Q & A: Understanding `return` in a Python PhoneBook Program

[Rajeev Bagra]

✅ Context

A learner is building a simple PhoneBook class in Python.
The goal is:

• Store contact names
• Allow multiple phone numbers for the same name
• Retrieve stored numbers later

Here is the initial code:

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]

---

❓ Q1) What is self.__persons storing?

✅ Answer:
self.__persons is a dictionary.

• Keys = contact names (str)
• Values = a list of phone numbers (list[str])

Example structure:

{
"Raj": ["999", "888"],
"Alex": ["123"]
}

---

❓ Q2) Why are we using an empty list [] when adding a new name?

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

So when a name is seen for the first time:

self.__persons[name] = []

Then every new number gets added using:

self.__persons[name].append(number)

---

❓ Q3) What does this line do?

return self.__persons[name]

✅ Answer:
It sends back the list of numbers stored for that name.

So if we do:

pb.get_numbers("Raj")

It could return:

["999", "888"]

---

❓ Q4) What if I write this instead?

number = self.__persons[name]

✅ Answer:
That is totally fine.

It stores the contact’s phone numbers inside a variable.

Example:

numbers = self.__persons[name]
return numbers

---

❓ Q5) What happens if I do NOT return anything at the end?

Example:

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

numbers = self.__persons[name]
# no return here

✅ Answer:
Then Python will automatically return None.

Because in Python:

If a function ends without return, it returns None by default.

---

❓ Q6) But I stored it in a variable… why doesn’t it work without return?

✅ Answer:
Because variables inside a function exist only inside that function.

So this line:

numbers = self.__persons[name]

only stores it temporarily inside the function.

If you want the caller to receive that value, you must return it.

---

❓ Q7) What will this print?

pb = PhoneBook()
pb.add_number("Raj", "999")
print(pb.get_numbers("Raj"))

✅ Answer:
If the function has this line:

return self.__persons[name]

Output will be:

['999']

But if you remove the return statement, output will be:

None

---

❓ Q8) Why does get_numbers() return None for a missing name?

✅ Answer:
Because this part runs:

if not name in self.__persons:
return None

So if a name is not found, it clearly signals:
✅ “This person doesn’t exist in the phonebook.”

---

✅ Quick Summary for Beginners

✅ self.__persons[name] is a list of numbers
✅ To send that list back, you need return
✅ If you don’t return anything, Python returns None automatically
✅ Storing in a variable is useful, but it does not automatically “send it back”

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