Understanding Dynamic File Paths in Python Using open()

[Rajeev Bagra]

When beginners first see code like this:

with open(f"{directory}/people.csv", encoding="utf-8") as f:

it may look like the CSV filename is being hardcoded.

At first glance, it seems Python is permanently tied to:

people.csv

However, this line is actually a combination of:

• dynamic path generation
• a fixed dataset structure

Understanding this distinction is very important when learning how real-world programs load files dynamically.

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The Original Code

directory = sys.argv[1] if len(sys.argv) == 2 else "large"

with open(f"{directory}/people.csv", encoding="utf-8") as f:
    reader = csv.DictReader(f)

---

What Is Happening Here?

The important part is:

f"{directory}/people.csv"

This is called an f-string in Python.

An f-string allows variables to be inserted directly into a string.

---

Example

Suppose:

directory = "large"

Then Python converts:

f"{directory}/people.csv"

into:

"large/people.csv"

If:

directory = "small"

then it becomes:

"small/people.csv"

---

Why This Is Not Fully Hardcoded

The program is not permanently tied to one folder.

Instead, the folder changes dynamically.

This means:

• small/people.csv can be loaded
• large/people.csv can be loaded
• another dataset folder could also be loaded

depending on user input.

---

Where the Dynamic Value Comes From

This line controls the dataset folder:

directory = sys.argv[1] if len(sys.argv) == 2 else "large"

If the user runs:

python degrees.py small

then:

directory = "small"

If the user runs:

python degrees.py large

then:

directory = "large"

---

Visual Folder Structure

The project expects a folder structure like this:

project/

    small/
        people.csv
        movies.csv
        stars.csv

    large/
        people.csv
        movies.csv
        stars.csv

So the filenames are fixed intentionally because the dataset schema is predefined.

---

What Would Real Hardcoding Look Like?

This would be much more rigid:

with open("large/people.csv") as f:

Now the program is permanently tied to the large dataset.

Changing datasets would require manually editing the code.

---

Why the CS50 Project Uses Fixed Filenames

The project assumes every dataset folder contains:

• people.csv
• movies.csv
• stars.csv

So the filenames are part of the dataset design.

The flexibility comes from changing the folder dynamically.

---

Key Learning Insight

This line:

with open(f"{directory}/people.csv")

is a mix of:

• dynamic directory selection
• fixed dataset naming conventions

This is a very common real-world programming pattern.

Programs often expect files to follow a known structure while still allowing the user to choose different datasets, environments, or folders dynamically.

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