Slice assignment allows mismatched length in Python 3.12.6 without raising ValueError

[AA15421]

Bug report

Bug description:

# Add a code block here, if required

Bug Description

#In Python 3.12.6, the following code does not raise an error:

l3 = [2, 3, 4, 5]
l3[::1] = [1, 3]
print(l3) 


Expected:
ValueError: attempt to assign sequence of size 2 to extended slice of size 4 

but showing output [1,3]

### CPython versions tested on:

3.12

### Operating systems tested on:

Windows

[StanFromIreland]

Please see the docs

s[i:j:k] = t
If k is not equal to 1, t must have the same length as the slice it is replacing.

[tim-one]

Yup, it's been that way forever, and won't change. Very useful, in fact! Slice assignment can change the length of the assigned-to target, replacing the specified slice with whatever the iterable on the right-hand produces (whether 0 or a billion items makes no difference).

The exception is if the specified slice has a non-unit step value. Then (and only then) the sizes must match.

[AA15421]

@tim-one and @StanFromIreland — thank you both so much for taking the time to explain this clearly. Your insights helped me understand the actual behavior of slice assignment in Python, especially the difference when using a step value. I really appreciate your time and knowledge!