What does for x, *y in list mean in python

Question

I recently came across a piece of python code that looked like this

groups = {}
    for d, *v in dishes:
        for x in v:
            groups.setdefault(x, []).append(d)

dishes represents a 2d array. What does the 1st for loop statement mean? What is *v? What does the asterisk before v indicate? What other situations is an asterisk before a variable used?

Answer 1

It's essentially a combination of tuple/list unpacking and *args iterable unpacking. Each iterable is getting unpacked on each iteration of the for loop.

First let's look at a simple tuple/list unpacking:

>>> x, y = (1, 2)
>>> x
1
>>> y
2

# And now in the context of a loop:
>>> for x, y in [(1, 2), (3, 4)]:
>>>     print(f'x={x}, y={y}')
"x=1, y=2"
"x=3, y=4"

Now consider the following (and imagine the same concept within the loop as shown above):

>>> x, y = (1, 2, 3)
ValueError: too many values to unpack (expected 2)

>>> x, *y = 1, 2, 3
>>> x
1 
>>> y 
[2, 3]

Note how * allows y to consume all remaining arguments.

This is similar to how you would use * in a function - it allows an unspecified number of arguments and it consumes them all. You can see more examples of (*args) usage here.

>>> def foo(x, *args):
>>>     print(x)
>>>     print(args)

>>>foo(1, 2, 3, 4)
1
[2, 3, 4]

As for practical examples, here is a quick one:

>>> names = ("Jack", "Johnson", "Senior")
>>> fist_name, *surnames =  names
>>> print(surnames)
["Johnson", "Senior"]

Answer 2

Basically * indicates n number of countless elements.

Example :

x=1,2,4,6,9,8
print(type(x))
print(x)

Output:

<class 'tuple'>
(1, 2, 4, 6, 9, 8)

y,*x=1,2,4,6,9,8
print(type(x))
print(x)

Output:

<class 'list'>
[2, 4, 6, 9, 8]