How to Unpack a Tuple or List in Python
In Python, you can assign elements of a tuple or list to multiple variables. This process is known as sequence unpacking.
See the following article for information on unpacking lists, tuples, and dictionaries (dict) as arguments in a function using * (asterisk).
Basics of tuple and list unpacking
You can assign elements from a tuple or list to multiple variables by placing comma-separated variables on the left-hand side.
t = (0, 1, 2)
a, b, c = t
print(a)
print(b)
print(c)
# 0
# 1
# 2
l = [0, 1, 2]
a, b, c = l
print(a)
print(b)
print(c)
# 0
# 1
# 2
The following examples use tuples, but the same approach applies to lists.
Since parentheses around tuples can be omitted, you can assign multiple values to variables in a single line, as shown below.
a, b = 0, 1
print(a)
print(b)
# 0
# 1
A ValueError occurs if the number of variables does not match the number of elements.
# a, b = t
# ValueError: too many values to unpack (expected 2)
# a, b, c, d = t
# ValueError: not enough values to unpack (expected 4, got 3)
If there are fewer variables than elements, you can prefix one variable with an asterisk * to collect the remaining elements in a list. This will be explained in more detail later.
Unpack a nested tuple or list
You can also unpack a nested tuple or list. To unpack inner elements, use parentheses () or brackets [] to mirror the nested structure.
t = (0, 1, (2, 3, 4))
a, b, c = t
print(a)
print(b)
print(c)
# 0
# 1
# (2, 3, 4)
print(type(c))
# <class 'tuple'>
a, b, (c, d, e) = t
print(a)
print(b)
print(c)
print(d)
print(e)
# 0
# 1
# 2
# 3
# 4
Unpack with _ (underscore)
In Python, by convention, _ (underscore) is often used to indicate values that are intentionally unused. It has no special syntactic meaning and is just a regular variable name.
t = (0, 1, 2)
a, b, _ = t
print(a)
print(b)
print(_)
# 0
# 1
# 2
Unpack with * (asterisk)
When unpacking, you can prefix one variable with an asterisk * to collect any remaining elements into a list.
This feature is available in Python 3 and is not supported in Python 2.
Individual elements are assigned to regular variables before and after the *-prefixed variable, while all remaining elements are collected into the *-prefixed variable as a list.
t = (0, 1, 2, 3, 4)
a, b, *c = t
print(a)
print(b)
print(c)
# 0
# 1
# [2, 3, 4]
print(type(c))
# <class 'list'>
a, *b, c = t
print(a)
print(b)
print(c)
# 0
# [1, 2, 3]
# 4
*a, b, c = t
print(a)
print(b)
print(c)
# [0, 1, 2]
# 3
# 4
For example, to assign only the first two elements of a tuple or list to variables, you can use the *-prefixed underscore (*_) to ignore the remaining elements.
a, b, *_ = t
print(a)
print(b)
print(_)
# 0
# 1
# [2, 3, 4]
The same operation can be written as follows.
a, b = t[0], t[1]
print(a)
print(b)
# 0
# 1
Only one variable can be prefixed with *. Using more than one causes ambiguity and results in a SyntaxError.
# *a, b, *c = t
# SyntaxError: two starred expressions in assignment
Even if only one element is collected by a *-prefixed variable, it is still assigned as a list.
t = (0, 1, 2)
a, b, *c = t
print(a)
print(b)
print(c)
# 0
# 1
# [2]
print(type(c))
# <class 'list'>
If there are no extra elements, an empty list is assigned.
a, b, c, *d = t
print(a)
print(b)
print(c)
print(d)
# 0
# 1
# 2
# []