Q&A: Understanding `.join()`, Lists, and Strings in Python

[Rajeev Bagra]

class FileHandler():
    def __init__(self, filename):
        self.__filename = filename

    def load_file(self):
        # ...

    def save_file(self, phonebook: dict):
        with open(self.__filename, "w") as f:
            for name, numbers in phonebook.items():
                line = [name] + numbers
                f.write(";".join(line) + "\n")

 

**Q1. Is `line` a string?**

Answer:
No.
line is a list that contains multiple string elements.
Example:

line = ["Alice", "12345", "67890"]

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Q2. What does the .join() method do?

Answer:
.join() combines all items in a list into one single string, inserting a separator between each item.
Example:

";".join(["A", "B", "C"])

Result:

"A;B;C"

---

Q3. Why is the separator written first, like ";".join(list)?

Answer:
Because .join() is a string method, not a list method.
The string before .join() becomes the separator (the glue) used between list items.

Example:

• ";" = separator
• .join(...) = join method
• list = items to join

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Q4. Can I write list.join(";") instead?

Answer:
No.
Lists do not have a .join() method.
Only strings (the separator) have the .join() method.

---

Q5. After joining, does the list become a string?

Answer:
No.
The list stays the same.
But .join() returns a new string containing all list elements.

Example:

line = ["Alice", "12345"]
result = ";".join(line)

Now:

• line → still a list
• result → "Alice;12345" (a string)

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Q6. Does .join() create only one string?

Answer:
Yes.
The output of .join() is one single string, no matter how many items were in the list.

---

Q7. What is the purpose of the separator (like ";")?

Answer:
Two purposes:

1. Human-readable formatting
   Helps visually separate values.

2. Machine-readable structure
   Lets Python later split the string back into a list using .split(";").

Without a separator, merged data would look like this:

Alice1234567890

which is impossible to separate correctly.

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Q8. Can we reverse .join()?

Answer:
Yes.
Use .split(separator) to restore the list.

"Alice;12345;67890".split(";")

Result:

["Alice", "12345", "67890"]

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Q9. Why do we add "\n" after join when writing to a file?

Answer:
To move to the next line in the file.
This ensures each record is stored on its own line.

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