python - Re.sub not working for me

Question

I'm trying to get re.sub to replace a pattern specified with a value for example

for lines in f:
    pattern='${2}'+key[0]+'${2}'
    re.search(pattern,lines)

this return the line where the pattern was found. For example this is one of the test returns if got

this is a $$test$$

The problem i'm having is when i do the following

re.sub(pattern,key[1],lines)

nothing happens. What am i missing? For more info key[0]=test and key[1]=replace so what i'm trying to do is whenever "$$test$$" is encountered it will replace it with "replace". I have no problem finding "$$test$$" but for some reason re.sub isn't replacing it.

Answer 1

You are assigning the result of re.sub back to a variable, right? e.g.

lines = re.sub(pattern, key[1], lines)

It's a string, so it can't be changed (strings are immutable in Python), therefore a new string is created and returned to you. If you don't assign it back to a name, you will lose it.