Why `ExerciseCounter()` Works as a Dictionary Value

[Rajeev Bagra]

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📘 Understanding Dictionaries with Objects in Python

When learning Python, most beginners first see dictionaries storing simple values like numbers or lists. But dictionaries can also store objects — and this is where programming becomes much more powerful.

In today’s lesson, we’ll understand:

• How a dictionary can store objects
• Why ExerciseCounter() appears twice in a dictionary
• How each student gets their own independent counter
• What actually happens in memory

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🧠 The Goal

We want to track how many exercises each student completes.

Instead of storing numbers directly in a dictionary, we’ll store objects.

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🔹 Step 1: Create the Class

class ExerciseCounter:
    def __init__(self):
        self.__exercises = 0

    def done(self):
        self.__exercises += 1

    def how_many(self):
        return self.__exercises

What This Class Does

• __init__() → Initializes exercise count to 0
• done() → Adds one completed exercise
• how_many() → Returns the total count

Each object created from this class has its own private counter.

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🔹 Step 2: Dictionary with Objects as Values

students = {
    "peter": ExerciseCounter(),
    "sarah": ExerciseCounter()
}

This line confuses many beginners.

Let’s break it down carefully.

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🔍 What Actually Happens Here?

When Python reads:

"peter": ExerciseCounter()

It:

1. Creates a NEW ExerciseCounter object
2. Stores it in memory
3. Assigns it to the key "peter"

Then Python reads:

"sarah": ExerciseCounter()

It:

1. Creates ANOTHER NEW object
2. Stores it separately in memory
3. Assigns it to "sarah"

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Important Rule

Every time you write:

ExerciseCounter()

Python creates a brand new object.

Even if the code looks the same, they are different objects.

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🔹 Visual Memory Diagram

After execution:

students (dictionary)
   │
   ├── "peter" ───▶ ExerciseCounter object A
   │                └── __exercises = 0
   │
   └── "sarah" ───▶ ExerciseCounter object B
                    └── __exercises = 0

Two keys.
Two separate objects.
Two independent counters.

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🔹 Step 3: Updating Exercise Counts

Now let’s increase Peter’s exercises:

students["peter"].done()
students["peter"].done()

Now:

Peter → 2
Sarah → 0

If we increase Sarah:

students["sarah"].done()

Now:

Peter → 2
Sarah → 1

They remain independent because they are separate objects.

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🔬 Proving They Are Different Objects

You can verify this using id():

print(id(students["peter"]))
print(id(students["sarah"]))

The two memory addresses will be different.

That proves they are not the same object.

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❌ Common Mistake (Shared Object)

This is wrong:

counter = ExerciseCounter()

students = {
    "peter": counter,
    "sarah": counter
}

Now both keys point to the same object.

If Peter does an exercise:

students["peter"].done()

Sarah’s count will increase too — because it’s the same object.

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🔹 Why Use Objects Instead of Numbers?

You could write:

students = {
    "peter": 0,
    "sarah": 0
}

But objects are better because:

✔ You can add more behavior
✔ You can protect data
✔ You can expand functionality later
✔ Code becomes more readable

Example of extension:

def reset(self):
    self.__exercises = 0

No need to change main program logic.

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🔹 Dictionary vs List

This is NOT a list of objects.

A list would look like:

[ExerciseCounter(), ExerciseCounter()]

But then you wouldn’t know which object belongs to which student.

A dictionary gives us:

Name → Object

That mapping is powerful.

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🔹 Key Concept to Remember

A dictionary:

Key → Value

And the value can be anything:

• int
• string
• list
• another dictionary
• object

In this case:

Key → ExerciseCounter object

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🎯 Final Takeaway

When you write:

students = {
    "peter": ExerciseCounter(),
    "sarah": ExerciseCounter()
}

You are creating:

• One dictionary
• Two separate objects
• Each object stores its own exercise count
• Each student “remembers” their own progress

Objects store the data.
The dictionary stores references to those objects.

That’s how Python keeps everything separate and organized.

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