Your First Class

The homework club at the community centre keeps its volunteer list in three columns on a clipboard: names down one side, grades down the next, hours down the third. Somebody crossed a name out in the middle of the sheet last term and did not cross out the matching hours. For six weeks the coordinator was thanking the wrong student.

Three lists that must be kept in step are a bug waiting for a busy afternoon. One class fixes it: a volunteer’s name, grade, and hours become one thing that moves together.

The program

# Volunteer records for the homework club that runs at the community
# centre on Tuesdays. The club needs a first name, a grade, and hours
# served. It does not need addresses, birthdates, or student numbers,
# so this program does not store them.
 
class Volunteer:
    """One person who has signed up to help at the homework club."""
 
    def __init__(self, name, grade):
        """Start a new volunteer with no hours served yet."""
        self.name = name
        self.grade = grade
        self.hours = 0
 
    def log_shift(self, length):
        """Add one completed shift, in hours, to this volunteer."""
        self.hours = self.hours + length
 
    def __str__(self):
        """Describe this volunteer in one line, for printing."""
        return f"{self.name} (Grade {self.grade}): {self.hours} h"
 
 
nadia = Volunteer("Nadia", 12)
rowan = Volunteer("Rowan", 11)
 
nadia.log_shift(2.5)
nadia.log_shift(2)
rowan.log_shift(1.5)
 
print(nadia)
print(rowan)
print(f"Nadia has served {nadia.hours} hours so far.")
print(f"Rowan is in grade {rowan.grade}.")

Nadia (Grade 12): 4.5 h
Rowan (Grade 11): 1.5 h
Nadia has served 4.5 hours so far.
Rowan is in grade 11.

How it works

class Volunteer: does not create a volunteer. It describes what every volunteer will be — the blueprint. The volunteers themselves arrive on the two lines near the bottom, and there are two of them, each with their own name, grade, and hours.

In the program	What it is called	What it does
class Volunteer:	The class	Describes the kind of thing
nadia, rowan	Objects, or instances	Two actual volunteers
self.name, self.hours	Attributes	What one volunteer knows
log_shift	A method	What one volunteer can do
__init__	The constructor	Runs once, when the object is made
self	The object itself	”This particular volunteer”

__init__ runs automatically. Volunteer("Nadia", 12) builds a fresh object, hands it to __init__ as self, and sets its three attributes. Notice that hours is not a parameter: nobody starts with hours already served, so the constructor decides that for everybody. Rules that hold for every object of the class belong in the constructor.

nadia.log_shift(2.5) reads as a sentence: this volunteer, log a shift of 2.5 hours. Python passes nadia in as self, so self.hours = self.hours + length changes her total and nothing else. Call it on rowan and only Rowan’s total moves. That separation is the entire point of objects, and it is what three parallel lists could never guarantee.

__str__ is the method Python looks for when you print an object. Delete it and print(nadia) produces something like <__main__.Volunteer object at 0x104a2d130> — accurate, useless. Writing __str__ is a small kindness to whoever reads your output at 4 p.m. on a Friday.

Why this program has no last names

The club needs to know who is coming and how long they stayed. It does not need a birthdate, an address, or a student number, so those fields do not exist here and cannot leak from here. Deciding what not to store is a design decision you make on purpose, once, at the start — see Encapsulation and Ethics, Security, and the Profession.

Change it

1. One line. In __str__, change {self.hours} h to {self.hours:.1f} h. The output becomes Nadia (Grade 12): 4.5 h and Rowan (Grade 11): 1.5 h — and a volunteer with exactly two hours now reads 2.0 h instead of 2 h. Formatting belongs at the point of display, never in the stored value.
2. A few lines. Add a method that answers a question the coordinator actually asks:

   def has_earned_reference(self):
       """True once this volunteer has served four hours or more."""
       return self.hours >= 4

   Then print(nadia.has_earned_reference()) prints True and print(rowan.has_earned_reference()) prints False. The rule now lives in one place; when the club raises the threshold to six hours, one line changes.
3. A real change. Give each volunteer a self.shifts = [] in the constructor and have log_shift append the length as well as adding it to the total. nadia.shifts then holds [2.5, 2], and max(nadia.shifts) gives 2.5. An object can hold a list as easily as a number — which is exactly what Objects in a List turns around.

Read the idea in Objects and Classes and Attributes and Methods, then make it dependable in Classes and Objects Practice.

Curriculum connection

C1.1

decompose a problem into modules, classes, or abstract data types (e.g., stack, queue, dictionary) using an object-oriented design methodology (e.g., CRC [Class Responsibility Collaborator] or UML [Unified Modeling Language]);

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C1.2

demonstrate the ability to apply data encapsulation in program design (e.g., classes, records, structures);

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A4.3

create fully documented program code according to industry standards (e.g., doc comments, docstrings, block comments, line comments);

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