Confused with 'flags'

[tester_V]

Greetings!
I'm confused about how to use 'flag', I read I can use it instead of 'ELSE'.
I'm playing with the code below to see how I can use "flags" but it does not work.

with open ('C:/02/somefile.txt','r') as f :
    flg = False
    for l in f :
        if '123' in l : 
            print (l)        
            flg = True 

    if flg == False :
        print (" 'ELSE' -->> "+l)

Thank you.