读/写文本文件

Question

I am trying to change a some lines in a text file without affecting the other lines. This is what's inside the text file called "text.txt"

this is  a test1|number1
this is a test2|number2
this is a test3|number2
this is a test4|number3
this is a test5|number3
this is a test6|number4
this is a test7|number5
this is a test8|number5
this is a test9|number5
this is a test10|number5

My objective is to change the line 4 and line 5 but keep the rest same.

mylist1=[]
for lines in open('test','r'):
    a=lines.split('|')
    b=a[1].strip()
    if b== 'number3':
        mylist1.append('{}|{} \n'.format('this is replacement','number7'))
    else:
         mylist1.append('{}|{} \n'.format(a[0],a[1].strip()))
myfile=open('test','w')
myfile.writelines(mylist1)

Even though the code works, I am wondering if there is any better and efficient way to do it? Is it possible to read the file just by line number?

Answer 1

There is not much you can improve. But you have to write all lines to a new file , either changed or unchanged. Minor improvements would be:

• using the with statement;
• avoiding storing lines in a list;
• writing lines without formatting in the else clause (if applicable).

Applying all of the above:

import shutil
with open('test') as old, open('newtest', 'w') as new:
    for line in old:
        if line.rsplit('|', 1)[-1].strip() == 'number3':
            new.write('this is replacement|number7\n')
        else:
            new.write(line)
shutil.move('newtest', 'test')

Answer 2

import fileinput

for lines in fileinput.input('test', inplace=True):
    # inplace=True redirects stdout to a temp file which will
    # be renamed to the original when we reach the end of the file. this
    # is more efficient because it doesn't save the whole file into memeory
    a = lines.split('|')
    b = a[1].strip()
    if b == 'number3':
        print '{}|{} '.format('this is replacement', 'number7')
    else:
        print '{}|{} '.format(a[0], a[1].strip())

Answer 3

否。文件是面向字节的，而不是面向行的，并且更改行的长度不会使后面的字节超前。

Answer 4

try this solution

with open('test', inplace=True) as text_file:
    for line in text_file:
         if line.rsplit('|', 1)[-1].strip() == 'number3':
             print '{}|{} \n'.format('this is replacement', 'number7')
         else:
             print line

Answer 5

It's not wholly clear whether your intent is to identify the lines to be replaced by their value , or by their line number .

If the former is your intent, you can get a list of lines like this:

with open('test','r') as f:
    oldlines = f.read().splitlines()

If there's a danger of trailing whitespace, you could also:

Then you can process them like this:

newlines = [ line if not line.strip().endswith('|number3') else 'this is replacement|number7' for line in oldlines]

Open the destination file (I'm assuming you want to overwrite the original, here), and write all the lines:

with open('test','w') as f:
    f.write("\n".join(newlines))

This is a general pattern that's useful for any kind of simple line-filtering.

If you meant to identify the lines by number, you could just alter the 'newlines' line:

 newlines = [ line if i not in (3, 4) else 'this is replacement|number7' for i, line in enumerate(oldlines)]