Odd slicing behavior?
dave.opstad at monotypeimaging.com
Wed Jun 22 22:01:24 CEST 2005
Take a look at this snippet:
>>> class L(list):
... def __init__(self, v):
... super(L, self).__init__(v)
... def __setitem__(self, key, value):
... print key.indices(len(self))
>>> v = L(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> v[::] = 
(0, 10, 1)
>>> v[0:10:1] = 
(0, 10, 1)
So both assignments result in slices with exactly the same start, stop
and step values: 0, 10 and 1. Also, the __setitem__ method is being
called, not the older __setslice__ method; we can tell this because it
hit the print statement.
However, actually trying to make a slice assignment on a regular list in
these two ways behaves differently:
>>> v2 = range(10)
>>> v2[0:10:1] = 
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ValueError: attempt to assign sequence of size 1 to extended slice of
>>> v2[::] = 
So given the equality of their slice representations, why do the v2[::]
and v2[0:10:1] assignments behave differently? My reading of section
5.3.3 of the manual suggests that these should behave the same.
If I'm just not seeing something obvious, please let me know!
More information about the Python-list