delimiting text file??

[Jim Bruer]

I've made it through the first 3 chapters of, Learning Python and
various online tutorials. With some excitement I'm trying to write my
FIRST program which will read a text file and comma delimit it, so it
can be read into a database table. I can read and write the file (line
by line) ok, but:

How do you insert a delimiter into a string at a given position?

Tried "f.insert(i,x)" then realized this is for mutable sequences.This
is so stupid I'm embarrassed to ask.