Using the sub() method with python

Question

I would like to replace text in a file using a regexp and Python. Using sed I can can do something like this on the command line

sed -r 's/([0-9]{1,3}\.)([0-9]{1,3}\.)([0-9]{1,3}\.)([0-9]{1,3})/\1\2xx.xx/' ./input/my_file > ./output/my_file_new

Which basically takes looks for a string of ip=[4 octets] and replaces the last two with xx.

The input file would look like

name=rockband&ip=176.4.23.71&releasedate=none
name=rockband2&ip=121.1.44.52&releasedate=none

The desired output file looks like

name=rockband&ip=176.4.xx.xx&releasedate=none
name=rockband2&ip=121.1.xx.xx&releasedate=none

I need to put this into a Python script I am using

import re
regexp = re.compile(r's/([0-9]{1,3}\.)([0-9]{1,3}\.)([0-9]{1,3}\.)([0-9]{1,3})/\1\2xx.xx/')

def replace(source_file_path):
fh, target_file_path = mkstemp()

with codecs.open(target_file_path, 'w', 'utf-8') as target_file:
    with codecs.open(source_file_path, 'r', 'utf-8') as source_file:
        for line in source_file:
            print(line)
            target_file.write( !! How to use sub in here )
remove(source_file_path)
move(target_file_path, source_file_path)

How can I use the sub() method to achieve what I want to do? I need to pass 3 arguments to this method and can only think of how to pass 2, I don't know what that third argument should be

target_file.write(re.sub(regexp, line))

So replacing every digit after third . into a x before the first &? — DirtyBit
– DirtyBit, Commented Jan 29, 2019 at 12:49
Unlike in case of sed syntax, your sub expect what to match and what to replace it as two arguments. — Ondrej K.
– Ondrej K., Commented Jan 29, 2019 at 12:49

Ondrej K. · Accepted Answer · 2019-01-29 12:56:38Z

Minimal required change to you code would be:

import re
regexp = re.compile(r'([0-9]{1,3}\.)([0-9]{1,3}\.)([0-9]{1,3}\.)([0-9]{1,3})')

def replace(source_file_path):
    fh, target_file_path = mkstemp()

    with codecs.open(target_file_path, 'w', 'utf-8') as target_file:
        with codecs.open(source_file_path, 'r', 'utf-8') as source_file:
            for line in source_file:
                print(line)
                target_file.write(regexp.sub(r'\1\2xx.xx', line))
    remove(source_file_path)
    move(target_file_path, source_file_path)

regexp only defines what to match. sub() has an argument of what to substitute with.

You can either call re.sub() which takes three required arguments: what to match, what to replace it with, which string to work on. Or as in the example above when you already have a precompiled regex, you can use its sub() method in which case need to say what to replace with and what string to work on.

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