pulling data from a list of lists if each list meets a condition

Question

I am trying to get a working method in a class I have constructed. This class accumulates a given student's course history. Each course is a list (name, grade, credits, etc) that's part of the total list of all courses for that student. Here is an example course history:

[['English 1', 4, 3, 'Fall', '2009'],
['English 2', 3.7, 3, 'Spring', '2010'],
['English 3', 2.7, 3, 'Fall', '2010'],
['English 4', 3.0, 3, 'Spring', '2011'],
['English 5', 3.7, 3, 'Fall', '2011'],
['English 6', 3.3, 3, 'Spring', '2012'],
['Math 1', 3.3, 3, 'Fall', '2009'],
['Math 2', 2.7, 3, 'Spring', '2010'],
['Science 1', 3.7, 4, 'Fall', '2009'],
['Science 2', 4, 4, 'Spring', '2010']]

I can figure out cumulative GPA, but I am finding it hard to determine a semester GPA. Here is my relevant code:

class StudentInfo(object):
    def __init__(self, last, first, dob):
        self.last=last
        self.first=first
        self.dob=dob
        self.gradehist=[]
    def __add__(self, course):
        return self.gradehist.append(course)
    def cumulativegpa(self):
        cred = [item[2] for item in self.gradehist]
        grad = [item[1] for item in self.gradehist]
        gc = [grad[i]*cred[i] for i in range(len(grad))]
        hrs = sum(cred)
        gpasum = sum(gc)
        cumgpa = gpasum/hrs
        return cumgpa
    def semgpa(self, x, y):
        semcred=[]
        semgrad=[]
        for i in self.gradehist:
            if self.gradehist[3] == x and self.gradehist[4] == y:
                semcred.append([item[2] for item in self.gradehist])
        for i in self.gradehist:
            if self.gradehist[3] == x and self.gradehist[4] == y:
                semgrad.append([item[1] for item in self.gradehist])
        return semcred, semgrad`

s1=StudentInfo('Bob','Dole', '1-1-1911')
s1 + ['English 1', 4, 3, 'Fall', '2009']
s1 + ['English 2', 3.7, 3, 'Spring', '2010']
etc

When I type s1.cumulativegpa() it returns the correct total gpa, but s1.semgpa('Fall','2009') returns "[],[]" Where am I going wrong?

edit: updated def semgpa(self, x, y): to include

for course in self.gradehist: if course[3] == x and course[4] == y: semcred.append([item[2] for item in course])

Answer 1

I'm not sure if this is what you want, but you can try it like this:

def semgpa(self, x, y):
    semcred=[]
    semgrad=[]
    for row in self.gradehist:
        if row[3] == x and row[4] == y:
            semcred.append(row[2])
            semgrad.append(row[1])
    return semcred, semgrad

First, you iterate over all rows. Then you only take the rows which belong to the selected semester. Then you can append the values from these rows to your new lists.