How to add line numbers to multiline string

Question

I have a multiline string like the following:

txt = """
some text

on several

lines
"""

How can I print this text such that each line starts with a line number?

Answer 1

I usually use a regex substitution with a function attribute:

def repl(m):
    repl.cnt+=1
    return f'{repl.cnt:03d}: '

repl.cnt=0          
print(re.sub(r'(?m)^', repl, txt))

Prints:

001: 
002: some text
003: 
004: on several
005: 
006: lines
007: 

Which allows you to easily number only lines that have text:

def repl(m):
    if m.group(0).strip():
        repl.cnt+=1
        return f'{repl.cnt:03d}: {m.group(0)}'
    else:
        return '(blank)'    

repl.cnt=0  
print(re.sub(r'(?m)^.*$', repl, txt))

Prints:

(blank)
001:    some text
(blank)
002:    on several
(blank)
003:    lines
(blank)