Calculate averages of stdin in python

Question

I want to read in python from stdin and it looks like this:

(Group),(Grade):
1gT,8
1gT,5
1gT,9
1gT,8
1gX,4
1gX,4
1gX,7
1gZ,2
1gZ,9
1gZ,10

Now I want to calculate the averages per group. I know I can read from stdin with

for line in sys.stdin:

And I know how to calculate the averages:

([Sum of all grades from one group] / [number of grades of one group])

But how can I read the grades per group and count the number of it in Python3?

Answer 1

Thank you all for thinking. Thanks to Watanabe.N for helping, with a few edits on your answer it worked.

This worked for me:

import sys

ave = 0
total = 0
count = 0
firstline = sys.stdin.readline()
group, grade = firstline.split()
currentGroup = group
grade = int(grade)
total += grade
count += 1

for line in sys.stdin:
    group, grade = line.split()
    grade = int(grade)
    if currentGroup != group:
        print(currentGroup, ave)
        count = 1
        total = 0 + grade
        currentGroup = group
        continue
    count += 1
    total += grade
    ave = total/count
else:
    print(currentGroup, ave)