Understanding sequences and mutability
A list is a sequence of values that can contain any data type. A string is a sequence of characters.
Two Kinds of Sequence
A sequence is an organized collection of values. In Python, a string is a sequence whose values are characters. A list is a sequence whose elements can be values of any data type. This difference makes lists suitable for collecting varied pieces of data, while strings represent text as characters.
A list is a sequence of values that can contain any data type. A string is a sequence of characters.
Building Lists with Brackets
The simplest way to create a list is to place its elements between square brackets and separate the elements with commas. The brackets show where the list begins and ends. The commas separate one element from the next.
| List form | What it represents |
|---|---|
| [5, 8, 13, 21] | A list of four integers |
| ['red', 'blue', 'green'] | A list of three strings |
| [] | An empty list with zero elements |
Square brackets create lists; commas separate their elements.
An empty list is a valid list that contains zero elements. It is written with square brackets and nothing between them. Empty lists can serve as starting points for building lists as a program runs.
Reading list structure
Consider the list [12, 'oak', 3.5]. What are its elements, and how many elements does it contain?
- 1Find the boundaries
The square brackets show that the complete expression is one list.
- 2Separate the elements
The commas separate 12, 'oak', and 3.5 into three individual elements.
- 3Identify the types
The elements are an integer, a string, and a floating-point value. They do not need to have the same type.
The list has three elements, and those elements have mixed data types.
Nested Lists and Containment
Because a list can contain any type of value, one of its elements can itself be another list. A list inside another list is called a nested list. The inner list remains a complete list; it is simply stored as one element of the outer list.
A nested list can organize data hierarchically. For example, an outer list might hold several values, while one element is a separate list containing related values. The outer list does not flatten the inner list; the inner list remains one element of the outer structure.
Names Referring to Lists
Lists can be assigned to variables just as numbers and strings can. After the assignment, the variable name refers to the entire list. This gives the list a convenient name that can be used to work with that list in a program.
Changing a List Over Time
Lists are useful when the collection of values can grow while a program runs. You can begin with an empty list and add elements later. This is the practical connection between lists and mutability in this lesson: the list's contents can be built up over time, while the variable name continues to be the name used to work with the list.
When you need a collection that will be filled as a program runs, start with an empty list. This makes the intended zero-element starting state explicit.
Mistakes Beginners Make
Common mistake Assuming every list element must have the same data type.
What it looks likeTreating a list containing an integer, a string, and a float as invalid.
Why it failsLists can contain mixed data types.
Do this insteadCheck each element separately; integers, strings, floats, and other values can coexist in one list.
Common mistake Treating a nested list as if its values were automatically separate elements of the outer list.
What it looks likeViewing [10, 20] inside an outer list as two outer elements.
Why it failsThe inner list is itself one complete element of the outer list.
Do this insteadDistinguish the outer list from the nested list it contains.
Common mistake Thinking square brackets with no contents do not form a list.
What it looks likeTreating [] as missing or incomplete data.
Why it fails[] is a valid empty list with zero elements.
Do this insteadRecognize [] as a useful starting list that can receive elements later.
Common mistake Confusing a variable name with the list data.
What it looks likeThinking the name items is an element inside the list it refers to.
Why it failsThe variable name refers to the entire list; it is not an element of that list.
Do this insteadSeparate the name used to work with the list from the list's elements.
Check Your Understanding
Describe the structure of this list: ['tea', 4, 1.5, [7, 8], []]. State how many elements the outer list has, identify the nested list, and identify the empty list.
Hints
- Count the comma-separated elements at the outermost level.
- A list inside the outer list counts as one outer element.
- An empty list is still a list, even though it has zero elements of its own.
What do you think happens?
Before checking the explanation, predict whether the outer list in ['tea', 4, 1.5, [7, 8], []] has three, four, or five elements.
Reveal answer
Five
The outer list contains the string 'tea', the integer 4, the float 1.5, the nested list [7, 8], and the empty list []. Both inner lists count as individual elements of the outer list.
Key Takeaways
- 1A list is a sequence that can contain values of any data type, while a string contains characters.
- 2Square brackets create lists, commas separate elements, and [] creates an empty list.
- 3Lists may contain mixed data types and may contain other lists as nested elements.
- 4A variable assigned a list refers to the entire list, not to one extra element inside it.
- 5An empty list can be used as a starting point for building a collection as a program runs.
At a glance
- Lists are sequences that can contain values of any data type; strings are sequences of characters.
- Square brackets create lists, including empty lists and lists containing other lists.
- A nested list is one complete element inside its outer list.
- A variable name refers to the entire list assigned to it.
- Lists can start empty and have their contents built as a program runs.