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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Use first.extend(second) to add every item in second to the end of first without nesting the lists. It changes first in place.
Use extend() to add the items
The Python 3.14 tutorial defines list.extend(iterable) as “Extend the list by appending all the items from the iterable.” Python’s list documentation shows the method’s behavior:
first = [1, 2]
second = [3, 4]
first.extend(second)
print(first) # [1, 2, 3, 4]Each item from second becomes an individual element at the end of first. The original first list is modified.
Why append() creates nesting
append() adds its argument as one object. So first.append(second) puts the entire second list inside the first:
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first = [1, 2]
second = [3, 4]
first.append(second)
print(first) # [1, 2, [3, 4]]Use this only when you deliberately want second to be a single nested element. To add one ordinary value, such as 5, use first.append(5).
Choose the operation that matches the result
| Goal | Code | Result |
|---|---|---|
Add every item from second to the existing first |
first.extend(second) |
Changes first in place |
| Create a new list containing both lists’ items | combined = first + second |
Creates a concatenated list; leaves both inputs unchanged |
Add second as one item |
first.append(second) |
Adds a nested list |
| Add one value | first.append(value) |
Adds that value as one element |
Python also supports first += second for in-place list extension. For beginners, first.extend(second) makes the intent explicit. Python’s augmented-assignment documentation describes the distinction between augmented assignment and ordinary addition.
Remember that extend() accepts any iterable
extend() is not limited to lists: it adds items from an iterable one at a time. For example, extending with a string adds its characters individually:
letters = []
letters.extend("cat")
print(letters) # ['c', 'a', 't']If the whole string should be one element, use letters.append("cat") instead.
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Do not assign the return value of extend()
extend() changes the list and returns None. Call it directly:
first.extend(second)Do not write first = first.extend(second); that replaces the list variable with None.
Version and scope
This guidance applies to Python’s built-in list type. The same append() and extend() distinction appears in the Python 3.11 tutorial as well as the current Python 3.14 tutorial; it should not be assumed for third-party arrays or custom containers with similarly named methods.
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