average.py

#!/usr/local/bin/python2.7

grad = {}
stu = {}
with open("input.txt") as f:
  for l in f:
    name, score = l.strip().split()
    if name not in grad:
      grad[name] = 0
    
    if name not in stu:
      stu[name] = 0
    grad[name] += int(score)
    stu[name] += 1

print grad
print stu

for x in stu:
  print x + " " + str( float(grad[x])/stu[x] )

input.txt

 tom  4
 peter 3
 tom 3
 jack 5
 peter 3

output.txt

{'peter': 6, 'jack': 5, 'tom': 7}
{'peter': 2, 'jack': 1, 'tom': 2}
peter 3.0
jack 5.0
tom 3.5