Replace text in multi-line string using text in the same line - python-3.x

Question

Here's a very dummy example of what I'm trying to accomplish, input is text:
{placeholder}, John Jones\n{placeholder}, Ben Franklin\n{placeholder}, George Washington
What I want to do is replace every instance of {placeholder} with the output of a function using data from the same line, e.g. the first initial and last name (would need to input first and last name to a function).
def initializer(name):
return f"{name.split(' ')[0][0]}. {name.split(' ')[1]}"
The desired result would be:
J. Jones, John Jones\nB. Franklin, Ben Franklin\nG. Washington, George Washington

Answer 1

A regular expression with a replacement function would work:
import re
s = "{placeholder}, John Jones\n{placeholder}, Ben Franklin\n{placeholder}, George Washington"
def repl_function(m):
return "{}. {}, {} {}".format(m.group(1)[0],m.group(2),m.group(1),m.group(2))
print(re.sub("\{placeholder},\s+(.*?)\s(.*)",repl_function,s))
prints:
J. Jones, John Jones
B. Franklin, Ben Franklin
G. Washington, George Washington
How does it work?
It captures {placeholder} and 2 words (until end of line, not matched by .* because re.DOTALL is not set and creates 2 groups (2 name parts).
On match, it calls the repl_function replacement function with the match object (second parameter of re.sub can be a string, a bytes object or a function which accepts one argument: the match object).
Just return the reformatted match object as string by shortening the first name and repeating the other information (this can be done in a lambda as well, but it's maybe more readable as a real function)

Answer 2

The following will work assuming you are using Python >= 3.6:
# it is the same function you showed in your question
def initializer(name):
return f"{name.split(' ')[0][0]}. {name.split(' ')[1]}"
s = '{placeholder}, John Jones\n{placeholder}, Ben Franklin\n{placeholder}, George Washington'
names = [name.rstrip() for name in s.split('{placeholder}, ') if name != '']
output = '\n'.join(f'{initializer(name)}, {name}' for name in names)
Output
'J. Jones, John Jones\nB. Franklin, Ben Franklin\nG. Washington, George Washington'