What is the important difference between lists and sets?

[Rajeev Bagra]

A learner working through the CS50 Degrees project noticed this line:

[code language="python"] names[row["name"].lower()] = {row["id"]} [/code]

and wondered:

“Why is the code not using append() here?”

At first glance, it may seem natural to expect something like:

[code language="python"] append(row["id"]) [/code]

However, understanding why append() is not used here helps clarify an important difference between Python lists and sets.

---

Q1. What is the original code?

[code language="python"] if row["name"].lower() not in names: names[row["name"].lower()] = {row["id"]} [/code]

This code appears inside the load_data() function of the CS50 Degrees project.

---

Q2. What does this line create?

[code language="python"] {row["id"]} [/code]

creates a Python set containing one value.

Example:

[code language="python"] {"102"} [/code]

So if:

[code language="python"] row["name"] = "Kevin Bacon" row["id"] = "102" [/code]

then Python creates:

[code language="python"] names["kevin bacon"] = {"102"} [/code]

---

Q3. Why is a set being used?

The project uses sets because multiple actors can share the same name.

Example:

[code] Chris Evans [/code]

could refer to more than one person.

So the structure may eventually become:

[code language="python"] names["chris evans"] = {"123", "456"} [/code]

Sets are useful because they:

• prevent duplicates automatically
• support fast lookup
• allow efficient additions

---

Q4. Why can’t append() be used?

The learner may expect something like:

[code language="python"] append(row["id"]) [/code]

However, append() only works on lists.

Example:

[code language="python"] mylist = []

mylist.append("102") [/code]

But the CS50 code is not using a list.

It is using a set.

---

Q5. What method do sets use instead?

Sets use:

[code language="python"] add() [/code]

Example:

[code language="python"] myset = set()

myset.add("102") [/code]

So:

• lists use append()
• sets use add()

---

Q6. Why does the first assignment not use add() either?

This line:

[code language="python"] names[row["name"].lower()] = {row["id"]} [/code]

is creating the set for the very first time.

Example:

[code language="python"] names["kevin bacon"] = {"102"} [/code]

At this stage:

• there is no existing set yet
• Python is assigning a brand new set directly

So add() is unnecessary here.

---

Q7. When does the code actually use add()?

Later, if another actor has the same name, this line runs:

[code language="python"] names[row["name"].lower()].add(row["id"]) [/code]

Suppose initially:

[code language="python"] names["chris evans"] = {"123"} [/code]

Then another actor with the same name appears:

[code language="python"] names["chris evans"].add("456") [/code]

Now the set becomes:

[code language="python"] {"123", "456"} [/code]

---

Q8. Why are sets better than lists here?

Suppose lists were used instead:

[code language="python"] names["chris evans"] = ["123"] [/code]

Then this could accidentally happen:

[code language="python"] append("123") [/code]

Result:

[code language="python"] ["123", "123"] [/code]

Now duplicates exist.

Sets automatically prevent this problem.

---

Q9. What is the important difference between lists and sets?

List example:

[code language="python"] ["102", "103"] [/code]

Uses:

[code language="python"] append() [/code]

Set example:

[code language="python"] {"102", "103"} [/code]

Uses:

[code language="python"] add() [/code]

This is one of the most important differences between lists and sets in Python.

---

Q10. What is the biggest conceptual takeaway?

The code does not use:

[code language="python"] append() [/code]

because the structure is a set, not a list.

Sets are ideal here because they:

• store unique actor IDs
• prevent duplicates automatically
• support efficient graph relationships

This design becomes especially important later when the CS50 Degrees project performs graph traversal and searches actor connections efficiently.

[Author]

Posts