Lecture notes · Programming in Python · Lecture 7

Decomposition, Abstraction, and Functions

MIT 6.100L Intro to CS and Programming using Python, lecture 7, explained in plain English

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Part 1 · 0:00–10:00

This part explains how big jobs get split into small, reusable pieces, and how hiding the details inside each piece (abstraction) makes that possible.

Key ideas

Abstraction means treating something as a black box. You care about what goes in and what comes out, not how the inside works. The smartphone is the example: most users only know which button does what. 0:35
Decomposition means splitting a large job into separate parts. Each part can be built on its own because everyone agrees on the inputs and outputs, which is the interface between the parts. 2:24
A function is a reusable piece of code that does one useful task. You write and test it once, then use it without worrying about how it works inside. 4:13
A specification, also called a docstring, is a written note that acts as a contract. It states the inputs, what the function does, and what it gives back. Python does not enforce it, so the writer has to make sure the function keeps its promise. 6:29
When you see the same code repeated in a large program, that is a clue to pull it out into its own function. The main program then just asks the functions for their answers and puts the results together. 8:41

Words to know

abstraction: Hiding the details of how something works, so you only need to know what it takes in and what it gives back.
decomposition: Breaking a big job or program into smaller separate parts that fit together.
function: A named, reusable piece of code that performs one task. You give it inputs and it gives back an output.
specification (docstring): A written description at the top of a function that explains its inputs, what it does, and its output. It works like a contract for anyone who uses the function.

Example

Think of a TV remote. You press the power button and the TV turns on, and you do not need to know how the circuits make that happen. The remote's buttons are the interface, and the inside is hidden, just like a function's docstring tells you what to put in and what you get back.

3 checked practice questions for this part are in the app.

Part 2 · 10:00–20:00

This part explains how to define a function in Python, how to write its docstring and body, and how a return statement sends a result back to whoever uses it.

Key ideas

Defining a function only tells Python that the function exists. The lines inside it do not run until you call the function. 10:20
A function has a name, inputs called parameters, a docstring that explains it, and a body of code. You write the body the same way you would write normal code, just wrapped inside the function. 12:07
The function's body ends with a return statement, which hands a final value back to whoever called the function. This is the function's output. 13:13
To build a function from a problem, first pick a descriptive name, decide the inputs, write the docstring, and then solve the problem in the body. The lecture's example is is_even, which takes one integer and returns True or False. 17:36
Once working code is written and tested, you can often make it shorter. For example, i % 2 == 0 already gives True or False, so you can return it directly. 19:30

Words to know

function: A named block of code you can reuse. You write it once, then run it as many times as you need.
parameter: An input that a function expects. It is listed inside the parentheses after the function's name. The lesson also calls these arguments.
docstring: A note written in triple quotes at the top of a function. It tells the user what the inputs are, what the function does, and what it gives back.
return: The keyword that ends a function and sends a value back to whoever used it, so that value can be used elsewhere.

Example

Think of a function like a recipe card. Writing the card does not cook anything. The cooking only happens when someone follows the recipe, and the finished dish is what they hand back. Each time you want that dish, you follow the card again, so you only write the recipe once.

3 checked practice questions for this part are in the app.

Part 3 · 20:00–30:00

This part explains how a function is run after you write it, and why you need a function call and a print statement to see any result.

Key ideas

Writing a function does not run its code. Python just stores the lines under the function's name until something calls it. 20:21
To run a function, you make a function call: type the function's name, add parentheses, and put in the inputs it expects. 21:26
A function call is replaced by the value the function returns, the same way Python swaps 3 plus 2 for 5 in an expression. 21:47
The name i in the definition is a formal parameter, a stand-in name. When you call the function with 3, Python swaps every i for 3 while the function runs. 24:21
Because you write a function once, you only have to debug it once, and then you can call it as many times as you like. 27:35

Words to know

function call: Using a function by typing its name, parentheses, and any inputs, which tells Python to run it now.
return value: The one answer a function hands back when it finishes, which then takes the place of the function call in the code.
formal parameter: The placeholder name written in a function's definition, like i, which stands for whatever value the caller gives later.
expression: A piece of code that Python works out into a single value, such as 3 plus 2 becoming 5.

Example

Think of a recipe card in a kitchen drawer. Writing the card does not make any food, and the card is only useful when someone follows it. When you do follow it, the finished dish is what you get back, and that dish takes the place of the cooking step.

3 checked practice questions for this part are in the app.

Part 4 · 30:00–40:00

This part shows how to write a function that gives back a answer with return, how a function is stored and used as a black box, and how to plan a function on paper before you write code.

Key ideas

A function should use return to give a value back, not print it. If a function has no return statement, Python gives back a special value called None. 30:00
When a test gives the wrong answer, check the logic inside the function. In this example, the remainder check was flipped, so the fix was to test n % d == 0. 31:25
When Python reads a def line, it only stores the function's code under its name. The code does not run until you call the function. 32:47
You can treat a function as a black box: you only need to know what goes in and what comes out, not how it works inside. 33:50
A function call acts like a value in the middle of other code, so you can use it inside a loop or an if statement. Before writing code for a new task, plan it on paper with one small example. 36:02

Words to know

function: A named piece of code you can run again whenever you need it by calling its name.
return: The word that sends a value back out of a function so the rest of the program can use it.
expression: A piece of code that gets worked out into a single value, like a function call that gives back true or false.
remainder: The amount left over after you divide one number by another. For example, 7 divided by 2 leaves a remainder of 1.

Example

Think of a vending machine. You put in money and press a button, and a snack comes out. You do not need to know how the machine moves the snack inside to use it. A function works the same way: you give it inputs, and it gives you back an output.

3 checked practice questions for this part are in the app.

Part 5 · 40:00–45:45

This part shows how to build a small, working piece of code step by step, then use a function so you can reuse it many times.

Key ideas

Start with a simpler version of the problem. Get the basic version working first, then add the extra details, called edge cases, one at a time. 40:43
Work out a few small examples by hand first, so you know what the right answer should be before you write code. 40:21
Test your code early with a tiny example. A test that fails shows you where the bug is. In this case, the for loop stopped one number before b, so the fix was to add 1 to the end of the range. 42:30
Add the last detail only after the basic version works. Here, that detail was checking whether each number is odd before adding it to the sum. 43:14
A function lets you abstract, or hide away, a task you will use many times. You create the function once, and then you can run it as many times as you need. 45:28

Words to know

abstract: To hide the details of a task behind a simple name, so you can use it without thinking about every step.
loop variable: A variable, often named i, that changes each time through a loop so the loop can visit each value in turn.
edge cases: Special or unusual inputs that can break code if you do not plan for them, such as the first or last value in a range.
specification: A written description of what a function should do, including its inputs and what it should give back.

Example

Think of a coffee machine. The company designs and builds the machine once, which is like creating a function. Then you press the button every morning to make a cup, which is like running the function. You do not rebuild the machine each time you want coffee.

3 checked practice questions for this part are in the app.

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Notes written by AI (Claude) from the lecture's official transcript, so check anything important against the lecture. MIT OpenCourseWare · CC BY-NC-SA 4.0 · not affiliated with or endorsed by MIT. These notes are shared under the same license.

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