Python PhoneBook: The Hidden Mistake in return self.__persons[name].append(number)

[Rajeev Bagra]

✅ Q&A: Understanding return self.__persons[name].append(number) in a PhoneBook Program

Context

In this program, a PhoneBook class is created to store people’s phone numbers.
Since one person can have multiple phone numbers, the phone book stores each name with a list of numbers.

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✅ Initial Code (Original Version)

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

def add_number(self, name: str, number: str):
if not name in self.__persons:
# add a new dictionary entry with an empty list for the numbers
self.__persons[name] = []

self.__persons[name].append(number)

def get_numbers(self, name: str):
if not name in self.__persons:
return None

return self.__persons[name]

✅ In this version:

• add_number() adds phone numbers
• get_numbers() retrieves the list of numbers for a person

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✅ Q&A

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Q1. What does this PhoneBook program store?

Answer:
It stores names and phone numbers using a dictionary like this:

{
"Ali": ["123", "456"],
"Sara": ["999"]
}

Each key is a name, and the value is a list of numbers.

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Q2. Why does the code use an empty list [] for new names?

Answer:
Because the program wants to store multiple numbers per person.

So when a person is first added, this happens:

self.__persons[name] = []

That creates a space to start adding numbers.

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Q3. What does this line do?

self.__persons[name].append(number)

Answer:
It adds the phone number into the list for that person.

Example:

self.__persons["Ali"].append("123")

Now "Ali" has:

["123"]

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Q4. What happens if we change add_number() to this?

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

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

Answer:
✅ The number will still be added successfully.
❌ But the function will return None.

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Q5. Why does it return None?

Answer:
Because list.append() in Python always returns None.

It modifies the list in-place, meaning it updates the existing list instead of creating a new one.

So this always happens:

return ["123"].append("456") # returns None

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Q6. Will the phone number still be saved even if it returns None?

Answer:
✅ Yes, the phone number is saved.
But the return value of the function is useless.

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Q7. What will be the output in this example?

pb = PhoneBook()
result = pb.add_number("Ali", "123")

print(result)
print(pb.get_numbers("Ali"))

Answer:

None
['123']

So the number gets stored correctly, but the returned value is None.

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Q8. What is the best correct way to write add_number()?

Answer:
The best way is to not return .append().

✅ Recommended:

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

This keeps the method clean and clear.

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Q9. What if we want add_number() to return something useful?

Answer:
You can return the updated list after appending.

✅ Useful version:

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

Now it returns:

['123']

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✅ Final Takeaway

ߓ Never do this:

return my_list.append(item)

Because it will always return:

✅ None

ߓ Instead:

• Append first
• Return something meaningful only if needed

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[Author]

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