List indexing and slicing in Python - Step-by-Step Execution

Concept Flow - List indexing and slicing

Start with a list

↓

Choose index or slice

↓

If index: get single element

↓

If slice: get sublist

↓

Return result

↓

End

You start with a list, pick an index or a slice, then get either one element or a sublist returned.

Execution Sample

Python

my_list = [10, 20, 30, 40, 50]
print(my_list[1])
print(my_list[1:4])

This code gets the element at index 1 and a slice from index 1 up to 4 (not including 4).

Execution Table

Step	Expression	Evaluation	Result	Explanation
1	my_list[1]	Index 1	20	Get element at index 1 (second element)
2	my_list[1:4]	Slice from 1 to 3	[20, 30, 40]	Get elements from index 1 up to 4 (excluding 4)
3	my_list[-1]	Index -1	50	Get last element using negative index
4	my_list[:3]	Slice from start to 2	[10, 20, 30]	Get first three elements
5	my_list[3:]	Slice from 3 to end	[40, 50]	Get elements from index 3 to end
6	my_list[::2]	Slice with step 2	[10, 30, 50]	Get every second element starting at index 0
7	my_list[4:1:-1]	Slice backwards	[50, 40, 30]	Get elements from index 4 down to 2 in reverse
8	my_list[5]	Index 5	Error	IndexError: index out of range, list has 5 elements (0-4)
9	my_list[1:10]	Slice from 1 to 9	[20, 30, 40, 50]	Slice stops at list end, no error

💡 IndexError occurs if index is out of range; slices gracefully handle out-of-range end.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	After Step 9
my_list	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]

Key Moments - 3 Insights

Why does my_list[1:4] include index 1 but not index 4?

Because slicing in Python includes the start index but excludes the end index, so it takes elements at 1, 2, and 3 only (see execution_table row 2).

What happens if I use a negative index like my_list[-1]?

Negative indexes count from the end, so -1 means the last element (see execution_table row 3).

Why does my_list[5] cause an error but my_list[1:10] does not?

Indexing with 5 is out of range and causes an error, but slicing stops at the list end without error (see execution_table rows 8 and 9).

Visual Quiz - 3 Questions

Look at the execution table, what is the result of my_list[3:] at step 5?

A [40, 50]

B [30, 40]

C [10, 20, 30]

D [50]

💡 Hint

Check the 'Result' column in execution_table row 5.

At which step does an IndexError occur?

A Step 2

B Step 8

C Step 4

D Step 9

💡 Hint

Look for 'Error' in the 'Result' column in execution_table.

If we change my_list[1:4] to my_list[1:5], what would be the result?

A [20, 30, 40]

B [10, 20, 30, 40]

C [20, 30, 40, 50]

D IndexError

💡 Hint

Slicing includes start index but excludes end index; see execution_table row 9 for similar slice.

Concept Snapshot

List indexing: my_list[index] gets one element.
Negative index counts from end (-1 last).
Slicing: my_list[start:end] gets sublist from start up to but not including end.
Omitting start or end means from start or to end.
Slices handle out-of-range gracefully; indexing does not.
Use step in slicing: my_list[start:end:step].

Full Transcript

This lesson shows how Python lists let you get single elements or parts of the list using indexing and slicing. Indexing uses a single number inside square brackets to get one element. Negative indexes count from the end. Slicing uses two numbers separated by a colon to get a sublist from the start index up to but not including the end index. You can omit start or end to slice from the beginning or to the end. Slices do not cause errors if the end is beyond the list length, but indexing does. You can also add a step to slice every nth element or reverse the list. The execution table shows examples and results step by step.

Practice

1. What does the expression my_list[2] return when my_list = [10, 20, 30, 40, 50]?

easy

A. 40

B. 20

C. 30

D. 50

Solution

1. Step 1: Understand list indexing

   Indexing starts at 0, so my_list[0] is 10, my_list[1] is 20, and my_list[2] is 30.

2. Step 2: Identify the value at index 2

   The value at index 2 is 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   Index 2 value = 30 [OK]

Hint: Remember indexing starts at zero, so count from 0 [OK]

Common Mistakes:

• Starting count from 1 instead of 0
• Confusing index 2 with index 3
• Mixing up values and indexes

2. Which of the following is the correct syntax to get the last element of a list nums using indexing?

easy

A. nums[last]

B. nums[1]

C. nums[len(nums)]

D. nums[-1]

Solution

1. Step 1: Recall negative indexing

   Negative indexes count from the end, so -1 means the last element.

2. Step 2: Check each option

   nums[-1] uses nums[-1], which correctly accesses the last element. nums[1] accesses the second element. nums[last] is invalid syntax. nums[len(nums)] causes an IndexError because list indexes go from 0 to len(nums)-1.

3. Final Answer:

   nums[-1] -> Option D

4. Quick Check:

   Last element index = -1 [OK]

Hint: Use -1 to get the last item in a list [OK]

Common Mistakes:

• Using len(nums) as index (out of range)
• Trying to use 'last' as an index
• Confusing positive and negative indexes

3. What is the output of this code?

letters = ['a', 'b', 'c', 'd', 'e']
print(letters[1:4])

medium

A. ['b', 'c', 'd']

B. ['c', 'd', 'e']

C. ['b', 'c', 'd', 'e']

D. ['a', 'b', 'c']

Solution

1. Step 1: Understand slicing syntax

   Slicing letters[1:4] means start at index 1 up to but not including index 4.

2. Step 2: Identify elements at indexes 1, 2, 3

   Index 1 is 'b', index 2 is 'c', index 3 is 'd'. So the slice is ['b', 'c', 'd'].

3. Final Answer:

   ['b', 'c', 'd'] -> Option A

4. Quick Check:

   Slicing excludes stop index [OK]

Hint: Slice stops before the stop index, not including it [OK]

Common Mistakes:

• Including the stop index element
• Confusing start and stop indexes
• Using wrong indexes for slicing

4. The code below causes an error. What is the problem?

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

medium

A. SyntaxError due to wrong brackets

B. IndexError because index 5 is out of range

C. TypeError because nums is not a list

D. No error, prints 5

Solution

1. Step 1: Check list length and valid indexes

   List nums has 5 elements with indexes 0 to 4.

2. Step 2: Understand index 5 usage

   Index 5 is outside the valid range, so accessing nums[5] causes an IndexError.

3. Final Answer:

   IndexError because index 5 is out of range -> Option B

4. Quick Check:

   Index must be less than list length [OK]

Hint: Indexes go from 0 to length-1 only [OK]

Common Mistakes:

• Thinking index 5 is valid for 5 elements
• Confusing SyntaxError with IndexError
• Assuming list indexes start at 1

5. Given data = [5, 10, 15, 20, 25, 30], which expression returns a new list with every second element in reverse order?

hard

A. data[::-2]

B. data[::2]

C. data[-2::-1]

D. data[1::2]

Solution

1. Step 1: Understand slicing with step and negative step

   The slice data[::-2] starts from the end and takes every second element backwards.

2. Step 2: Check what data[::-2] returns

   It returns [30, 20, 10], which is every second element in reverse order.

3. Step 3: Verify other options

   data[::2] returns every second element forward: [5, 15, 25]. data[-2::-1] returns from index -2 backwards: [25, 20, 15, 10, 5]. data[1::2] returns every second element starting at index 1: [10, 20, 30].

4. Final Answer:

   data[::-2] -> Option A

5. Quick Check:

   Negative step reverses and skips elements [OK]

Hint: Use negative step to reverse and skip elements [OK]

Common Mistakes:

• Using positive step for reverse
• Confusing start index with step sign
• Misunderstanding slice boundaries