From Diagram to Code

On this page

Define the Model Create the Object Snapshot

Translate the model one responsibility at a time: fields become instance attributes; operations become methods; associations become references to other objects.

The UML activity diagram shows the translation workflow; rounded boxes are actions read from the start circle to the end bullseye.

Define the Model

None means no grade has been assigned. A list comprehension builds a new list by reading the enrollment links. raise ValueError(...) rejects an input that breaks a model rule.

class Student:
    def __init__(self, student_id, name):
        self.student_id = student_id
        self.name = name
        self._enrollments = []

    def enroll(self, course):
        for enrollment in self._enrollments:
            if enrollment.course is course:
                raise ValueError("Already enrolled")
        enrollment = Enrollment(self, course)
        self._enrollments.append(enrollment)
        course._enrollments.append(enrollment)
        return enrollment

    def course_codes(self):
        return [enrollment.course.course_code for enrollment in self._enrollments]


class Course:
    def __init__(self, course_code, title, credits):
        if not isinstance(credits, int) or isinstance(credits, bool) or credits <= 0:
            raise ValueError("Credits must be a positive integer")
        self.course_code = course_code
        self.title = title
        self.credits = credits
        self._enrollments = []

    def student_names(self):
        return [enrollment.student.name for enrollment in self._enrollments]


class Enrollment:
    def __init__(self, student, course):
        self.student = student
        self.course = course
        self._grade = None

    @property
    def grade(self):
        return self._grade

    def set_grade(self, value):
        if value not in {"A", "B", "C", "D", "F"}:
            raise ValueError("Grade must be A, B, C, D or F")
        self._grade = value

Listing 346 — Define the Model

The definitions produce no visible output. Use Student.enroll to create and register a link on both sides; directly constructing Enrollment would not register it.

Create the Object Snapshot

Continue with all three class definitions above:

ada = Student("S001", "Ada Lovelace")
cs101 = Course("CS101", "Intro to CS", 6)
enr1 = ada.enroll(cs101)
enr1.set_grade("A")
print(ada.course_codes())
print(cs101.student_names())
print(enr1.grade)

Listing 347 — Create the Object Snapshot

Output

['CS101']
['Ada Lovelace']
A

The returned enrollment links Ada and CS101; its grade becomes A. Each query builds a new list from those links.

Keep one Course object per catalog code: duplicate detection here uses object identity. Data is held only in memory.

Try it: enroll Grace in CS101, grade her enrollment B, and check that Ada’s grade remains A. Then reject grade Z and confirm that the stored grade is unchanged.

The model now supports enrollment and grading. Application design begins by asking who uses these operations and what they want to achieve.