Question

I have the following code, which modifies each line of the file test.tex by making a regular expression substitution.

import re
import fileinput

regex=re.compile(r'^([^&]*)(&)([^&]*)(&)([^&]*)')

for line in fileinput.input('test.tex',inplace=1):
    print regex.sub(r'\3\2\1\4\5',line),

The only problem is that I only want the substitution to apply to certain lines in the file, and there’s no way to define a pattern to select the correct lines. So, I want to display each line and prompt the user at the command line, asking whether to make the substitution at the current line. If the user enters “y”, the substitution is made. If the user simply enters nothing, the substitution is not made.

The problem, of course, is that by using the code inplace=1 I’ve effectively redirected stdout to the opened file. So there’s no way to show output (e.g. asking whether to make the substitution) to the command line that doesn’t get sent to the file.

Any ideas?

Answer 1

The file input module is really for dealing with more than one input file.
You can use the regular open() function instead.

Something like this should work.

By reading the file then resetting the pointer with seek(), we can override the file instead of appending to the end, and so edit the file in-place

import re

regex = re.compile(r'^([^&]*)(&)([^&]*)(&)([^&]*)')

with open('test.tex', 'r+') as f:
    old = f.readlines() # Pull the file contents to a list
    f.seek(0) # Jump to start, so we overwrite instead of appending
    for line in old:
        s = raw_input(line)
        if s == 'y':
            f.write(regex.sub(r'\3\2\1\4\5',line))
        else:
            f.write(line)

http://docs.python.org/tutorial/inputoutput.html