第三章-Python基础 课后答案

3-1

python是动态语言，它的对象的类型和内存都是运行时确定的；在创建新对象时，解释器会根据语法和右侧的操作数来决定新对象的类型。

3-2

python不用去声明函数的返回类型，是由其“若类型”的语言特性决定的。python是把储存的数据直接放到内存，然后再去用一个变量名引用这个数据。

3-3

因为python系统定义的名字是__xxx__，所以在变量名的开始和结尾使用双下划线容易引起混淆。

3-4

可以，但是要用";"分隔开。

3-5

可以，一行过长的语句可以使用反斜杠( \ )分解成几行；使用闭合操作符时，单一语句也可以跨多行，例如：在含有小括号、中括号、花括号时可以书写多行。另外就是三引号包括下的字符串也可以跨行书写。

3-6

(a) 1,2,3

(b) 2,3,1

3-7

查文档

3-8

makeTextFile.py

'makeTextFile.py -- create text file'

import os
ls = os.linesep

#get filename
while True:

    if os.path.exists('C:\\Users\\Administrator\\Desktop\\python\\xue\\weixing.txt'):
        print "ERROR: '%s' already exists" % ('C:\\Users\\Administrator\\Desktop\\python\\xue\\weixing.txt')
    else:
        break

#get file content (text) lines
all = []
print "\nEnter lines ('.' by itself to quit).\n"

# loop until user terminates input
while True:
    entry = raw_input('input something you want to write: ')
    if entry == '.':
        break
    else:
        all.append(entry)

# write lines to file with proper line-ending
fobj = open('C:\\Users\\Administrator\\Desktop\\python\\xue\\weixing.txt', 'w')
fobj.writelines(['%s%s' % (x, ls) for x in all])
fobj.close()
print 'DONE!'

readTextFile.py

'readTextFile.py -- read and display text file'

# get filename
fname = raw_input('Enter filename: ')
print

# attempt to open file for reading
try:
    fobj = open(fname, 'r')
except IOError, e:
    print "*** file open error:", e
else:
    #display contents to the screen
    for eachLine in fobj:
        print eachLine,
    fobj.close()

3-9

os.linesep给出当前平台使用的行终止符

windows平台使用 '\r\n'

linux平台使用 '\n'

mac平台使用 '\r'

3-10

makeTextFile2.py

import os
ls = os.linesep

#get filename
while True:
    fname = raw_input("Enter file name:")
    try:
        open(fname, 'r')
        print "*** ERROR:'%s' already exists" % fname
    except IOError:
        break
        fname.close()

# get file content (text) lines
all = []
print "\nEnter lines ('.' by itself to quit).\n"

# loop until user terminates input
while True:
    entry = raw_input("input something you want to write: ")
    if entry == '.':
        break
    else:
        all.append(entry)

# write lines to file with proper line-ending
fobj = open(fname, 'w')
fobj.writelines(['%s%s' % (x, ls) for x in all])
fobj.close()
print 'DONE!'

readTextFile2.py

import os

'readTextFile.py -- read and display text file'

# get filename
fname = raw_input('Enter filename: ')
print

# attempt to open file for reading
if os.path.exists(fname):
    fobj = open(fname, 'r')
    for eachLine in fobj:
        print eachLine,
    fobj.close()
else:
    print "Error: can't find the file!"
    

3-11

'readTextFile.py -- read and display text file'

# get filename
fname = raw_input('Enter filename: ')
print

# attempt to open file for reading
try:
    fobj = open(fname, 'r')
except IOError, e:
    print "*** file open error:", e
else:
    #display contents to the screen
    for eachLine in fobj:
        print eachLine.strip()
    fobj.close()

3-12

import os
ls = os.linesep

def makeTextFile():
    while True:
        fname = raw_input("Enter file name:")
        try:
            open(fname, 'r')
            print "*** ERROR:'%s' already exists" % fname
        except IOError:
            break
            fname.close()

    # get file content (text) lines
    all = []
    print "\nEnter lines ('.' by itself to quit).\n"

    # loop until user terminates input
    while True:
        entry = raw_input("input something you want to write: ")
        if entry == '.':
            break
        else:
            all.append(entry)

    # write lines to file with proper line-ending
    fobj = open(fname, 'w')
    fobj.writelines(['%s%s' % (x, ls) for x in all])
    fobj.close()
    print 'DONE!'

def readTextFile():
    fname = raw_input('Enter filename: ')
    print

    # attempt to open file for reading
    if os.path.exists(fname):
        fobj = open(fname, 'r')
        for eachLine in fobj:
            print eachLine,
        fobj.close()
    else:
        print "Error: can't find the file!"

if __name__ == "__main__":
    while True:
        print """Choose one:
            1)makeTextFile
            2)readTextFile
            3)quit
        """

        getcode = raw_input("which you choose:")
        if getcode == "1":
            makeTextFile()
        elif getcode == "2":
            readTextFile()
        elif getcode == "3":
            break
        else:
            print "Please repeat one!"