Could anyone explain this unexpected behaviour of the interface?

edit

The display of lists seems to get extra newline characters when they go over a certain number of elements.

But you can get a unified behaviour by using print, str or repr.

sage: a = list(range(22))
sage: b = list(range(23))

sage: print(a)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]
sage: print(b)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]

sage: repr(a)
'[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]'
sage: repr(b)
'[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]'

sage: str(a)
'[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]'
sage: str(b)
'[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]'

sage: a.__repr__()
'[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]'
sage: b.__repr__()
'[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]'

sage: a.__str__()
'[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]'
sage: b.__str__()
'[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]'

Edit (after reading @Sébastien's answer and the stackoverflow answer mentioned in it). Note that what makes the pretty printing of the list get extra newline characters is not the length of the list, but whether its string representation goes over 79 characters.

sage: c = list(range(12345678901234, 12345678901239))
sage: len(c)
5
sage: len(str(c))
80
sage: c
[12345678901234,
 12345678901235,
 12345678901236,
 12345678901237,
 12345678901238]
sage: print(c)
[12345678901234, 12345678901235, 12345678901236, 12345678901237, 12345678901238]

Note that this was asked before on Ask Sage:

• Ask Sage question #37477: long list goes vertical

and there is a Sage Trac ticket about this

• Sage Trac ticket #22968: Sage does not handle %pprint correctly