I need a code which sums different numbers - python-2.7

Question

I got a very difficult assignment as I am new to python, I hope you will be able to help me.
I wrote this code:
def hours_per_student(student_course,course_hours):
new={}
for key in student_course.keys():
for val in student_course.values():
for m in range(len(val)):
if not new.has_key(key):
new[key]=course_hours[val[m]]
else:
new[key]=new[key]+course_hours[val[m]]
return new
for these dictionaries:
student_course = {'rina' : ['math', 'python'], 'yossi' : ['chemistry', 'biology'], 'riki' : ['python']}
course_hours = {'math' : 4, 'python' : 4, 'chemistry' : 6, 'biology' : 5}
And I need to get this:
hours_per_student(student_course, course_hours)
to return this:
{'rina': 8, 'yossi': 11, 'riki': 4}
But I keep getting identical numbers for each key.

Answer 1

You shouldn't be iterating over .values() if you're already iterating over .keys(), just use the key to get the value. Or where you have for m in len(val), just do for m in val and then reference m instead of val[m](the naming here stinks but I discuss that later). Python is much better at iteration than that. For instance, instead of the line
for val in student_course.values():
you should try something like
for courses in student_course[key]:
for course in courses:
if key not in new:
new[key] = course_hours[course]
else:
new[key] += course_hours[course]
Naming your variables intelligently will make it easier for you to keep track of what's happening. For example, each value in student_course is a list of courses, so you should name it that, not something ambiguous like val. Similarly, each element in courses is the name of a course, so name it as such.

Answer 2

Here you go:
solution = {student: sum([hours.get(course, 0) for course in s_courses]) for student, s_courses in student_course.items()}

Answer 3

Here are the things I found lacking in your code:
When iterating over your students, you could have just created a key for each of them, and then add the hours per course.
Naming variables so you understand what they mean is confused, refrain from using new, key or val.
You don't have to use keys() function, iterating a dictionary using for key in dictionary works the same way.
Here's a fixed code snippet:
def hours_per_student(students, course_hours):
total_hours = {}
for student in students:
courses = students[student]
total_hours[student] = 0
for course in courses:
total_hours[student] += course_hours[course]
return total_hours