Why Does line += person.numbers() Add Numbers to the List?

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April 15, 2026 at 5:30 pm #6393
Rajeev Bagra
Keymaster
When learning Python lists, this line can look confusing:
```
line += person.numbers()
```
How does Python know to add phone numbers into line?

Short Answer

Because line is a list, and += on lists means:

Extend the list by adding each item from another list.

Example Setup
```
line = ["Eric"]
person.numbers() == ["123", "456"]
```
Then:
```
line += person.numbers()
```
Becomes:
```
["Eric", "123", "456"]
```
Step-by-Step

1. Start list
```
line = ["Eric"]
```
2. Numbers list
```
["123", "456"]
```
3. Python extends the list
```
line += ["123", "456"]
```
Result:
```
["Eric", "123", "456"]
```
Why Not Use append()?
```
line.append(person.numbers())
```
Would give:
```
["Eric", ["123", "456"]]
```
This creates a nested list, which is not what we want.

Better Equivalent
```
line.extend(person.numbers())
```
This does the same as:
```
line += person.numbers()
```
Mental Model
```
line += list_of_numbers
```
Means:

Add each item from the second list into the first list.

In the PhoneBook Example
```
line = ["Eric"]
line += ["123", "456"]
```
Final result:
```
["Eric", "123", "456"]
```
Later converted into file text:
```
Eric;123;456
```
One-Line Takeaway

For Python lists, += means extend the list with items from another list.
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