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❯ py t.py
Traceback (most recent call last):
  File "/Users/bartosz.slawecki/Python/cpython/t.py", line 10, in <module>
    l[::2] = MyGen()
    ~^^^^^
ValueError: attempt to assign sequence of size 3 to extended slice of size 2

The error message from this approach leaks internal implementation details. MyGen() (from the PR) is not a sequence of size 3, it's a generator.

@johnslavik updated based off your feedback. Please check again.

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How does bytearray and array.array handle those cases? likewise what about assigning with itself? does it change the current behavior? a[::2] = a should not be behave differently or if so we should test it

For me on 3.14.3 and on the latest head of this branch this scenario works the same:

In [1]: a = list(range(10))

In [2]: a
Out[2]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

In [3]: a[::2] = a
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[3], line 1
----> 1 a[::2] = a

ValueError: attempt to assign sequence of size 10 to extended slice of size 5

In [4]:

tbd on bytearray and arrray.array

@picnixz I think i addressed your points. Thanks!