Why would you assign a variable to “”?

Question

so i'm in the middle of the Python course on the Treehouse website and question asks exactly this:

Create a function named most_classes that takes a dictionary of teachers. Each key is a teacher's name and their value is a list of classes they've taught. most_classes should return the teacher with the most classes.

Here I have posted the correct code below that I have found from a resource on the Treehouse forums and I have asked this same question but got no reply - So what exactly does assigning teacher = "" do? I am so confused

 # The dictionary will be something like:
 # {'Jason Seifer': ['Ruby Foundations', 'Ruby on Rails Forms', 'Technology Foundations'],
 #  'Kenneth Love': ['Python Basics', 'Python Collections']}

 # Often, it's a good idea to hold onto a max_count variable.
 # Update it when you find a teacher with more classes than
 # the current count. Better hold onto the teacher name somewhere
 # too!

def most_classes(my_dict):
    count = 0
    teacher = "" #this is where I am confused!
    for key in my_dict: 
        if(len(my_dict[key]) > count):
            count = len(my_dict[key])
            teacher = key   

    return teacher

Answer 1

它为教师分配默认值，该默认值将替换为代码中的实际教师姓名。

Answer 2

teacher = "" ensures if my_dict is empty you won't exit the for loop without ever setting teacher = key . Otherwise, if my_dict is empty, teacher will be returned without ever having been set.

If you comment out that line, then call your function like this:

most_classes({})

You'll get this (since teacher never gets initialized before it is returned):

UnboundLocalError: local variable 'teacher' referenced before assignment

Answer 3

Why not remove that line and test it?

def most_classes(my_dict):
    count = 0
    teacher = "" # this is where I am confused!
    for key in my_dict:
        if(len(my_dict[key]) > count):
            count = len(my_dict[key])
            teacher = key

    return teacher


def most_classes_cavalier(my_dict):
    count = 0
    for key in my_dict:
        if(len(my_dict[key]) > count):
            count = len(my_dict[key])
            teacher = key

    return teacher


if __name__ == "__main__":
    dic = {'Jason Seifer': ['Ruby Foundations', 'Ruby on Rails Forms', 'Technology Foundations'],
           'Kenneth Love': ['Python Basics', 'Python Collections']}
    print "The teacher with most classes is - " + most_classes(dic)
    print "The teacher with most classes is - " + most_classes_cavalier(dic)
    dic = {}
    print "The teacher with most classes is - " + most_classes(dic)
    print "The teacher with most classes is - " + most_classes_cavalier(dic)

This is what I get when I run the program -

The teacher with most classes is - Jason Seifer
The teacher with most classes is - Jason Seifer
The teacher with most classes is -
Traceback (most recent call last):
  File "experiment.py", line 30, in <module>
    print "The teacher with most classes is - " + most_classes_cavalier(dic)
  File "experiment.py", line 20, in most_classes_cavalier
    return teacher
UnboundLocalError: local variable 'teacher' referenced before assignment

I see that @martijn-pieters has already provided the explanation, but in this case the Python interpreter would have done that for you much quicker.