packing and unpacking tuples in python

Unlike a list or a dictionary, in Python, tuples are immutable.¹ This is clear when investigating the methods available on the tuple object. Conspicuously absent are any methods for adding or removing elements:

>>> dir(tuple)
['__add__', '__class__', '__contains__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmul__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'count', 'index']

So, how can you add a new item to a tuple? Technically you can’t. But what you can do is reassign the variable to a new tuple.

This is where unpacking comes in handy. Unpacking is comparable to object destructuring from Javascript. There’s even a “rest operator” equivalent: the *.

Let’s look at an example of a family.

>>> family = 'Stephen', 'Kate', 'Finn' # packing a tuple
>>> *adults, animals = family # unpacking
>>> adults
['Stephen', 'Kate']
>>> animals
'Finn'
>>> id(family)
4326772672

Interestingly, unlike destructuring in Javascript, the “rest” portion of this can occur at any point. In this example, the adults are leading the group, and I want to pull them out.

Below, the family gets a new addition, a cat named Garfield.

>>> family = *family, 'Garfield'
>>> family
('Stephen', 'Kate', 'Finn', 'Garfield')
>>> id(family)
4326761632

The first thing we do is create a new tuple. This is done by unpacking the original tuple and then adding a new member (Garfield). We know it’s a new tuple because the id that Python’s tracking for its position in memory is different.

Let’s split this group into a few different factions:

>>> *original_cast, new_addition = family
>>> original_cast
['Stephen', 'Kate', 'Finn']
>>> new_addition
'Garfield'
>>> *adults, dog, cat = family
>>> adults
['Stephen', 'Kate']
>>> dog
'Finn'
>>> cat
'Garfield'
>>> dad, mom, *animals = family
>>> dad
'Stephen'
>>> mom
'Kate'
>>> animals
['Finn', 'Garfield']

These examples better show the various ways a tuple can be unpacked.

In case you missed it, packing is the term used to describe the creation of the tuple. E.g.,:

my_tuple = ('a', 'b', 'c') # this is packing
my_other_tuple = 1, 2, 3 # this is also packing

Wrap Up

Tuples may be immutable, but with the spread unpacking operator, it’s easy enough to modify them. Just remember that you’re creating a brand new list - with all of the performance implications therein.

Footnotes

• ¹ For what it’s worth, I always thought a tuple was a pair. Turns out that’s not the case. Per Wikipedia, “a tuple is finite ordered lists (sequence) of elements.” The point is that it’s finite, not that it’s a pair.