Unpacking And Multiple Assignment in

About Unpacking And Multiple Assignment

Unpacking refers to the act of extracting the elements of a collection, such as a list, tuple, or dict, using iteration. Unpacked values can then be assigned to variables within the same statement. A very common example of this behavior is for item in list, where item takes on the value of each list element in turn throughout the iteration.

Multiple assignment is the ability to assign multiple variables to unpacked values within one statement. This allows for code to be more concise and readable, and is done by separating the variables to be assigned with a comma such as first, second, third = (1,2,3) or for index, item in enumerate(iterable).

The special operators * and ** are often used in unpacking contexts. * can be used to combine multiple lists/tuples into one list/tuple by unpacking each into a new common list/tuple. ** can be used to combine multiple dictionaries into one dictionary by unpacking each into a new common dict.

When the * operator is used without a collection, it packs a number of values into a list. This is often used in multiple assignment to group all "leftover" elements that do not have individual assignments into a single variable.

It is common in Python to also exploit this unpacking/packing behavior when using or defining functions that take an arbitrary number of positional or keyword arguments. You will often see these "special" parameters defined as def some_function(*args, **kwargs) and the "special" arguments used as some_function(*some_tuple, **some_dict).

Multiple assignment

In multiple assignment, the number of variables on the left side of the assignment operator (=) must match the number of values on the right side. To separate the values, use a comma ,:

>>> a, b = 1, 2
>>> a
1

If the multiple assignment gets an incorrect number of variables for the values given, a ValueError will be thrown:

>>> x, y, z = 1, 2

ValueError: too many values to unpack (expected 3, got 2)

Multiple assignment is not limited to one data type:

>>> x, y, z = 1, "Hello", True
>>> x
1

>>> y
'Hello'

>>> z
True

Multiple assignment can be used to swap elements in lists. This practice is pretty common in sorting algorithms. For example:

>>> numbers = [1, 2]
>>> numbers[0], numbers[1] = numbers[1], numbers[0]
>>> numbers
[2, 1]

Since tuples are immutable, you can't swap elements in a tuple.

Unpacking

In Python, it is possible to unpack the elements of list/tuple/dictionary into distinct variables. Since values appear within lists/tuples in a specific order, they are unpacked into variables in the same order:

>>> fruits = ["apple", "banana", "cherry"]
>>> x, y, z = fruits
>>> x
"apple"

If there are values that are not needed then you can use _ to flag them:

>>> fruits = ["apple", "banana", "cherry"]
>>> _, _, z = fruits
>>> z
"cherry"

Deep unpacking

Unpacking and assigning values from a list/tuple inside of a list or tuple (also known as nested lists/tuples), works in the same way a shallow unpacking does, but often needs qualifiers to clarify the values context or position:

>>> fruits_vegetables = [["apple", "banana"], ["carrot", "potato"]]
>>> [[a, b], [c, d]] = fruits_vegetables
>>> a
"apple"

>>> d
"potato"

You can also deeply unpack just a portion of a nested list/tuple:

>>> fruits_vegetables = [["apple", "banana"], ["carrot", "potato"]]
>>> [a, [c, d]] = fruits_vegetables
>>> a
["apple", "banana"]

>>> c
"carrot"

If the unpacking has variables with incorrect placement and/or an incorrect number of values, you will get a ValueError:

>>> fruits_vegetables = [["apple", "banana"], ["carrot", "potato"]]
>>> [[a, b], [d]] = fruits_vegetables

ValueError: too many values to unpack (expected 1)

Unpacking a list/tuple with *

When unpacking a list/tuple you can use the * operator to capture "leftover" values. This is clearer than slicing the list/tuple (which in some situations is less readable). For example, we can extract the first element and pack the remaining values into a new list without the first element:

>>> fruits = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
>>> x, *last = fruits
>>> x
"apple"

>>> last
["banana", "cherry", "orange", "kiwi", "melon", "mango"]

We can also extract the values at the beginning and end of the list while grouping all the values in the middle:

>>> fruits = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
>>> x, *middle, y, z = fruits
>>> y
"melon"

>>> middle
["banana", "cherry", "orange", "kiwi"]

We can also use * in deep unpacking:

>>> fruits_vegetables = [["apple", "banana", "melon"], ["carrot", "potato", "tomato"]]
>>> [[a, *rest], b] = fruits_vegetables
>>> a
"apple"

>>> rest
["banana", "melon"]

Unpacking a dictionary

Unpacking a dictionary is a bit different from unpacking a list/tuple. Iteration over dictionaries defaults to the keys. So when unpacking a dict, you can only unpack the keys and not the values:

>>> fruits_inventory = {"apple": 6, "banana": 2, "cherry": 3}
>>> x, y, z = fruits_inventory
>>> x
"apple"

If you want to unpack the values then you can use the <dict>.values() method:

>>> fruits_inventory = {"apple": 6, "banana": 2, "cherry": 3}
>>> x, y, z = fruits_inventory.values()
>>> x
6

If both keys and values are needed, use the <dict>.items() method. <dict>.items() generates an iterable view containing key-value pairs. These can be unpacked into a tuple:

>>> fruits_inventory = {"apple": 6, "banana": 2, "cherry": 3}
>>> x, y, z = fruits_inventory.items()
>>> x
("apple", 6)

Packing

Packing is the ability to group multiple values into one list that is assigned to a variable. This is useful when you want to unpack values, make changes, and then pack the results back into a variable. It also makes it possible to perform merges on 2 or more lists/tuples/dicts.

