✅ Understanding `+` in Python — List Concatenation vs String Concatenation

[Rajeev Bagra]

✅ Code Example (Initial Code)

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")

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✅ Q&A

Q1) Why is the + operator used in this code?

✅ Answer:
The + operator is used to combine things, but what it actually does depends on the data type.

In this code, it is used in two places, and it behaves differently in each place.

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Q2) How does + work here?

line = [name] + numbers

✅ Answer:
Here, + is doing list concatenation, because both sides are lists.

• [name] is a list containing the name
• numbers is a list containing phone numbers

So Python joins them into one list.

Example:

["Amit"] + ["9876", "1234"]
# Output: ["Amit", "9876", "1234"]

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Q3) Why is the name written as [name] instead of just name?

✅ Answer:
Because numbers is a list, and Python allows concatenation only like:

✅ list + list

So we convert the name into a list:

[name]

This makes the concatenation possible.

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Q4) How does + work here?

f.write(";".join(line) + "\n")

✅ Answer:
Here + is doing string concatenation, because both sides are strings.

• ";".join(line) produces a string like "Amit;9876;1234"
• "\n" is also a string (newline)

So Python joins them into one final string to write to the file.

Example:

"Amit;9876;1234" + "\n"
# Output: "Amit;9876;1234\n"

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Q5) What does ";".join(line) actually do?

✅ Answer:
It converts the list into a single text line separated by semicolons.

Example:

line = ["Amit", "9876", "1234"]
";".join(line)

Output:

Amit;9876;1234

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Q6) Why do we add "\n" at the end?

✅ Answer:
To make sure each contact is written on a new line in the file.

Without it, all contacts may get written in one long line.

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Q7) So is + a concatenation operator in both cases?

✅ Answer:
Yes ✅
But it concatenates different things:

• List + List → creates a bigger list
• String + String → creates a bigger string

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Q8) What’s the simple rule to remember?

✅ Answer:

Data type on both sides	+ means
list + list	list concatenation
string + string	string concatenation
number + number	arithmetic addition

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