What is the difference between append() and extend() on a list?
⚡ 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.
☕Coffee Chat Question
Concept Made Simple
“What is the difference between append() and extend() on a list?”
🧠Mind Map Answer
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⌨️Hands-on Keyboard
Learn by Doing
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][1, 2, [3, 4]] [1, 2, 3, 4]
🔥What If?
Think Beyond the Expected
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:
⚠️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
- 1How is `list += other` different from `list = list + other`?
- 2What's the time complexity of append?
- 3How do you flatten a list of lists?
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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.
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