❓ Why does a dictionary value in Python change from a list to a string in this PhoneBook program?

[Rajeev Bagra]

Context (Code First)

Consider this simple phonebook design in Python:

class PhoneBook:
    def __init__(self):
        self.__persons = {}

    def add_number(self, name: str, number: str):
        if not name in self.__persons:
            # Create a list to store multiple phone numbers
            self.__persons[name] = []

        self.__persons[name].append(number)

This design allows one person to have multiple phone numbers.

A student asked:

What happens if we replace the last line with
self.__persons[name] = number instead of .append(number)?

---

❓ Q1. What does self.__persons = {} actually mean?

Answer:
It means self.__persons is a dictionary.
But it does not mean its values must be lists.

A Python dictionary can store any type as its values:

d = {
    "a": [],
    "b": "hello",
    "c": 42
}

So even if self.__persons starts empty {}, its values can later become lists, strings, numbers, or anything else.

---

❓ Q2. Why does the original code use a list?

Answer:
Because one person can have more than one phone number.

So the structure is:

{
    "Alice": ["1234", "5678"],
    "Bob":   ["9999"]
}

Each name points to a list of numbers, not a single number.

---

❓ Q3. What happens if we write self.__persons[name] = number?

Answer:
It replaces the list with a string.

Example:

add_number("Alice", "1234")

Dictionary becomes:

{"Alice": "1234"}

Then:

add_number("Alice", "5678")

Dictionary becomes:

{"Alice": "5678"}

The old number "1234" is lost.

So the phonebook silently changes from:

One person → many numbers
to
One person → only one number

---

❓ Q4. Why does Python allow this?

Answer:
Because dictionaries do not enforce the type of their values.

This is allowed:

d["x"] = []
d["x"] = "hello"

Python assumes you know what you are doing.
It will not stop you from replacing a list with a string — even if that breaks your design.

---

❓ Q5. Why is .append(number) the correct design?

Answer:
Because you first created a list:

self.__persons[name] = []

So you must add to that list, not replace it:

self.__persons[name].append(number)

This keeps all phone numbers for that person.

---

✅ Final takeaway

Your original design:

self.__persons[name].append(number)

creates a real phonebook:

One dictionary → many people → each with many numbers

Replacing it with:

self.__persons[name] = number

destroys that structure and turns it into:

One dictionary → many people → each with only one number

This is why that one line matters so much in data structure design.

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