Find line matching an expression, copy entire line, insert in new line and replace the expression - awk

Question

I have several files (yml, tf, xml) for which I need to find a string i.e. var1, and then insert a new line with foo2, the rest of the line is unchanged.
Example
variable "my_vars" {
type = "map"
default = {
var1 = "10.48.225.160/28"
var2 = "10.48.225.160/28"
var3 = "10.48.225.160/28"
var4 = "10.48.225.160/28"
}
}
I tried the code below but I need the edit in place.
import sys
import string
def find(substr, replstr, infile):
f = open(infile,"rw")
lines = f.readlines()
for i in range(len(lines)):
if substr in lines[i]:
j = string.replace(lines[i], substr, replstr)
lines.insert(i + 1, j)
print "\n".join(lines)
old_env = sys.argv[1]
new_env = sys.argv[2]
file = sys.argv[3]
find(old_env, new_env, file)

Answer 1

import sys
import string
def find(substr, replstr, infile):
f = open(infile,"r")
lines = f.readlines()
for i in range(len(lines)):
if substr in lines[i]:
j = string.replace(lines[i], substr, replstr)
lines.insert(i + 1, j)
print "".join(lines)
f.close()
f = open(infile,"w")
k = "".join(lines)
f.writelines(k)
f.close()
old_env = sys.argv[1]
new_env = sys.argv[2]
file = sys.argv[3]
find(old_env, new_env, file)
The one caveat is there is a match on the last line of the file, the iterator will miss this.