Accessing list elements with indexing
A list is a sequence of values that can contain any data type, unlike strings which contain only characters.
Try it: Indexing and Slicing
How Python indexes a sequence from 0 (or from the end with negative numbers) and how a slice start:stop:step picks positions — with the exact CPython rules for omitted and out-of-range bounds.
How it works
- Positions run 0 … len−1; negative positions count back from the end (−1 is the last).
- An index outside the string raises IndexError.
- A slice resolves start and stop (omitted → the ends; out of range → clamped), then walks by step, stopping before stop.
- A negative step walks backwards; a step of 0 is a ValueError.
Default run (7 steps): 'PYTHONIC'[1:6:2]: a slice copies characters from start up to (not including) stop, moving by step. … 'PYTHONIC'[1:6:2] → 'YHN' (positions 1, 3, 5).
Simplified: Works on a short string (up to 16 printable ASCII characters); lists slice by exactly the same rules.
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From Sequence to Element
A list is a sequence of values. Because the values have positions in that sequence, you can access an individual element by using its index. The important starting point is to recognize that a list is not one single value in the same sense as a string: it is a container holding separate elements.
A list is a sequence of values that can contain any data type. A string, by contrast, contains only characters.
Reading Positions
Indexing connects a position number to the value stored at that position. In Python, the first position is index 0, the next position is index 1, and the positions continue upward through the list. Therefore, the index is not the value itself; it is the position used to select the value.
Selecting an Element by Position
Consider the list ['red', 'green', 'blue']. Which element is selected by index 1?
Identify the positions: The list is a sequence, so its elements have ordered positions. Counting from 0 gives red at index 0, green at index 1, and blue at index 2.
Use the requested index: Index 1 refers to the second element in the sequence.
Read the element: The value at index 1 is green.
Index 1 selects the element green.
Lists and Strings
| Value type | What it contains | How its elements are represented |
|---|---|---|
| List | Values that can have any data type | Separate elements, written between square brackets and separated by commas |
| String | Characters only | Characters forming the string |
The difference matters when interpreting an index. An index into a list selects one value from the list, and that value might be an integer, a string, a float, or another list. An index into a string selects a character because strings contain only characters.
Building List Structures
Square brackets create lists. Write elements inside the brackets and separate them with commas. The brackets can contain several values, no values, or another list.
- A list of integers can contain four integer elements.
- A list of strings can contain three string elements.
- A mixed list can contain integers, strings, floats, and other types together.
- An empty list is written as [] and contains zero elements.
- A nested list is a list contained as an element inside another list.
Reading a Nested Element
Consider the mixed list containing the string spam, the float 2.0, the integer 5, and the nested list [10, 20]. What is the fourth element of the outer list?
Count the outer elements: The outer list has four elements: spam, 2.0, 5, and [10, 20].
Separate the levels: The list [10, 20] is one complete element of the outer list. Its two integers belong to the inner list.
Apply the outer position: The fourth outer element is at index 3 because the first outer element has index 0.
The fourth element of the outer list is the nested list [10, 20].
Naming the List
A list can be assigned to a variable just as a number or string can. The variable name refers to the entire list, not automatically to one particular element. After assigning a list to a variable, use that name when working with the list and use an index when selecting one of its elements.
Mistakes with Positions
Treating the first element as index 1
Python list indexing starts at 0, so the first element has index 0.
Fix:
Map the first element to 0, the second to 1, and continue upward.Assuming every list element must have the same data type
Lists can contain mixed data types, including integers, strings, floats, and other types.
Fix:
Identify each element by its own value and data type.Counting the contents of a nested list as separate outer elements
A nested list is itself one complete element of the outer list.
Fix:
Count the nested list once when identifying outer positions, then inspect its contents separately.Confusing a variable name with an individual element
A variable assigned to a list refers to the entire list.
Fix:
Use an index after the variable name when you need one particular element.
Check Your Indexing
A list contains the values ['tea', 7, 3.5, ['hot', 'cold']]. Identify the number of elements in the outer list, state the index of 3.5, and explain whether ['hot', 'cold'] is one outer element or two.
Hints
- Count the outer values without opening the nested list.
- Remember that the first outer element has index 0.
- The nested list is itself an element of the outer list.
What do you think happens?
In the list ['tea', 7, 3.5, ['hot', 'cold']], what does index 2 select?
Reveal answer
Answer: 3.5
Counting from index 0 gives tea at index 0, 7 at index 1, 3.5 at index 2, and the nested list at index 3.
Key Takeaways
- A list is a sequence of values and can contain any data type, while a string contains only characters.
- Square brackets create lists; [] creates a valid empty list.
- Lists can contain mixed values and can contain other lists as nested elements.
- A variable assigned to a list refers to the entire list.
- Indexing selects an element by position, with the first position represented by index 0.
Key Takeaways
- Lists are sequences whose elements can have different data types.
- Square brackets create lists, including empty and nested lists.
- A variable name can refer to an entire list rather than one element.
- Indexing uses a position to select one element, and the first position is index 0.
- A nested list counts as one element of its outer list.