Really need help understanding this, could someone explain in detail?

Ok

Dicts are great fun! However starting them can be a little tricky, I came a cropper with the same question.

The bit you are missing is to use dict.items().

So this gives you back the key AND the values. So to explain further

If I have a dict like this {'keith': ['ginger', 'English', 'Chubby'], 'mywife': ['blonde', 'polish', 'slim']}

So we have a dict that has two keys, keith and mywife, it has two values which in this case are lists.

to get at the key I can write "for key in dictionary:"I will not get the dict passed as a variable. I would then below have to say dictionary[key] to get at each list in the for loop.

The code below will give me the length of the list ie the number of things in the list that pertains to the key in the dictionary

for key in dictionary:
   print(len(dictionary[key]))

To make life easier python lets you call a function on a dictionary, items()

for key, value in dictionary.items:
   print(len(value ))

So try this,

def most_classes(teachers):
  name_of_busiest_teacher =''
  max_classes = 0
  for teacher, classes in teachers.items():
    if len(classes) > max_classes:
      name_of_busiest_teacher = teacher
      max_classes = len(classes)
  return name_of_busiest_teacher

def num_teachers(teachers):
  number_of_teachers = 0
  for teacher in teachers:
    number_of_teachers += 1
  return number_of_teachers

def stats(teachers):
  teachers_and_classes_list = []
  for teacher, classes in teachers.items():
    # so append the list with another list containing the teachers name 
    # (teacher the key of the dict, and the number(len) of classesfrom the value (classes)
    teachers_and_classes_list.append([teacher, len(classes)])
  return teachers_and_classes_list

def courses(teachers):
    course_list = []
    # I'm going to do this a differnt way so you can see dictionary useage without items()
    for teacher in teachers:
        for course in teachers[teacher]: # see the key in the name of the dict[key] so that lets use iterate over the values.
            course_list.append(course)
    return course_list

• Hey Jan! Don't fret too much about it. I was in the same boat a day or two ago, but it came full circle for me with the help of the Treehouse Community and several hours focusing on the code. You'll get it!

• As for your for loop question: "for key in dictionary: print(len(dictionary[key]))" the len is referencing the values that belong to those [keys] in the dictionary. That's just how the syntax is set up. I agree, it is a little counterintuitive (at least at first).

• The dict.items() is a method of dictionaries that, I personally, found to be quite helpful once I found out about it (just yesterday from a MOD on here). The nice thing about it is that it grabs both the key and the value from the dictionary To my understanding, the "key, value" and, the comma, that you ask about are simply the specific syntax setup you use when executing the .items() method on a dict in a for loop (i.e., when your wanting to grab both the key and the values from the dict) I'm confident that this is why python knows what your are talking about when you use the word key in this instance (because it knows you are using a for loop with the .items() method, on a dictionary. So the syntax is rigid yet powerful.

I don't know Kenneth really went over this in the videos yet, or if he did I must have missed it. But a quick google search of .items() method will give info on what it does:

• It returns (or grabs) both the keys and values of a dictionary
• specifically, it returns as list of tuple pairs (i.e. key, value pairs) from a given dictionary (My guess: this is why the comma must be present)

Keep working at it Jan. You'll have your "light-bulb" moments and progress as long as you stick with it! :)