[Tut] Convert List to Tuple | The Most Pythonic Way

[Tut] Convert List to Tuple | The Most Pythonic Way - Printable Version

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[Tut] Convert List to Tuple | The Most Pythonic Way - xSicKxBot - 06-22-2020

Convert List to Tuple | The Most Pythonic Way

Answer: The simplest, most straightforward, and most readable way to convert a list to a tuple is Python’s built-in

Code:
tuple(list)

function. You can pass any iterable (such as a list, another tuple, or a set) as an argument into this so-called constructor function and it returns a new tuple data structure that contains all elements of the iterable.

Converting a list to a tuple seems trivial, I know. But keep reading and I’ll show you surprising ways of handling this problem. I guarantee that you’ll learn a lot of valuable things from the 3-5 minutes you’ll spend reading this tutorial!

Problem: Given a list of elements. Create a new tuple with the same elements—thereby converting the list to a tuple.

Example: You have the following list.

Code:
lst = [1, 2, 3]

You want to create a new tuple data structure that contains the same integer elements:

Code:
(1, 2, 3)

Let’s have a look at the different ways to convert a list to a tuple—and discuss which is the most Pythonic way in which circumstance.

You can get a quick overview in the following interactive code shell. Explanations for each method follow after that:

https://trinket.io/embed/python/dc9e315394

Exercise: Run the code. Skim over each method—does any of them confuse you?

Method 1: Tuple Constructor

The simplest, most straightforward, and most readable way to convert a list to a tuple is Python’s built-in

Code:
tuple(iterable)

Here’s an example:

Code:
lst = [1, 2, 3]
t = tuple(lst)

The result is the following tuple:

Code:
print(t)
# (1, 2, 3)

This is the most Pythonic way if a flatconversion of a single list to a tuple is all you need. But what if you want to convert multiple lists to a tuple?

Method 2: Unpacking

There’s an alternative that works for one or more lists. In other words, you can use this general method to convert one or more lists into a tuple. This method is equally efficient and it takes less characters than Method 1 (at the costs of readabilityfor beginner coders). Sounds interesting? Let’s dive into unpacking and the asterisk operator!

The asterisk operator

Code:
*

is also called “star operator” and you can use it as a prefix on any list. The operator will “unpack” all elements of the list into the outer structure—so make sure you call it only within argument lists or within an enclosing container type such as a list or a tuple.

Here’s how it works to unpack all elements of a list into an enclosing tuple—thereby converting the original list to a new tuple.

Code:
t = (*lst,)
print(t)
# (1, 2, 3)

You unpack all elements in the

Code:
lst

into the outer structure

Code:
(*lst,)

. The outer parentheses with the comma indicate that it’s a tuple and not the parentheses of a simple expression. If you’d forget to add the comma, Python would interpret the expression

Code:
(*lst)

as being a simple precedence relation and it would throw an error because there’s no outer container data structure that “catches” the unpacked elements.

The strength of this approach is—despite being even conciser than the standard

Code:
tuple(...)

function—that you can unpack multiple values into it!

Method 3: Unpacking to Convert Multiple Lists to a Single Tuple

Let’s have a look at how you’d create a tuple from multiple lists:

Code:
lst_1 = [1, 2, 3]
lst_2 = [4, 5, 6]
t = (*lst_1, *lst_2)
print(t)
# (1, 2, 3, 4, 5, 6)

The expression

Code:
(*lst_1, *lst_2)

unpacks all elements in

Code:
lst_1

and

Code:
lst_2

into the outer tuple structure. This allows you to convert multiple lists to a single tuple.

Method 4: Generator Expression to Convert Multiple Lists to Tuple

If you have multiple lists stored in a list of lists and you want to convert them to a single tuple, you can use a short generator expression statement to go over all inner lists and over all elements of each inner list. Then, you place each of those elements into the tuple structure:

Code:
lst = [[1, 2], [3, 4], [5, 6, 7]]
t = tuple(x for l in lst for x in l)
print(t)
# (1, 2, 3, 4, 5, 6, 7)

This is the most Pythonic way to convert a list of lists to a tuple. It’s short and efficient and readable. You don’t create any helper data structure that takes space in memory.

But what if you want to save a few more characters?

Method 5: Generator Expression + Unpacking

Okay, you shouldn’t do this last method using the asterisk operator—it’s unreadable—but I couldn’t help including it here:

Code:
lst = [[1, 2], [3, 4], [5, 6, 7]]
t = (*(x for l in lst for x in l),)
print(t)
# (1, 2, 3, 4, 5, 6, 7)

Rather than using the

Code:
tuple(...)

function to convert the generator expression to a tuple, you use the

Code:
(...,)

helper structure to indicate that it’s a tuple you want—and unpack all elements from the generator expression into the tuple. Sure, it’s not very readable—but you could see such a thing in practice (if pro coders want to show off their skills ;)).

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