I think the accepted answer is great, but why don't you do it explicitly? I feel more people would understand your code, and that is in agreement with PEP 8:

max_value = max(my_list)
max_index = my_list.index(max_value)

This method is also about three times faster than the accepted answer:

import random
from datetime import datetime
import operator

def explicit(l):
    max_val = max(l)
    max_idx = l.index(max_val)
    return max_idx, max_val

def implicit(l):
    max_idx, max_val = max(enumerate(l), key=operator.itemgetter(1))
    return max_idx, max_val

if __name__ == "__main__":
    from timeit import Timer
    t = Timer("explicit(l)", "from __main__ import explicit, implicit; "
          "import random; import operator;"
          "l = [random.random() for _ in xrange(100)]")
    print "Explicit: %.2f usec/pass" % (1000000 * t.timeit(number=100000)/100000)

    t = Timer("implicit(l)", "from __main__ import explicit, implicit; "
          "import random; import operator;"
          "l = [random.random() for _ in xrange(100)]")
    print "Implicit: %.2f usec/pass" % (1000000 * t.timeit(number=100000)/100000)

Results as they run in my computer:

Explicit: 8.07 usec/pass
Implicit: 22.86 usec/pass

Other set:

Explicit: 6.80 usec/pass
Implicit: 19.01 usec/pass

It is easiest to just apply logic to the entire list. You can get the max value of a list using max(list), and you can use the same index function you use above to get the index of this number in the list.

max_val = max(list_c)
idx_max = list_c.index(max_val)

If you want to loop over the values to find the max, then take note of the following:

1. list.index(value) will find the index of that value in the list. In your case you are writing list.index(index_number) which doesnt make sense. 'i' already represents the list index, so just set idx_max = 'i'.

2. Further, you should note that if you set max_val to zero, if all the list items are less than zero, it will stay at zero during the loop and never find the max value. It is better to start by setting max_val to the first item of the list ie list_c[0].

    list_c = [-14, 7, -9, 2]
   
    max_val = list_c[0]
    idx_max = 0
   
    for i in range(len(list_c)):
        if list_c[i] > max_val:
            max_val = list_c[i]
            idx_max = i
   

NB 'return' is used to return values from a function. If you want to print the values, use print(list_c, max_val, idx_max).