonobject.

5. Nowthe

entry

variableisadictionarywithfamiliar-lookingkeys and values.

The

pickle.dump() / pickle.load()

cycleresultsinanewdatastructurethatisequaltotheoriginaldata

structure.

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>>> shell

①

>>> with open('entry.pickle', 'rb') as f:

②

...

entry2 = pickle.load(f)

③

...

>>> entry2 == entry

④

True

>>> entry2 is entry

④

False

>>> entry2['tags']

⑥

('diveintopython', 'docbook', 'html')

>>> entry2['internal_id']

b'\xDE\xD5\xB4\xF8'

1. SwitchbacktoPythonShell#1.

2. Openthe

entry.pickle

file.

3. Load d theserializeddataintoanewvariable,

entry2

4. Pythonconfirms s thatthetwodictionaries,

entry

and

entry2

,areequal.Inthisshell,youbuilt

entry

fromthe

groundup,startingwithanemptydictionaryandmanuallyassigningvaluestospecific keys.Youserializedthis

dictionary and storeditin the

entry.pickle

file.Nowyou’vereadtheserializeddatafromthatfileand

createdaperfectreplicaoftheoriginaldatastructure.

5. Equalityisnotthesameasidentity.Isaidyou’vecreatedaperfectreplicaoftheoriginaldatastructure,which

istrue.Butit’s stillacopy.

6. Forreasons s thatwillbecomeclearlaterin this chapter,Iwanttopointoutthatthevalueofthe

'tags'

key

isatuple,and thevalueofthe

'internal_id'

keyisa

bytes

object.

⁂

13.4.P

ICKLING

ITHOUTA

ILE

Theexamples intheprevious sectionshowed howtoserializeaPythonobjectdirectlytoafileondisk.But

whatifyoudon’twantorneedafile?Youcanalsoserializetoa

bytes

objectinmemory.

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>>> shell

>>> b = pickle.dumps(entry)

①

>>> type(b)

②

>>> entry3 = pickle.loads(b)

③

>>> entry3 == entry

④

True

1. The

pickle.dumps()

function(notethe

's'

attheendofthefunctionname) performsthesameserialization

asthe

pickle.dump()

function.Insteadoftakingastreamobjectandwritingtheserializeddatatoafileon

disk,itsimplyreturnstheserializeddata.

2. Sincethepickleprotocolusesabinarydataformat,the

pickle.dumps()

functionreturnsa

bytes

object.

3. The

pickle.loads()

function(again,notethe

's'

attheend ofthefunctionname)performsthesame

deserialization as the

pickle.load()

function.Insteadoftakingastreamobjectandreadingtheserializeddata

fromafile,ittakesa

bytes

objectcontainingserializeddata,suchastheonereturnedbythe

pickle.dumps()

function.

4. Theendresultisthesame:aperfectreplicaoftheoriginaldictionary.

⁂

13.5.B

YTESAND

TRINGS

EAR

HEIR

GLY

EADS

GAIN

Thepickleprotocolhas beenaroundformany years,andithasmatured as Pythonitselfhasmatured.There

arenow

fourdifferentversionsofthepickleprotocol.

• Python1.xhadtwopickleprotocols,atext-based d format(“version0”)andabinary format(“version1”).

• Python2.3introducedanewpickleprotocol(“version2”)tohandlenewfunctionalityinPythonclass

objects.Itisabinaryformat.

• Python3.0introducedanotherpickleprotocol(“version3”) ) withexplicitsupportfor

bytes

objectsandbyte

arrays.Itisabinaryformat.

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Ohlook,

thedifferencebetweenbytesandstringsrears its uglyhead again.(Ifyou’resurprised,you haven’t

beenpayingattention.)Whatthismeansinpracticeisthat,whilePython3canread datapickledwith

protocolversion2,Python2cannotreaddatapickledwithprotocolversion3.

⁂

13.6.D

EBUGGING

ICKLE

ILES

Whatdoes thepickleprotocollooklike?Let’sjumpoutofthePythonShellforamomentandtakealook

atthat

entry.pickle

filewecreated.

you@localhost:~/diveintopython3/examples$ ls -l entry.pickle

-rw-r--r-- 1 you you 358 Aug 3 13:34 entry.pickle

you@localhost:~/diveintopython3/examples$ cat entry.pickle

comments_linkqNXtagsqXdiveintopythonqXdocbookqXhtmlq?qX publishedq?

