✅ Understanding the + (Concatenation) Operator in Python Lists (Phonebook Example)

[Rajeev Bagra]

Initial Code (Example)

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) What does the + operator do in this line?

line = [name] + numbers

✅ Answer:
Here, + is used for list concatenation, meaning it joins two lists into one list.

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Q2) Why is [name] written in square brackets?

✅ Answer:
Because [name] turns the name into a list containing one item.
Example:

name = "Amit"
[name]   # becomes ["Amit"]

This is required because Python can only concatenate list + list.

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Q3) What type of value should numbers be?

✅ Answer:
numbers must be a list of strings, like:

numbers = ["9876543210", "9123456789"]

So it can be combined with [name].

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Q4) What happens after concatenation?

✅ Answer:
Suppose:

name = "Amit"
numbers = ["9876543210", "9123456789"]

Then:

line = [name] + numbers

becomes:

["Amit", "9876543210", "9123456789"]

So the name and phone numbers become one single list.

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Q5) Why is line created this way before writing to the file?

✅ Answer:
Because the next line uses:

";".join(line)

join() works only on one list of strings, so we create a combined list first.

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Q6) What does this line do?

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

✅ Answer:
It writes the data into the file in this format:

Amit;9876543210;9123456789

• ";".join(line) joins list items with ;
• + "\n" adds a new line after each contact

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Q7) Can we concatenate a list with a string?

✅ Answer:
❌ No. Python will throw an error.

Example (Wrong):

["Amit"] + "9876543210"

✅ Correct way:

["Amit"] + ["9876543210"]

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Q8) What is the practical use of this + operator in file saving?

✅ Answer:
It makes the file clean and structured by storing:

Name + all phone numbers in one row, separated by semicolons.

This is very useful for saving phonebook/contact data in a readable format.

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If you want, I can also create the matching load_file() function that reads this exact format back into a dictionary ✅

[Author]

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