Need help with "IndentationError: expected an indented block"

[hequ]

Hello!
Since python has a min(array) command I tried to make one manually. However I ran into a problem. Here is my code (file name is "listan.minimi.py"):

import sys


arr = [1,2,3]
def print2Smallest(arr):

arr_size = len(arr)
if arr_size < 2:
    print "invalid output"
    return

first = second = sys.maxint #antaa muuttujille first ja second suurimman mahdollisen lista-arvon pythonissa
for i in range(0, arr_size)

    if arr(i) < first:
    second = first
    first = arr[i]

    elif(arr[i]<second and arr[i]=!first):
        second = arr[i];

if (second == sys.maxint):
    print "No second smallest element"
else:
    print 'The smallest element is', first 'and'\
		' second smallest element is', second

Here is what I get out:

File "listan.minimi.py", line 7
arr_size = len(arr)
^
IndentationError: expected an indented block

I tried replacing tabs with spaces but still I don't seem to understand the problem here. Thanks!