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Python中文件处理和序列化
1.文件和流
Python将每个文件都视为一个顺序的字节流。每个文件都结束于一个EOF（文件尾）标记，或结束于一个特定字节编号（由系统维护的一个管理数据结构记 录）。程序“打开"文件时，Python会创建一个对象，并将一个”流“与那个对象关联。
Python程序开始执行时，会创建3个文件流，包括sys.stdin,sys.stdout,sys.stderr。这些流在程序和特定文件/设备之间建立了沟通渠道。
2.创建顺序访问文件
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# Fig. 14.3: fig14_03.py
# Opening and writing to a file.
import sys
# open file
try:
file = open( "clients.dat", "w" ) # open file in write mode
except IOError, message: # file open failed
print >> sys.stderr, "File could not be opened:", message
sys.exit( 1 )
print "Enter the account, name and balance."
print "Enter end-of-file to end input."
while 1:
try:
accountLine = raw_input( "? " ) # get account entry
except EOFError:
break # user entered EOF
else:
print >> file, accountLine # write entry to file
file.close() |
模式 说明
"a" 所有输出都写到文件尾，如果指定的文件不存在，就创建一个
"r" 打开文件以便输入，如果文件不存在，就引发IOError异常
"r+" 打开文件以便输入和输出，如果文件不存在，就引发IOError异常
"w" 打开文件以便输出，如果文件存在，就删除其中所有数据，如果文件不存在，就创建一个
"w+" 打开文件以便输入和输出，如果文件存在，就删除其中所有数据，如果文件不存在，就创 建一个
"ab","rb","r+b" 打开文件以便进行二进制（也就是非文本形式）输入或输出。注意，只有在Windows和 "wb","w+b" Macintosh平台才支持这些模式
常用的文件对象的方法有close(),flush()等，可在Python中输入help(file)来获取更详细的信息。
3.从顺序访问文件读取数据
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# Fig. 14.6: fig14_06.py
# Reading and printing a file.
import sys
# open file
try:
file = open( "clients.dat", "r" )
except IOError:
print >> sys.stderr, "File could not be opened"
sys.exit( 1 )
records = file.readlines() # retrieve list of lines in file
print "Account".ljust( 10 ),
print "Name".ljust( 10 ),
print "Balance".rjust( 10 )
for record in records: # format each line
fields = record.split()
print fields[ 0 ].ljust( 10 ),
print fields[ 1 ].ljust( 10 ),
print fields[ 2 ].rjust( 10 )
file.close() |
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# Fig. 14.7: fig14_07.py
# Credit inquiry program.
import sys
# retrieve one user command
def getRequest():
while 1:
request = int( raw_input( "\n? " ) )
if 1 <= request <= 4:
break
return request
# determine if balance should be displayed, based on type
def shouldDisplay( accountType, balance ):
if accountType == 2 and balance < 0: # credit balance
return 1
elif accountType == 3 and balance > 0: # debit balance
return 1
elif accountType == 1 and balance == 0: # zero balance
return 1
else: return 0
# print formatted balance data
def outputLine( account, name, balance ):
print account.ljust( 10 ),
print name.ljust( 10 ),
print balance.rjust( 10 )
# open file
try:
file = open( "clients.dat", "r" )
except IOError:
print >> sys.stderr, "File could not be opened"
sys.exit( 1 )
print "Enter request"
print "1 - List accounts with zero balances"
print "2 - List accounts with credit balances"
print "3 - List accounts with debit balances"
print "4 - End of run"
# process user request(s)
while 1:
request = getRequest() # get user request
if request == 1: # zero balances
print "\nAccounts with zero balances:"
elif request == 2: # credit balances
print "\nAccounts with credit balances:"
elif request == 3: # debit balances
print "\nAccounts with debit balances:"
elif request == 4: # exit loop
break
else: # getRequest should never let program reach here
print "\nInvalid request."
currentRecord = file.readline() # get first record
# process each line
while ( currentRecord != "" ):
account, name, balance = currentRecord.split()
balance = float( balance )
if shouldDisplay( request, balance ):
outputLine( account, name, str( balance ) )
currentRecord = file.readline() # get next record
file.seek( 0, 0 ) # move to beginning of file
print "\nEnd of run."
file.close() # close file |
4.将数据写入shelve文件
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# Fig. 14.9: fig14_09.py
# Writing to shelve file.
import sys
import shelve
# open shelve file
try:
outCredit = shelve.open( "credit.dat" )
except IOError:
print >> sys.stderr, "File could not be opened"
sys.exit( 1 )
print "Enter account number (1 to 100, 0 to end input)"
# get account information
while 1:
# get account information
accountNumber = int( raw_input(
"\nEnter account number\n? " ) )
if 0 < accountNumber <= 100:
print "Enter lastname, firstname, balance"
currentData = raw_input( "? " )
outCredit[ str( accountNumber ) ] = currentData.split()
elif accountNumber == 0:
break
outCredit.close() # close shelve file |
5.从shelve文件获取数据
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# Fig. 14.9: fig14_09.py
# Reading a shelve file.
import sys
import shelve
# print formatted credit data
def outputLine( account, aList ):
print account.ljust( 10 ),
print aList[ 0 ].ljust | | |
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