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1)
4.6.4 Mutable Sequence Types
| s[i:j] = t | slice of s from i to j is replaced |
| | by the contents of the iterable t |
>>> lst = list('abc')
>>> barr = bytearray(b'abc')
>>> lst[:1] = 4
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: can only assign an iterable
>>> barr[:1] = 4
>>> barr
bytearray(b'\x00\x00\x00\x00bc')
>>>
there is no info about this feature in the documentation
2)
4.6.4 Mutable Sequence Types
| s.extend(x) | same as s[len(s):len(s)] = x |
>>> lst = list('abc')
>>> barr = bytearray(b'abc')
>>> lst.extend(4)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable
>>> barr.extend(4)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable
>>> lst[len(lst):len(lst)] = 4
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: can only assign an iterable
>>> barr[len(barr):len(barr)] = 4
>>> barr
bytearray(b'abc\x00\x00\x00\x00')
>>>
barr.extend(x) != barr[len(barr):len(barr)] = x

To my eye, this looks like a bytearray API bug in the bytearray implementation. ISTM, the rhs of a slice assignment needs to be restricted to iterable inputs.
I'm marking this as low priority because the documented behaviors (i.e. normal slice assignment from an iterable) appear to be working fine. So, there is no rush to take away the gratuitous API extension that accepts an integer input where a slice is usually expected. The extension is not entirely harmless though -- we do lose the error-checking for the common mistake of writing s[i:j]=x instead of s[i:j]=[x].

Yes, this is a bug in bytearray and should be fixed.

This was fixed in the latest versions.
>>> b = bytearray(b'457')
>>> b[:1] = 4
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: can assign only bytes, buffers, or iterables of ints in range(0, 256)
>>>