You should not use key=int in c.sort(key=int) as c is already a list of ints due to you creating it via c.append(int(digit)). 
However the key issue is that c.sort() is in-place and as such returns None, instead use sorted which returns a sorted list:
csort = sorted(c)
And then you can print the boolean result of the comparsion via:
print c == csort
solved The digits of a number in ascending order. (comparing) [closed]