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❯ py t.pyTraceback (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.

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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]: aOut[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] = aValueError: attempt to assign sequence of size 10 to extended slice of size 5In [4]:

tbd on bytearray and arrray.array

@picnixz I think i addressed your points. Thanks!