I/O Demo for http://unix.stackexchange.com/a/145916/60298

unix_so_145916_60298.py

#This input:

infile = '''
key0=0
key1=1
 key1 = 1
key2=2 # comment1
#key3=3
  #key4=4
#key5=5 # comment
  #key6=6 # comment
key7=7
key8 = 8
'''

# used with:

replacements = {
    'key1' :'11repl',
    'key2' :'22repl',
    'key3' :'33repl',
    'key4' :'44repl',
    'key5' :'55repl',
    'key6' :'66repl',
    }

    
# lead to the following file:

key0=0
key1=11repl
key1=11repl
key2=22repl# comment1
#key3=3
  #key4=4
#key5=5 # comment
  #key6=6 # comment
key7=7
key8= 8