The Immutable Tuple

Tuples are lists that never change.


The tuple is a data structure very similar to a list, except for being immutable. The text of this lesson used to be part of the list lesson, but I’ve moved it here just to keep the wall of text manageable.

There’s not much to say specifically about tuples, since they are so similar to lists. But their one big difference from lists is useful for describing the concept of immutable Python objects.

Table of contents

What is immutability?

Before we studied lists, most of the Python objects we've dealt with so far, including numbers and strings, have been immutable.

The values of immutable objects do not change. That means for string objects, we can be sure that all of the methods that we call from a string, including upper() and replace()do not affect the actual string value.

An example of not mutating a string

Consider the Python code below, which you can run from your interactive Python shell. The mytxt variable is assigned to the string of "Hello world". And the variables a and b point to the results of the mytxt string object calling the upper() and replace() methods, respectively:

>>> mytxt = "Hello world"
>>> a = mytxt.upper()
>>> b = mytxt.replace("o", "X")

The strings that a and b point to are clearly different from mytxt. But the original string that mytxt was assigned to has not changed.

>>> print(a)
>>> print(b)
HellX wXrld
>>> print(mytxt)
Hello world

The above examples don't prove that strings are immutable – their immutability is just a fact of Python. One consequence of strings being immutable, though, is that none of their methods will modify the calling string. Instead, string methods always return a copy of the string.

An example of mutating a list

It's easier to explain immutability by showing an example of mutability. Lists are mutable. They have in-place methods which do modify the lists that call them. One such list method is pop(), which removes the last member of a list:

>>> mylist = [1, 2, 3]
>>> mylist.pop()
>>> len(mylist)
>>> print(mylist)
[1, 2]

That's mutability in a nutshell. The lesson on lists, has more examples of how to mutate a list object.

What is a tuple

Tuples are lists that are immutable.

I like Al Sweigart's characterization of a tuple as being the "cousin" of the list. The tuple is so similar to the list that I am tempted to just skip covering it, especially since it's an object that other popular languages – such as Ruby, Java, and JavaScript – get by just fine without having. However, the tuple is an object that is frequently used in Python programming, so it's important to at least be able to recognize its usage, even if you don't use it much yourself.

Syntax for declaring a tuple

Like a list, a tuple is a collection of other objects. The main visual difference, in terms of syntax, is that instead of enclosing the values in we use __parentheses, not square brackets, to enclose the values of a tuple:

List Tuple
[1, 2, "hello"] (1, 2, "hello")

Otherwise, the process of iterating through a tuple, and using square bracket notation to access or slice a tuple's contents is the same as it is for lists (and pretty much for every other Python sequence object, such as range…but we'll gloss over that detail for now).

Parentheses overload?

Word of warning: if you're relatively new to programming and reading code, the use of parentheses as delimiters might seem like a potential syntactic clusterf—, as parentheses are already used in various other contexts, such as function calls. What differentiates a tuple declaration, e.g.

mytuple = ("hello", "world")

– from the parentheses-enclosed values of a function call, e.g.

print("hello", "world")

Well, that's easy – the latter set of parentheses-enclosed objects immediately follows a function name, i.e. print. The potential confusion from the similar notation isn't too terrible, in practice.

One strange thing you might come across is this:

>>> mytuple = ("hello", )
>>> type(mytuple)
>>> len(mytuple)

Having a trailing comma is the only way to denote a tuple of length 1. Without that trailing comma, mytuple would be pointing to a plain string object that happens to be enclosed in parentheses:

>>> mytuple = ("hello")
>>> type(mytuple)

Tuples are immutable

Besides the different kind of brackets used to delimit them, the main difference between a tuple and a list is that the tuple object is immutable. Once we've declared the contents of a tuple, we can't modify the contents of that tuple.

For example, here's how we modify the 0th member of a list:

>>> mylist = [1, 2, 3]
>>> mylist[0] = 999
[999, 2, 3]

That will not work with a tuple:

>>> mytuple = (1, 2, 3)
>>> mytuple[0] = 999
TypeError: 'tuple' object does not support item assignment

And while the list object has several methods for adding new members, the tuple has no such methods.

In other words, "immutability" == "never-changing".

Similarly, when a program alters the contents of a mutable object, it's often described as "mutating" that object.

Why tuples? Why immutability?

As I said earlier, I'm not naturally inclined to focus a lot on tuples, as I had never used them before in other languages, and they're so similar in concept to lists.

So why do they exist? Because their presence in a program indicates that this here is something that will never change. If you're new to programming and haven't written any massively complicated programs yet, it will likely make no sense why anyone would want to impose this restriction upon their code, e.g. what kind of harebrained idiot creates a data object that they don't want to change, but then accidentally change? Can't you just get your life together rather than create a whole new data type?

Ha ha, you'll see how easy it is to confuse yourself with your own code…but the precaution is not just to protect a programmer from messing up their own code, but for the situation in which you use someone else's code and want to be assured that a particular collection of objects will not change.

There's also some performance optimizations that come from objects that are declared as immutable…but we're not doing anything at such a scale that we would benefit from such optimization. The optimization here is solely on the human side – i.e. your ability to read code and be confident about what it does and does not do.

When immutable tuples contain mutable objects

So this is a bit of trivia. In fact, it's something I constantly forget about: Tuples may be immutable, but like lists, they can contain sequences of any other kind of Python object, including mutable objects.

So what's the big deal? Not much, practically speaking. It just means that we can effectively "mutate" a tuple by changing (i.e. mutating) one of its mutable objects.

In the following snippet, a tuple is declared with a list as its second element. I then mutate the second element of that list, effectively changing the contents of the tuple:

>>> mytuple = (0, ["hello", "world"], 999)
>>> print(mytuple[1])
['hello', 'world']
>>> mytuple[1][1] = "NINJA TURTLE"        # << this is naughty
>>> print(mytuple[1])
['hello', 'NINJA TURTLE']

What does this have to do with anything? Not much, in terms of our day-to-day practical programming. You have to actually go out of your way to do this, as in, you want to prove a point. But generally, there is never a reason to create a tuple that includes a list among its members, nevermind then futzing about with that list and changing its contents.

But if you're curious about the seeming contradiction, here's a good explanation: Why can tuples contain mutable items?

Whew! That was a lot of information about a topic that frankly, I'm going to try to avoid ever dealing with explicitly. In fact, for the purposes of this guide and its exercises, you may not have to worry about all the nuances. However, it's because I'm trying to design the exercises in such a way as to avoid the implications of "immutable vs. mutable". Sometimes my attempts at avoidance results in somewhat convoluted algorithms, so I'm hoping that at least knowing about this concept will help you understand some of my recommended design decisions.

If you came here from the lesson on lists, now you know a bit about immutability and can go back to learning about lists and how they are mutable.

References and Related Readings

Lists, mutability, and in-place methods
Lists are ordered collections of other objects. And lists are mutable.
Chapter 4: Lists
A great, concise walkthrough of the list and its "cousin", the tuple.
Mutation in Python
Why can tuples contain mutable items?
If a tuple is immutable then why can it contain mutable items?