3.1. Collection Types - Strings, Lists and Tuples¶

3.1.1. Strings Revisited¶

Throughout the first chapters of this book we have used strings to represent words or phrases that we wanted to print out. Our definition was simple: a string is simply some characters inside quotes. In this chapter we explore strings in much more detail.

3.1.2. A Collection Data Type¶

So far we have seen built-in types like: int, float, bool, str, int, float, and bool are considered to be simple or primitive data types because their values are not composed of any smaller parts. They cannot be broken down. On the other hand, strings are different from the others because they are made up of smaller pieces. They are made up of smaller strings each containing one character.

Types that are comprised of smaller pieces are called collection data types. Depending on what we are doing, we may want to treat a collection data type as a single entity (the whole), or we may want to access its parts. This ambiguity is useful.

Strings can be defined as sequential collections of characters. This means that the individual characters that make up the string are assumed to be in a particular order from left to right.

A string that contains no characters, often referred to as the empty string, is still considered to be a string. It is simply a sequence of zero characters and is represented by '' or "" (two single or two double quotes with nothing in between).

3.1.3. Lists¶

A list is a sequential collection of Python data values, where each value is identified by an index. The values that make up a list are called its elements. Lists are similar to strings, which are ordered collections of characters, except that the elements of a list can have any type and for any one list, the items can be of different types.

There are several ways to create a new list. The simplest is to enclose the elements in square brackets ( [ and ]). This is often referred to as the literal form of a list.

[10, 20, 30, 40]
["spam", "bungee", "swallow"]
[]

The first example is a list of four integers. The second is a list of three strings. As we said above, the elements of a list don’t have to be the same type. The following list contains a string, a float, an integer, and another list.

["hello", 2.0, 5, [10, 20]]

A list within another list is said to be nested and the inner list is often called a sublist. Finally, there is a special list that contains no elements. It is called the empty list and is denoted [].

In [1]: empty = []

In [2]: type(empty)
Out[2]: list

As you would expect, we can also assign list values to variables and pass lists as parameters to functions.

Check your understanding

rec-5-19: A list can contain only integer items.
• (A) False
• Yes, unlike strings, lists can consist of any type of Python data.
• (B) True
• Lists are heterogeneous, meaning they can have different types of data.

3.1.4. Tuples¶

So far you have seen two types of sequential collections: strings, which are made up of characters; and lists, which are made up of elements of any type. The third type of built-in sequence is a tuple, which like a list, is a sequence of items of any type. Syntactically, a tuple is a comma-separated sequence of values. Although it is not always necessary, it is conventional to enclose tuples in parentheses:

julia = ("Julia", "Roberts", 1967, "Duplicity", 2009, "Actress", "Atlanta, Georgia")

Tuples are useful for representing what other languages often call records — some related information that belongs together, like your student record. There is no description of what each of these fields means, but we can guess. A tuple lets us “chunk” together related information and use it as a single thing.

Because tuples are delimited by parentheses, we need some special syntax which would otherwise be interpreted as an arithmetic expression. Specifically, a single item tuple needs an added comma, or it will be interpreted as an arithmetic expression.

In [3]: one_item = ("one",)

In [4]: one_item
Out[4]: ('one',)

In [5]: type(one_item)
Out[5]: tuple

In [6]: one_item_no_comma = ("one")

In [7]: one_item_no_comma
Out[7]: 'one'

In [8]: type(one_item_no_comma)
Out[8]: str

rec-5-20: Which of the following is NOT a tuple?
• (A) (1,2,3)
• A tuple is surrounded by parentheses and separated by commas
• (B) ()
• This is the empty tuple
• (C) (2)
• A length 1 tuple needs a comma
• (D) (2,)
• A length 1 tuple needs a comma