My code works in IDE why not here?

# The dictionary will look something like:
# {'Andrew Chalkley': ['jQuery Basics', 'Node.js Basics'],
#  'Kenneth Love': ['Python Basics', 'Python Collections']}
#
# Each key will be a Teacher and the value will be a list of courses.
#
# Your code goes below here.


def num_teachers(arg):
    count = 0
    for key in arg:
        count += 1

    return count

def num_courses(arg):
    count = 0
    for key in arg:
        count += len(arg[key])
    return count

def courses(arg):
    ret_courses = []
    for key in arg:
        ret_courses.extend(arg[key])

    return ret_courses

def most_courses(arg):
    most_teacher_val = max(arg[key] for key in arg)
    for key in arg:
        if arg[key] == most_teacher_val:
            return key