Search and replace is adding not replacing

April 6, 2023

I am trying to find this string in a file:

/home/pi/dew-heater/get-temp.py

And replace it with

#/home/pi/dew-heater/get-temp.py

But there are cases where it already reads #/home/pi/dew-heater/get-temp.py. So my program is replacing it with a double ## so it looks like this:

##/home/pi/dew-heater/get-temp.py

How can I prevent the double ## ??

with open('/home/pi/dew-heater/getweather.sh', 'r') as fp:
    data = fp.read()
    typos = data.replace('/home/pi/dew-heater/get-temp.py', '#/home/pi/dew-heater/get-temp.py')
with open('/home/pi/dew-heater/getweather.sh', 'w') as fp:
    fp.write(typos)

>Solution :

Use a regular expression with a negative lookbehind.

import re

with open('/home/pi/dew-heater/getweather.sh', 'r') as fp:
    data = fp.read()
    typos = re.sub(r'(?<!#)/home/pi/dew-heater/get-temp\.py', '#/home/pi/dew-heater/get-temp.py', data)

with open('/home/pi/dew-heater/getweather.sh', 'w') as fp:
    fp.write(typos)