EasyExperience level: 0-2 years1 min read

What is the difference between append() and extend() on a list?

Reviewed by Gurusankar M.

Asked inTCSInfosysCognizant
#list#append#extend#python basics#collections
Report issue

Short Answer

`append(x)` adds x as a single element (the list grows by one, even if x is itself a list). `extend(iterable)` adds each element of the iterable individually (the list grows by len(iterable)). Use append for one item, extend to merge another sequence in.

On this page

Concept Made Simple

Coffee Chat Question

“What is the difference between append() and extend() on a list?”

Remember It Faster

Mind Map Answer

append(x)
adds ONE element (x itself)
extend(it)
adds each element of it
a + b
returns a NEW list (no mutation)

Learn by Doing

Hands-on Keyboard

python
nums = [1, 2]
nums.append([3, 4])   # nested
print(nums)           # [1, 2, [3, 4]]

nums = [1, 2]
nums.extend([3, 4])   # flattened in
print(nums)           # [1, 2, 3, 4]
Output
[1, 2, [3, 4]]
[1, 2, 3, 4]

Think Beyond the Expected

What If?

What does `nums.append([3, 4])` produce and why does it surprise people?

It produces `[1, 2, [3, 4]]` — the whole list is added as a single nested element, not merged. People expect flattening; that's what `extend` (or `+=`) does. append always adds exactly one item, whatever its type.

Real World

Accidentally using append where you meant extend creates nested lists that break later iteration — a very common beginner bug when building up results from multiple sources.

Interviewer's Expectation

Keywords they're listening for:

  • append adds one element
  • extend adds each element
  • append([...]) nests
  • += behaves like extend

Common Mistakes

  • Using append when you meant extend (nesting)
  • Assuming a + b mutates a
  • Confusing extend with a shallow copy

Best Practices

  • append for a single item, extend to merge
  • Use += for in-place extend
  • Prefer list comprehension to flatten nested lists

Follow-up Questions

  1. How is `list += other` different from `list = list + other`?
  2. What's the time complexity of append?
  3. How do you flatten a list of lists?

Continue Learning with AI

Take this question deeper with your favourite AI assistant. Pick a depth, copy the prompt, or open it directly — AI is your learning companion, not a shortcut.

Plain-language foundations

I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner.

Topic: Lists (Python)
Interview question: "What is the difference between append() and extend() on a list?"

Please:
1. Explain the core idea in simple, plain language, using an everyday analogy.
2. Define any technical terms you use.
3. Walk through one small, concrete example.
4. Finish with a single sentence I can easily remember.

Keep the tone friendly and assume I'm new to this topic.
Open inChatGPTGeminiClaude

Was this answer helpful?

Affiliate links

Tools for Your Developer Workspace

Equipment for a comfortable, productive setup.

As an Amazon Associate I earn from qualifying purchases.