Does the CS50 Degrees Project Use 4 Dictionaries? Understanding Main vs Nested Dictionaries in Python

[Rajeev Bagra]

https://cs50.harvard.edu/ai/projects/0/degrees/

Many learners studying the Degrees of Separation project from :contentReference[oaicite:0]{index=0} ask an excellent question:

“If I see dictionaries inside dictionaries, does that mean there are 4 dictionaries in the program?”

The short answer is: No. There are 3 main dictionaries, while some values inside them are nested dictionaries.

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The 3 Main Dictionaries in the Program

At the top level, the program stores data in these three dictionary variables:

names = {}
people = {}
movies = {}

These are the core data structures loaded from CSV files.

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1. The names Dictionary

names["tom hanks"] = {"158"}

This maps a person’s lowercase name to one or more IDs.

• Name = key
• Set of IDs = value

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2. The people Dictionary

people["158"] = {
    "name": "Tom Hanks",
    "birth": "1956",
    "movies": {"12", "37"}
}

This maps a person ID to another dictionary containing that person’s details.

Important: The inner part is a nested dictionary.

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3. The movies Dictionary

movies["12"] = {
    "title": "Forrest Gump",
    "year": "1994",
    "stars": {"158", "991"}
}

This maps a movie ID to another dictionary containing movie details.

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Why It Looks Like More Than 3 Dictionaries

Because these lines are true:

type(people)         # dict
type(people["158"])  # dict

So yes, a dictionary exists inside another dictionary. But that inner dictionary is a value, not a separate top-level program variable.

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Best Analogy: Cupboards and Folders

Imagine your program has 3 cupboards:

• names cupboard
• people cupboard
• movies cupboard

Inside the people cupboard, each folder contains more organized sheets.

Still only 3 cupboards.

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Why This Matters in Real Programming

Understanding nested dictionaries helps with:

• JSON APIs
• Flask and Django apps
• Configuration files
• Database-like Python structures
• Graph problems like CS50 Degrees

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Final Answer

The program uses 3 main dictionaries. Some entries inside those dictionaries are themselves dictionaries, called nested dictionaries.

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Learning Tip: When you see a dictionary inside another dictionary, ask:

Is this a new variable?
Or just a value stored inside another structure?

That question will sharpen your Python thinking quickly.

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