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Generator functions return an iterator object that contains traversable values. To retrieve the next value from a generator object, we can use the Python built-in function next() which will cause the generator function to resume its execution until the next yield expression is found. After the next yield expression is found, the function will pause execution again.

If no additional yield expressions are found in a generator function, that means the code has finished and a StopIteration is raised.

Generator functions are not limited to just single yield statements. They can also include loops where the yield occurs.

To see this in action, imagine we have a dictionary of students and their student ID numbers. We want to hold a raffle where every student whose student ID is a multiple of 3 wins prize A and every student whose ID is a multiple of 5 wins prize B. Any student whose ID is both a multiple of 3 and 5 wins prize C.

Here is what it might look like:

def prize_generator():
  student_info = {
    "Joan Stark": 355,
    "Billy Mars": 45,
    "Tori Rivers": 18,
    "Kyle Newman": 25
  }

  for student in student_info:
    name = student
    id = student_info[name]
    if id % 3 == 0 and id % 5 == 0:
      yield student + " gets prize C"
    elif id % 3 == 0:
      yield student + " gets prize A"
    elif id % 5 == 0:
      yield student + " gets prize B"

Since this is a generator function, the local variable dictionary, student_info is preserved while the function executes with each next() call. We can see this by creating a variable prizes that calls the prize_generator() function and then calling next() on it. Let’s have a look:

prizes = prize_generator()
print(next(prizes))
print(next(prizes))
print(next(prizes))
print(next(prizes))

Running this code will produce the following output:

Joan Stark gets prize B
Billy Mars gets prize C
Tori Rivers gets prize A
Kyle Newman gets prize B

If we were to call next() one additional time, we would see a StopIteration exception raised since the student_info dictionary will have been exhausted:

print(next(prizes))

Running this code will produce the following output:

StopIteration

Now, let’s practice retrieving values from a generator object!

Instructions

A list student_standings of four student’s class standings is inside the generator function student_standing_generator().

Finish the function by adding a for loop that traverses through the student_standings list and yields 500 for each 'Freshman' value.

Outside the function, retrieve the iterator object by calling student_standing_generator() and set it to a variable called standing_values.

Print out the values within the returned standing_values generator using the Python built-in function next().

Two values of 500 should be retrieved since our student_standings list contained two 'Freshman' values.

Use next() one more time on the generator object. What occurs?

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