Lists — append, slice, sort, comprehensions

Lists are ordered, mutable, indexed. The most-used Python data structure.

script.py v3.1

# Create
nums = [3, 1, 4, 1, 5, 9, 2, 6]
print('original:', nums)

# Indexing (0-based, negative from end)
print('first:', nums[0], 'last:', nums[-1])

# Slicing [start:stop:step] — stop is EXCLUSIVE
print('first 3:', nums[:3])
print('last 3:', nums[-3:])
print('every other:', nums[::2])
print('reversed:', nums[::-1])

# Mutating
nums.append(7)
print('after append:', nums)
nums.extend([8, 9])
print('after extend:', nums)
nums.insert(0, 0)
print('after insert:', nums)
nums.remove(1)  # removes FIRST 1
print('after remove(1):', nums)

# Sorting
sorted_nums = sorted(nums)
print('sorted:', sorted_nums)
nums.sort(reverse=True)
print('reverse sorted in place:', nums)

# Comprehension
doubled = [x * 2 for x in nums]
print('doubled:', doubled)

Python output click Run to execute

Click "Run" to execute the code on the left.
The output will appear here.

🧠 How it works

Lists are arrays that grow/shrink. Indexed 0, 1, 2, ... and -1, -2, -3 from the end. slice [start:stop:step] — start INCLUSIVE, stop EXCLUSIVE. nums[1:4] gives indices 1, 2, 3. Omitting start/stop means 'from beginning' / 'to end'. nums[::-1] reverses. append(x) adds to end. extend(iter) adds all from iter. insert(i, x) puts x at index i. remove(x) removes the FIRST x. pop() removes and returns the last. sort() sorts in place (returns None). sorted() returns a new sorted list (doesn't mutate). Comprehension [expr for x in list if cond] — faster than a for+append loop.

✏️ Try these changes

Change nums[:3] to nums[3:]
→ last 5 instead of first 3
Change nums[::2] to nums[::3]
→ every 3rd element
Change nums.sort(reverse=True) to nums.sort()
→ ascending order
Change [x * 2 for x in nums] to [x * 2 for x in nums if x > 4]
→ only doubles values > 4
Add nums.clear()
→ empties the list

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