Difference Between append() and extend() in Python
append() adds one element as-is (even a whole list becomes one item), extend() adds each element individually. Examples and output proof inside.
The one-line answer
append() adds its argument as a SINGLE item at the end; extend() adds EACH element of its argument individually. The difference only becomes visible when the argument is itself a list or other iterable.
a = [1, 2, 3]
b = [1, 2, 3]
a.append([4, 5]) # whole list becomes ONE item
b.extend([4, 5]) # each element added separately
print("append:", a)
print("extend:", b)append: [1, 2, 3, [4, 5]]
extend: [1, 2, 3, 4, 5]
Look carefully: after append the list has 4 items (the last one is a nested list), after extend it has 5 items.
Length proof
a = [1, 2, 3]
a.append([4, 5])
print(len(a), a[3]) # 4 [4, 5] <- nested list!
b = [1, 2, 3]
b.extend([4, 5])
print(len(b), b[3]) # 5 4 <- plain number4 [4, 5]
5 4
extend() works with any iterable
letters = ["a", "b"]
letters.extend("cd") # string is iterable: adds 'c','d'
letters.extend(("e", "f")) # tuple works
letters.extend({"g"}) # set works
print(letters)['a', 'b', 'c', 'd', 'e', 'f', 'g']
Trap:
letters.append("cd") would add the whole string "cd" as one item, but letters.extend("cd") splits it into 'c' and 'd' — because a string is an iterable of characters. This exact question appears in written tests.Comparison table
| Point | append(x) | extend(iterable) |
|---|---|---|
| Adds | x as one single item | Every element of the iterable |
| Argument | Anything | Must be iterable |
| Length change | Always +1 | +len(iterable) |
| Nested result | Yes, if x is a list | Never nested |
| Equivalent | — | list1 += list2 |
When to use which?
- append() — adding one record:
students.append("Aman"), or intentionally keeping a sub-list:matrix.append([1, 2, 3]). - extend() — merging two lists:
all_marks.extend(class_b_marks). - Interview line: "append is +1 item, extend is +N items."
One-line answer
append() अपने argument को अंत में एक SINGLE item की तरह जोड़ता है; extend() argument के हर element को अलग-अलग जोड़ता है. यह अंतर तभी दिखता है जब argument खुद list या कोई iterable हो.
a = [1, 2, 3]
b = [1, 2, 3]
a.append([4, 5]) # पूरी list एक item बन गई
b.extend([4, 5]) # हर element अलग-अलग जुड़ा
print("append:", a)
print("extend:", b)append: [1, 2, 3, [4, 5]]
extend: [1, 2, 3, 4, 5]
ध्यान से देखें: append के बाद list में 4 items हैं (आखिरी एक nested list है), extend के बाद 5 items.
Length proof
a = [1, 2, 3]
a.append([4, 5])
print(len(a), a[3]) # 4 [4, 5] <- nested list!
b = [1, 2, 3]
b.extend([4, 5])
print(len(b), b[3]) # 5 4 <- सादा number4 [4, 5]
5 4
extend() किसी भी iterable के साथ काम करता है
letters = ["a", "b"]
letters.extend("cd") # string iterable है: 'c','d' जुड़े
letters.extend(("e", "f")) # tuple चलता है
letters.extend({"g"}) # set चलता है
print(letters)['a', 'b', 'c', 'd', 'e', 'f', 'g']
Trap:
letters.append("cd") पूरी string "cd" को एक item की तरह जोड़ता, लेकिन letters.extend("cd") उसे 'c' और 'd' में तोड़ देता है — क्योंकि string characters की iterable है. यही question written tests में आता है.Comparison table
| Point | append(x) | extend(iterable) |
|---|---|---|
| जोड़ता है | x को एक single item की तरह | Iterable का हर element |
| Argument | कुछ भी | Iterable होना ज़रूरी |
| Length change | हमेशा +1 | +len(iterable) |
| Nested result | हां, अगर x list है | कभी nested नहीं |
| Equivalent | — | list1 += list2 |
कब कौन-सा use करें?
- append() — एक record जोड़ना:
students.append("Aman"), या जानबूझकर sub-list रखना:matrix.append([1, 2, 3]). - extend() — दो lists merge करना:
all_marks.extend(class_b_marks). - Interview line: "append है +1 item, extend है +N items."
Frequently Asked Questions
What is the difference between append and extend?
append(x) adds x as one single item (a list becomes a nested list), while extend(iterable) adds each element of the iterable individually to the end.
What happens if we extend a list with a string?
The string is split into individual characters because it is an iterable — ["a"].extend("bc") gives ["a","b","c"].
Is list1 += list2 same as append or extend?
It behaves like extend: each element of list2 is added individually to list1.