Putting It All Together¶

At this point, you have seen all the fundamental pieces of Python: objects, names, expressions, types, and structure. Now we bring them together into a single program that demonstrates how Python works as a system.

A Complete Example¶

```python name = "Alice" age = 20 scores = [80, 90, 85]

average = sum(scores) / len(scores) status = "adult" if age >= 18 else "minor"

print(f"{name} is {status} with average score {average}") ```

Output:

Alice is adult with average score 85.0

Mapping to Core Concepts¶

Step-by-Step Breakdown¶

Step 1 --- Objects and Names. "Alice", 20, and [80, 90, 85] are objects. The names name, age, and scores are bound to them.

Step 2 --- Expressions. sum(scores) / len(scores) composes smaller expressions: sum(scores) produces 255, len(scores) produces 3, and 255 / 3 produces 85.0. Each step creates new objects.

Step 3 --- Type-Dependent Behavior. >= is an operator that calls a method on age (an int). The result is a Boolean that determines which string the conditional expression selects.

Step 4 --- Output. The f-string constructs a new string from all the computed values, and print sends it to the console.

The Big Idea¶

This entire program follows one principle:

Python programs transform objects through expressions, under the control of structure.

With this model, Python stops being a collection of rules. You can now explain:

• why "1" + "1" produces "11" (string concatenation)
• why True + True produces 2 (Boolean is a subclass of int)
• why modifying a list can affect multiple variables (shared references)
• why a + b depends on the type of a (method dispatch)

These are not exceptions---they are consequences of the system.

When Things Go Wrong¶

The model also explains failures. Consider this broken version:

python scores = [80, 90, "85"] average = sum(scores) / len(scores)

This raises TypeError: unsupported operand type(s) for +: 'int' and 'str'. The sum() function calls + on each element, and int.__add__ does not know how to add a string. The type mismatch is caught at runtime, not at definition time.

Another common failure:

python scores = [] average = sum(scores) / len(scores)

This raises ZeroDivisionError: division by zero because len([]) is 0. The model predicts this: sum([]) returns 0, len([]) returns 0, and 0 / 0 is undefined.

Understanding the model lets you predict where and why errors occur---not just what went wrong.

Mental Model Checklist¶

When reading any Python program, ask:

1. What objects are involved?
2. What names refer to them?
3. What expressions are being evaluated?
4. What types determine behavior?
5. What structure controls execution?

If you can answer these five questions, you understand the program.

The Python Mental Model¶

Every program you write follows this flow:

flowchart LR
    A[Names] --> B[Objects]
    B --> C[Types]
    C --> D[Expressions]
    D --> E[Evaluation]
    E --> F[New Objects]
    F --> G[Statements]
    G --> H[Output]

Python programs transform objects through expressions, under the control of structure.

Exercises¶

Exercise 1. Predict the output of the following program without running it, then verify your answer.

python width = 5 height = 3 area = width * height perimeter = 2 * (width + height) print("Area:", area) print("Perimeter:", perimeter)

Exercise 2. Write a short program that converts a temperature from Celsius to Fahrenheit. Define a variable celsius = 25, compute the Fahrenheit value using the formula \(F = C \times \frac{9}{5} + 32\), and print the result.

Exercise 3. A student writes the following program and expects the output Total: 10. Instead, the program produces Total: 55. Identify the bug and fix it.

python price = 5 quantity = "10" total = price + quantity print("Total:", total)

Exercise 4. Explain the difference between an expression and a statement in Python. Give one example of each.

Exercise 5. Write a program that defines three variables -- your name (a string), your age (an integer), and your height in meters (a float) -- and prints a single sentence containing all three values using string concatenation or an f-string.