Python PhoneBook: Should You Return After .append()?

[Rajeev Bagra]

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✅ Q&A: Python PhoneBook add_number() and return with .append()

Q1) What does this code do?

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

    def add_number(self, name: str, number: str):
        if not name in self.__persons:
            self.__persons[name] = []

        self.__persons[name].append(number)

✅ Answer:
This code creates a phonebook where:

• self.__persons is a dictionary
• Each name stores a list of phone numbers
• If the name does not exist, it creates an empty list
• Then it adds the new number into the list using .append()

Example result:

{
  "Rajeev": ["98765", "12345"]
}

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Q2) What happens if I add return to the last line like this?

return self.__persons[name].append(number)

✅ Answer:
The number will still be added ✅
BUT the return value will be None ❌

Because:

📌 .append() always returns None.

So this will work:
✅ it will store the number

But if you do:

result = phonebook.add_number("Rajeev", "99999")
print(result)

Output will be:

None

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Q3) Will adding return affect the functionality of storing the number?

✅ Answer:
No, it will not affect storing the number.

The number will still be stored properly in the list.

The only change is:
➡️ The method will now return something (which will be None if you return .append()).

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Q4) Why does .append() return None?

✅ Answer:
Because .append() modifies the list in-place.

Python list methods like:

• .append()
• .sort()
• .reverse()

usually return None to avoid confusion and to show they modify the original list directly.

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Q5) What if I want add_number() to return something meaningful?

✅ Answer:
Then use one of these approaches:

✅ Option A: Return the updated list

self.__persons[name].append(number)
return self.__persons[name]

Output example:

["99999"]

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✅ Option B: Return True after successful adding

self.__persons[name].append(number)
return True

Output example:

True

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Q6) What if I accidentally write this (wrong approach)?

return number

✅ Answer:
This will return the new number string, but it does not return the whole list.

It’s not wrong, but usually not useful because your phonebook is supposed to store multiple numbers per person.

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Q7) What is the best version of add_number() for a phonebook?

✅ Answer (Recommended):

def add_number(self, name: str, number: str):
    if name not in self.__persons:
        self.__persons[name] = []

    self.__persons[name].append(number)

Simple ✅
Does the job ✅
No unnecessary return ✅

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