Packing a list/tuple with *

Packing a list/tuple can be done using the * operator. This will pack all the values into a list/tuple.

>>> fruits = ("apple", "banana", "cherry")
>>> more_fruits = ["orange", "kiwi", "melon", "mango"]

# fruits and more_fruits are unpacked and then their elements are packed into combined_fruits
>>> combined_fruits = *fruits, *more_fruits

# If there is no * on to the left of the "=" the result is a tuple
>>> combined_fruits
("apple", "banana", "cherry", "orange", "kiwi", "melon", "mango")

# If the * operator is used on the left side of "=" the result is a list.
# Note the trailing comma. 
>>> *combined_fruits_too, = *fruits, *more_fruits
>>> combined_fruits_too
['apple', 'banana', 'cherry', 'orange', 'kiwi', 'melon', 'mango']

# A list literal can be used instead, but might not be as readable.
>>> [*combined_fruits_too] = *fruits, *more_fruits
>>> combined_fruits_too
['apple', 'banana', 'cherry', 'orange', 'kiwi', 'melon', 'mango']

For more details on the use of * and **, check out PEP 3132 and PEP 448.

Packing a dictionary with **

Packing a dictionary is done by using the ** operator. This will pack all key-value pairs from one dictionary into another dictionary, or combine two dictionaries together.

>>> fruits_inventory = {"apple": 6, "banana": 2, "cherry": 3}
>>> more_fruits_inventory = {"orange": 4, "kiwi": 1, "melon": 2, "mango": 3}

# fruits_inventory and more_fruits_inventory are unpacked into key-values pairs and combined.
>>> combined_fruits_inventory = {**fruits_inventory, **more_fruits_inventory}

# then the pairs are packed into combined_fruits_inventory
>>> combined_fruits_inventory
{"apple": 6, "banana": 2, "cherry": 3, "orange": 4, "kiwi": 1, "melon": 2, "mango": 3}

Usage of * and ** with functions

Packing with function parameters

When you create a function that accepts an arbitrary number of arguments, you can use *args or **kwargs in the function definition. *args is used to pack an arbitrary number of positional (non-keyworded) arguments and **kwargs is used to pack an arbitrary number of keyword arguments.

Usage of *args:

# This function is defined to take any number of positional arguments

>>> def my_function(*args):
...   print(args)

# Arguments given to the function are packed into a tuple

>>> my_function(1, 2, 3)
(1, 2, 3)

>>> my_function("Hello")
("Hello")

>>> my_function(1, 2, 3, "Hello", "Mars")
(1, 2, 3, "Hello", "Mars")

Usage of **kwargs:

# This function is defined to take any number of keyword arguments

>>> def my_function(**kwargs):
...   print(kwargs)

# Arguments given to the function are packed into a dictionary

>>> my_function(a=1, b=2, c=3)
{"a": 1, "b": 2, "c": 3}

*args and **kwargs can also be used in combination with one another:

>>> def my_function(*args, **kwargs):
...   print(sum(args))
...   for key, value in kwargs.items():
...       print(str(key) + " = " + str(value))

>>> my_function(1, 2, 3, a=1, b=2, c=3)
6
a = 1
b = 2
c = 3

You can also write parameters before *args to allow for specific positional arguments. Individual keyword arguments then have to appear before **kwargs.

>>> def my_function(a, b, *args):
...   print(a)
...   print(b)
...   print(args)

>>> my_function(1, 2, 3, 4, 5)
1
2
(3, 4, 5)

Writing arguments in an incorrect order will result in an error:

>>> def my_function(*args, a, b):
...   print(args)

>>>my_function(1, 2, 3, 4, 5)
Traceback (most recent call last):
  File "c:\something.py", line 3, in <module>
    my_function(1, 2, 3, 4, 5)
TypeError: my_function() missing 2 required keyword-only arguments: 'a' and 'b'

Unpacking into function calls

You can use * to unpack a list/tuple of arguments into a function call. This is very useful for functions that don't accept an iterable:

>>> def my_function(a, b, c):
...   print(c)
...   print(b)
...   print(a)

numbers = [1, 2, 3]
>>> my_function(*numbers)
3
2
1

Using * unpacking with the zip() function is another common use case. Since zip() takes multiple iterables and returns a list of tuples with the values from each iterable grouped:

>>> values = (['x', 'y', 'z'], [1, 2, 3], [True, False, True])
>>> a, *rest = zip(*values)
>>> rest
[('y', 2, False), ('z', 3, True)]