import dis

def one(filepath):
    with open(filepath, 'r') as fp:
        line = fp.readline()
        count = 1
        while line:
             # do processing
             count +=1
             line = fp.readline()

def two(filepath):
    with open(filepath, 'r') as fp:
        for i, line in enumerate(fp):
            # do processing
            pass


print("function one")
print("- - - - - - - - - - - ")
dis.dis(one)

print("function two")
print("- - - - - - - - - - - ")
dis.dis(two)


here is the python bytecode of the respective loops
(without the regex 'work' which should be the same)

function one
        >>   26 LOAD_FAST                2 (line)
             28 POP_JUMP_IF_FALSE       48
# do processing is here
  9          30 LOAD_FAST                3 (count)
             32 LOAD_CONST               2 (1)
             34 INPLACE_ADD
             36 STORE_FAST               3 (count)

 10          38 LOAD_FAST                1 (fp)
             40 LOAD_ATTR                1 (readline)
             42 CALL_FUNCTION            0
             44 STORE_FAST               2 (line)
             46 JUMP_ABSOLUTE           26

function two
        >>   22 FOR_ITER                 8 (to 32)
             24 UNPACK_SEQUENCE          2
             26 STORE_FAST               2 (i)
             28 STORE_FAST               3 (line)
# do processing is here
 16          30 JUMP_ABSOLUTE           22