Python program : String manipulations

The possible grades are A, AB, B, BC, C, CD, D with corresponding grade points 10, 9, 8, 7, 6, 5 and 4. The grade point average of a student is the sum of his/her grade points divided by the number of courses. For instance, if a student has taken two courses with grades A and C, the grade point average is 8 = (10+6)÷2. If a student has not completed any courses, the grade point average is defined to be 0.
You may assume that the data is internally consistent. For every grade, there is a corresponding course code and roll number in the input data.
Each section of the input starts with a line containing a single keyword. The first section begins with a line containing Courses. The second section begins with a line containing Students. The third section begins with a line containing Grades. The end of the input is marked by a line containing EndOfInput.
Write a Python program to read the data as described above and print out a line listing the grade point average for each student in the following format:

Roll Number~Full Name~Grade Point Average

Your output should be sorted by Roll Number. The grade point average should be rounded off to 2 digits after the decimal point. Use the built-in function round().
Here is a sample input and its corresponding output.
Sample Input

Courses
TRAN~Transfiguration~1~2011-2012~Minerva McGonagall
CHAR~Charms~1~2011-2012~Filius Flitwick
Students
SLY2301~Hannah Abbott
SLY2302~Euan Abercrombie
SLY2303~Stewart Ackerley
SLY2304~Bertram Aubrey
SLY2305~Avery
SLY2306~Malcolm Baddock
SLY2307~Marcus Belby
SLY2308~Katie Bell
SLY2309~Sirius Orion Black
Grades
TRAN~1~2011-2012~SLY2301~AB
TRAN~1~2011-2012~SLY2302~B
TRAN~1~2011-2012~SLY2303~B
TRAN~1~2011-2012~SLY2305~A
TRAN~1~2011-2012~SLY2306~BC
TRAN~1~2011-2012~SLY2308~A
TRAN~1~2011-2012~SLY2309~AB
CHAR~1~2011-2012~SLY2301~A
CHAR~1~2011-2012~SLY2302~BC
CHAR~1~2011-2012~SLY2303~B
CHAR~1~2011-2012~SLY2305~BC
CHAR~1~2011-2012~SLY2306~C
CHAR~1~2011-2012~SLY2307~B
CHAR~1~2011-2012~SLY2308~AB
EndOfInput

Sample Output

SLY2301~Hannah Abbott~9.5
SLY2302~Euan Abercrombie~7.5
SLY2303~Stewart Ackerley~8.0
SLY2304~Bertram Aubrey~0
SLY2305~Avery~8.5
SLY2306~Malcolm Baddock~6.5
SLY2307~Marcus Belby~8.0
SLY2308~Katie Bell~9.5
SLY2309~Sirius Orion Black~9.0

def gpa(l):
  n=len(l)
  m=0
  M=0
  for i in range(0,n):
    if l[i]=='A':
      m=10
    elif l[i]=='AB':
      m=9
    elif l[i]=='B':
      m=8
    elif l[i]=='BC':
      m=7
    elif l[i]=='C':
      m=6
    elif l[i]=='CD':
      m=5
    elif l[i]=='D':
      m=4
    elif l[i]=='E':
      return(0)
    M += m
    #M_round=round(M ,2)
  gpa = (M/n)
  return(round(gpa,2))


n=None
c=''
s=''
g=''
gl=''
while(n!='EndOfInput'):      
  n=input()
  c += n +'\n'
  if (n=='Students'):
    while(n!='Grades'):
      n=input()
      s += n +'~'+'\n'
  if (n=='Grades'):
    while(n!='EndOfInput'):
      n=input()
      g += n + '\n'
      gl += n
      

#print(c)
#print(s)
#print(g)
G=g[ :-12]
#print(gl)
#print(G)
S=s[ :-8]
gs=G.split('\n')
#print(G,end='')
#print(gs)
#print(S,end='')
stu=S.split('\n')
#print(stu)
stud=stu[ : -1]
#print(stud)
stut=[]
for a in range(0,len(stud)):
  stut += [stud[a].split('~')]
#print(stut)

sorted_stut= sorted(stut)
#print(sorted_stut)


for p in range(0,len(stut)):
  if gl.find(stut[p][0])== -1:
    gs += ['t~t~t~' + str(stut[p][0])+'~E']

#print(gs)

gsl=[]
for i in range(0,len(gs)):
  gsl += [gs[i].split('~')]
  
#print(gsl)
gslm=[]
for j in range(0,len(gsl)):
  gslm += [{gsl[j][3]:gsl[j][4]}]
#print(gslm)

r={}
for d in gslm:
  for k,v in d.items():
    r.setdefault(k,[]).append(v)

#print(r)
R={}
R=sorted(r.items())
#print(R)
grades=[]
for z in range(0,len(R)):
  grades += [R[z][1]]

#print(grades)


glist=[]
for x in range(0,len(grades)):
  glist += [gpa(grades[x])]
#print(glist)

outputlist=[]
for x in range(0,len(R)):
  outputlist += [[sorted_stut[x][0]] +['~']+[sorted_stut[x][1]]+['~']+ [str(glist[x])]+['\n']]
#print(outputlist)
for x in range(0,len(outputlist)):
    print(outputlist[x][0]+outputlist[x][1]+outputlist[x][2]+ outputlist[x][3]+outputlist[x][4])
 

Thanks

Happy Programming !