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Python Python Collections (2016, retired 2019) Dictionaries Teacher Stats

Benjamin Bradshaw
Benjamin Bradshaw
3,208 Points

Some kinda strangeness is happening.

I'm on task 5 but somehow, whenever I submit my answer it says that task 4 is no longer working. When I go back to task 4 it works just fine. My question is, why is this happening and how does my task 5 look?

teachers.py
# 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.
teachers = {'Benj': ['Omnisciences'], 'Heather': ['Social Therory'] , 'Chris': ['Game Therory', 'Food Studies'], 'Abe': ['studystudies', 'allstudies']}


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

def num_courses(teachers):
    num = 0
    for courses in teachers.values():
        num += len(courses)
    return num

def courses(teachers):
    new_list = []
    for value in teachers.values():
        new_list += value
    return new_list

def most_courses(teachers):
    max_count = 0
    most_courses = str()
    for teacher, course in teachers.items():
        if len(course) > max_count:
            max_count += len(course)
            most_courses = teacher
        return most_courses

def stats(teachers):
    stat_list = []
    for item in teachers:
        current_teacher = []
        current_teacher = [item, len(teachers[item])]
        stat_list.append(current_teacher)
    return stat_list      

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,166 Points

When a previously passing task now fails, it usually means that a new syntax error was introduced while working on the current task. In this case, it looks like the return statement for the previous most_courses function has gotten indented inside the for loop.

Also it looks like the max_count is using a += instead of a simple assignment.

Benjamin Bradshaw
Benjamin Bradshaw
3,208 Points

The syntax was the problem. It worked with the += so I'm wondering why I would want to use an assignment instead. should it not have worked? Is there and advantage to using an assignment over a += ?

Chris Freeman
Chris Freeman
Treehouse Moderator 68,166 Points

Revisiting this question got me thinking....

The fact your code passes with the "+= in place is a lucky coincidence due to the order the teachers are retrieved from the teachers dict test code. Running the code with slightly differing data yields wrong results. I added print message to your code to watch for a failing case. Below is a example of a failing case.

>>> def most_courses(teachers):
...     max_count = 0
...     most_courses = str()
...     for teacher, course in teachers.items():
...         print(teacher, course)  # print current teacher and course list
...         if len(course) > max_count:
...             max_count += len(course)
...             print("max_count:", max_count)  # print current max_count
...             most_courses = teacher
...     return most_courses
...

# defining teachers where "jupiter" has the most courses 
>>> teachers = {'venus': ['a1', 'a2'], 'mercury': ['b1', 'b2', 'b3'], 'jupiter': ['c1', 'c2', 'c3', 'c4']}
>>> most_courses(teachers)
venus ['a1', 'a2']
max_count: 2
jupiter ['c1', 'c2', 'c3', 'c4']
max_count: 6
mercury ['b1', 'b2', 'b3']
'jupiter'
# jupiter is seen second while max_count is 2 so it becomes the most_courses teacher

# renaming "jupiter" to "earth", but still with the max courses, changes the dict hash order
>>> teachers = {'venus': ['a1', 'a2'], 'mercury': ['b1', 'b2', 'b3'], 'earth': ['c1', 'c2', 'c3', 'c4']}
>>> most_courses(teachers)
venus ['a1', 'a2']
max_count: 2
mercury ['b1', 'b2', 'b3']
max_count: 5
earth ['c1', 'c2', 'c3', 'c4']
'mercury'
# earth is seen last, but the max_count has already grown to 5 so mercury stays most_courses teacher

You can see from the above example using += has undesired behavior. Using = is the correct solution.

Tagging Kenneth Love to review the challenge test data to be sure to catch the misuse of +=.