XlinkXJhttp://diveintomark.org/archives/2009/03/27/dive-into-history-2009-edition

q Xpublished_dateq

ctime

struct_time

?qRqXtitleqXDive into history, 2009 editionqu.

Thatwasn’tterriblyhelpful.Youcanseethestrings,butotherdatatypesendupasunprintable(oratleast

unreadable)characters.Fieldsarenotobviouslydelimited bytabsorspaces.Thisis notaformatyouwould

wanttodebugbyyourself.

317

>>> shell

>>> import pickletools

>>> with open('entry.pickle', 'rb') as f:

...

pickletools.dis(f)

0: \x80 PROTO 3

2: } EMPTY_DICT

3: q BINPUT 0

5: ( MARK

6: X BINUNICODE 'published_date'

25: q BINPUT 1

27: c GLOBAL 'time struct_time'

45: q BINPUT 2

47: ( MARK

48: M BININT2 2009

51: K BININT1 3

53: K BININT1 27

55: K BININT1 22

57: K BININT1 20

59: K BININT1 42

61: K BININT1 4

63: K BININT1 86

65: J BININT -1

70: t TUPLE (MARK at 47)

71: q BINPUT 3

73: } EMPTY_DICT

74: q BINPUT 4

76: \x86 TUPLE2

77: q BINPUT 5

79: R REDUCE

80: q BINPUT 6

82: X BINUNICODE 'comments_link'

100: q BINPUT 7

102: N NONE

318

103: X BINUNICODE 'internal_id'

119: q BINPUT 8

121: C SHORT_BINBYTES 'ÞÕ´ø'

127: q BINPUT 9

129: X BINUNICODE 'tags'

138: q BINPUT 10

140: X BINUNICODE 'diveintopython'

159: q BINPUT 11

161: X BINUNICODE 'docbook'

173: q BINPUT 12

175: X BINUNICODE 'html'

184: q BINPUT 13

186: \x87 TUPLE3

187: q BINPUT 14

189: X BINUNICODE 'title'

199: q BINPUT 15

201: X BINUNICODE 'Dive into history, 2009 edition'

237: q BINPUT 16

239: X BINUNICODE 'article_link'

256: q BINPUT 17

258: X BINUNICODE 'http://diveintomark.org/archives/2009/03/27/dive-into-history-2009-edition'

337: q BINPUT 18

339: X BINUNICODE 'published'

353: q BINPUT 19

355: \x88 NEWTRUE

356: u SETITEMS (MARK at 5)

357: . STOP

highest protocol among opcodes = 3

Themostinterestingpieceofinformationinthatdisassemblyisonthelastline,becauseitincludesthe

version ofthepickleprotocolwithwhichthisfilewassaved.Thereisnoexplicitversionmarkerin the

pickleprotocol.Todeterminewhichprotocolversionwas usedtostoreapicklefile,youneedtolookat

themarkers(“opcodes”)withinthepickled dataandusehard-codedknowledgeofwhichopcodes were

introducedwitheachversion ofthepickleprotocol.The

pickle.dis()

functiondoesexactlythat,andit

319

printstheresultinthelastlineof thedisassemblyoutput.Hereisafunctionthatreturnsjusttheversion

number,withoutprintinganything:

import pickletools

def protocol_version(file_object):

maxproto = -1

for opcode, arg, pos in pickletools.genops(file_object):

maxproto = max(maxproto, opcode.proto)

return maxproto

Andhereitisinaction:

>>> import pickleversion

>>> with open('entry.pickle', 'rb') as f:

...

v = pickleversion.protocol_version(f)

>>> v

⁂

13.7.S

ERIALIZING

YTHON

BJECTSTOBE

EADBY

THER

ANGUAGES

Thedataformatusedbythe

pickle

moduleis Python-specific.Itmakes noattempttobecompatiblewith

otherprogramminglanguages.Ifcross-languagecompatibility isoneofyourrequirements,youneedtolook

atotherserializationformats.Onesuchformatis

JSON

.“

JSON

”standsfor“JavaScriptObjectNotation,”but

don’tletthenamefoolyou—

JSON

isexplicitlydesignedtobeusableacross multipleprogramming

languages.

Python3includesa

json

moduleinthestandard library.Likethe

pickle

module,the

json

modulehas

functionsforserializingdatastructures,storingtheserializeddataondisk,loadingserialized datafromdisk,

320

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