Average calculator to run python scripts with arguments

Question

I am trying to make a simple average calculation but trying my best to run on CMD. So far this is what I've came out

import sys

myList = [a,b,c]

myList[0] = int(sys.argv[1])

myList[1] = int (sys.argv[2])

myList[2] = int(sys.argv[3])

print 'Average:'  + sum(myList) / len(myList)

my question is; how do set a variable in a list to give them a value?

EDIT:

   import sys

    myList = [a,b,c]

    a = int(sys.argv[1])

    b = int (sys.argv[2])

    c = int(sys.argv[3])

    print 'Average:'  + sum(myList) / len(myList)

whats wrong with this code?

EDIT:

I want to allow users to run this program with three input arguments by passing three values to the program: a, b and c.

Edit:

this is my final edit, anyone can help me with this

import sys

a = float(sys.argv[1])
b = float(sys.argv[2])
c = float(sys.argv[3])
if a == str or  b == str or c == str:
   print 'Your input is invalid'
else:   
    print 'Average: %.2f ' % ((a + b + c) / 3)

Answer 1

Are you looking for something like:

import sys
my_list = [float(arg) for arg in sys.argv[1:]]
print('Average: {}'.format(sum(my_list) / len(my_list))

[float(arg) for arg in sys.argv[1:]] is a list-comprehension (one-liner for that returns a list). It parse args into float (float(arg)) and loop from the second element of sys.argv (first is name of you script file).

Float type is forced because Python trunk results of division if both numerator and denominator are int.

Btw, PEP8 required snake_case (my_list, not myList) and string concat should never be done with +, but with format or join().

Answer 2

You could create an empty list and use method append.

3 / 2 equals 1 in Python 2 so you want to work with floats.

You can't concatenate floats and strings so you want to use % or format.

Here is your code after corrections:

my_list = []
my_list.append(float(sys.argv[1]))
my_list.append(float(sys.argv[2]))
my_list.append(float(sys.argv[3]))
print 'Average: %s' % (sum(my_list) / len(my_list))

Or shorlty:

my_list = map(float, sys.argv[1:])
print 'Average: %s' % (sum(my_list) / len(my_list))

Or if you want to unpack arguments in separate variables:

a, b, c = map(float, sys.argv[1:])
print 'Average: %s' % ((a + b + c) / 3)