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Ifthisisn’tenoughforyou,the
pickle
moduleisalsoextensible.Ifyou’reinterested inextensibility,check
outthelinksinthe
FurtherReadingsectionattheendofthechapter.
13.1.1.AQ
UICK
N
OTE
A
BOUT
T
HE
E
XAMPLES IN
T
HIS
C
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
D
ATATOA
P
ICKLE
F
ILE
The
pickle
moduleworkswithdatastructures.Let’sbuild one.
311
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>>> shell
1
>>> 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:
...
pickle.dump(entry, f)
...
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
D
ATAFROMA
P
ICKLE
F
ILE
Nowswitchtoyoursecond PythonShell—i.e.nottheonewhereyoucreatedthe
entry
dictionary.
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>>> shell
2
>>> entry
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:
...
entry = pickle.load(f)
...
>>> entry
{'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:
...
entry2 = pickle.load(f)
...
>>> entry2 == entry
True
>>> entry2 is entry
False
>>> entry2['tags']
('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
W
ITHOUTA
F
ILE
Theexamples intheprevious sectionshowed howtoserializeaPythonobjectdirectlytoafileondisk.But
whatifyoudon’twantorneedafile?Youcanalsoserializetoa
bytes
objectinmemory.
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>>> shell
1
>>> b = pickle.dumps(entry)
>>> type(b)
<class 'bytes'>
>>> entry3 = pickle.loads(b)
>>> entry3 == entry
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
S
TRINGS
R
EAR
T
HEIR
U
GLY
H
EADS
A
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
P
ICKLE
F
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
  Xpublished_dateq
ctime
struct_time
?qRqXtitleqXDive into history, 2009 editionqu.
Thatwasn’tterriblyhelpful.Youcanseethestrings,butotherdatatypesendupasunprintable(oratleast
unreadable)characters.Fieldsarenotobviouslydelimited bytabsorspaces.Thisis notaformatyouwould
wanttodebugbyyourself.
317
>>> shell
1
>>> 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
3
13.7.S
ERIALIZING
P
YTHON
O
BJECTSTOBE
R
EADBY
O
THER
L
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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