👉 Why My Phone Book Search Failed: Understanding Lists vs Strings in Python

[Rajeev Bagra]

✅ Q&A: Phone Book Expansion – Search by Number

❓ Question

I’m extending the phone book application to allow searching by number.
In my PhoneBook class I wrote:

if self.__entries[name] = number:
    return name

But it doesn’t work. Why is this wrong, and what is the correct way to implement search by number?

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✅ Context: Initial program structure

The phone book stores multiple numbers per person.

Example internal structure:

self.__entries = {
    "Eric": ["02-123456", "045-4356713"],
    "Anna": ["040-999999"]
}

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

The template already supports:

• adding entries
• searching by name

Now we are adding:

👉 searching by number

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❌ Problem in the original line

You wrote:

if self.__entries[name] = number:

There are two issues here.

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🔴 Issue 1: = is assignment, not comparison

Python uses:

=   assign value
==  compare values

Inside an if, you must compare.

This causes a syntax error:

if x = 5:   # ❌ invalid

Correct:

if x == 5:  # ✅ valid

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🔴 Issue 2: You compared a list to a string

This:

self.__entries[name]

returns a list of numbers.

Example:

["02-123456", "045-4356713"]

But you compared it to:

"02-123456"

A list will never equal a string:

["02-123456"] == "02-123456"  # False

We don’t want to compare the whole list.

We want to check if the number exists inside the list.

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✅ Correct solution

We must search each list and check membership:

def search_by_number(self, number: str):
    for name, numbers in self.__entries.items():
        if number in numbers:
            return name
    return None

Key idea:

👉 "number in numbers" checks if the number exists in that person’s list.

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🧠 How the loop works

For each person:

Eric → ["02-123456", "045-4356713"]
Anna → ["040-999999"]

We ask:

Does this number appear in their list?

If yes → return the name.

If no match → return None.

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✅ Alternative version (same logic)

You could also write:

def search_by_number(self, number: str):
    for name in self.__entries:
        if number in self.__entries[name]:
            return name
    return None

This is equivalent — just different syntax.

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✅ Expected behavior

command: 3
number: 02-123456
Eric

command: 3
number: 0100100
unknown number

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