Python Tuple Unpacking: *rest, First and Last Values
Sequence unpacking assigns items from an ordered sequence to names. A starred name collects extra items into a list.
Read Unpacking Values first. You should know list unpacking and why the number of names normally matches the number of values.
The goal is to unpack sequences whose length can vary, using one starred name. Each example runs on its own in the lesson editor; no imports or setup are needed.
Unpack a tuple or string
A tuple holds values in order, like a list, but its items cannot be replaced. You can write one with commas inside parentheses and unpack it like a list:
city, temperature = ("Hobart", 12)
print(city, temperature)
This prints Hobart 12. Unpacking also works on a string, one character per name:
first, second = "hi"
print(first)
print(second)
This prints h, then i. More generally, unpacking accepts an iterable:
an object that supplies items one at a time. Use an ordered sequence when
positions have meaning.
Collect the remaining values
Use ordinary unpacking when you need exactly one item per name.
Use * before one name when the number of remaining items can vary:
first, *rest = (3, 5, 2)
print(first)
print(rest)
This prints 3, then [5, 2]. The starred name always receives a list,
even when the source is a tuple or string.
Without the star, first, rest = (3, 5, 2) raises ValueError because two
ordinary names require exactly two items. With the star, rest collects all
items left after first, rather than receiving just the second item.
Keep the first and last values
A starred name can go in the middle. Names before it receive the first items; names after it receive the last items:
first, *middle, last = [3, 5, 2, 6]
print(first, last)
print(middle)
This prints 3 6, then [5, 2]. Use this when the ends have fixed meanings
but the number of items between them varies.
The starred part can be empty
Only the ordinary names require an item. The starred name can receive an empty list:
first, *middle, last = [3, 6]
print(first, last)
print(middle)
This prints 3 6, then []. Changing the source to [3] raises ValueError:
there are still two ordinary names to fill. Try it to see the error.
Note: Only one name can have a star in the same unpacking pattern. Python cannot decide how to split the extra items between two starred names.
See also: Unpacking Values, Lists.
Your turn
Unpack record into name, a starred scores list, and status.
Print each variable on its own line. Use the same pattern with empty_record,
then print its scores to check the empty case.
record = ("Ada", 8, 6, 9, "ready")
empty_record = ("Bo", "waiting")
# unpack and print record's three parts, then unpack empty_record and print its scores
Expected output
Ada
[8, 6, 9]
ready
[]