Question

this is my python script below, I try to use sub() to replace all the values between <StartNum> and </StartNum> by using a variable continous number n Then, write the results to a new text file.
any people could point out the mistake?

import re
f1 = open('name.xml', 'r')
f2 = open('result.txt', 'w')
data=f1.read()
n=5000
for line in f1:
      f2.write(re.sub('<StartNum>(.*)</StartNum>', r'<StartNum>%s</StartNum>' % str(n), data))
      if "<StartNum>" in line:
          n=n+1
f1.close() 
f2.close()

Answer 1

You have a few problems with your code.

1. You are reading the file, f1, into data but you are continuing to us f1 for the loop. Once you have read the data, you can close the file.
2. You would probably want to split the lines up. You can do this a number of ways. The readlines method is probably the most straight forward.
3. When you are evaluating a regex in a loop, it can be very slow because it has to be recompiled each iteration. Instead, you can compile the regex ahead of time and use it instead.
4. In your re.sub, you were using data as the source, while you actually want the line.

There are other things you could do to improve the code, but the ones listed above are pretty necessary.

I am guessing this will likely do what you want.

import re
f1 = open('name.xml', 'r')
f2 = open('result.txt', 'w')
data = f1.readlines()
f1.close()
n = 5000
rex = re.compile('<StartNum>(.*)</StartNum>')
for line in data:
    f2.write(rex.sub(r'<StartNum>%s</StartNum>' % str(n), line))
    if "<StartNum>" in line:
        n += 1
f2.close()

Given names.xml of

<root>
    <StartNum>1</StartNum>
    <StartNum>5</StartNum>
    <StartNum>8</StartNum>
    <StartNum>9</StartNum>
    <StartNum>13</StartNum>
    <StartNum>33</StartNum>
    <foo>
        <bar baz="5" />
    </foo>
</root>

result.txt will be:

<root>
    <StartNum>5000</StartNum>
    <StartNum>5001</StartNum>
    <StartNum>5002</StartNum>
    <StartNum>5003</StartNum>
    <StartNum>5004</StartNum>
    <StartNum>5005</StartNum>
    <foo>
        <bar baz="5" />
    </foo>
</root>