← Python

Lists – Slicing

A slice [start:stop] extracts a sub-list from a list

What it does: Slice from index 1 up to (not including) index 4. Step -1 traverses the list backwards, producing a reversed copy. Start at index 1, stop before 4: 20(1) 30(2) 40(3). [:2] takes indices 0 and 1: 10, 20. [2:] starts at index 2 and takes everything to the end. Slicing always returns a new list — the original is unchanged. Step of -1 reverses the list. Step of 2 starts at index 0: items at 0, 2, 4 = 1, 3, 5.

Common mistakes: Use colon : for slicing, not comma. Use square brackets for slicing, not parentheses.

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Learning

Duo

[10,20,30,40,50][1:4] returns?

[20, 30, 40][10, 20, 30, 40]

Matches

❌ nums = [1,2,3,4,5] nums[1,3]

↓

Error: Use colon for slicing

Applying

Meanings

Term

list[start:stop]

Meaning

Get items from start up to but not including stop.

Practice

nums = [10, 20, 30, 40, 50]

Testing

Gaps

print(nums[:3])

Check Answer

Spots

1nums = [10, 20, 30]

2part = nums[0,2]

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Quick Questions

1. [10,20,30,40,50][1:4] returns?

A. [20, 30, 40] ✓

B. [10, 20, 30, 40]

Start at index 1, stop before 4: 20(1) 30(2) 40(3)

2. [10,20,30,40][:2] returns?

A. [10, 20, 30]

B. [10, 20] ✓

[:2] takes indices 0 and 1: 10, 20

3. [10,20,30,40][2:] returns?

A. [30, 40] ✓

B. [10, 20]

[2:] starts at index 2 and takes everything to the end

4. Does list slicing modify the original?

A. Yes

B. No creates a new list ✓

Slicing always returns a new list — the original is unchanged

5. [1,2,3,4,5][::-1] returns?

A. [5, 4, 3, 2, 1] ✓

B. [1, 2, 3, 4, 5]

Step of -1 reverses the list

Match the Pairs

nums = [1,2,3,4,5] nums[1,3] → Error: Use colon for slicing

nums = [1,2,3] nums[::0] → Error: Slice step cannot be zero

nums = [1,2,3,4,5] nums[1:4] → Valid: Returns [2, 3, 4]

nums[::-1] → Ok: Reverses the list

n = [10,20,30,40,50] print(n[:3]) → Output: [10, 20, 30]

n = [1,2,3,4,5] print(n[::2]) → Output: [1, 3, 5]

Key Term

list[start:stop]

Get items from start up to but not including stop.~~list[:stop]

Fill in the Blanks

1. nums = [10, 20, 30, 40] print(nums[0:3])

2. nums = [10, 20, 30, 40] print(nums[2:4])

3. nums = [1, 2, 3, 4, 5] print(nums[::-1])

4. nums = [1, 2, 3] copy = nums[:]

5. nums = [10, 20, 30, 40, 50] print(nums[:2])

Spot the Error

Question 1

A nums = [10, 20, 30]

B part = nums[0,2]

C print('Slice:', part)

D print('Done')

Line B — Use colon : for slicing, not comma

Question 2

A nums = [10, 20, 30, 40]

B part = nums(1:3)

C print('Slice:', part)

D print('Done')

Line B — Use square brackets for slicing, not parentheses

Question 3

A nums = [1, 2, 3, 4, 5]

B part = nums[1:4:0]

C print('Slice:', part)

D print('Done')

Line B — Step cannot be 0: causes ValueError

Question 4

A nums = [1, 2, 3]

B copy = nums

C copy[0] = 99

D # expects nums unchanged

Line B — copy = nums doesn't copy: both point to the same list. Use nums[:] to copy.

Question 5

A nums = [10, 20, 30, 40, 50][-1:-4]

B label = 'Slice'

C print('Slice:', nums)

D print('Done')

Line A — Start must come before stop: -4 comes before -1

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