On 1/26/2011 9:20 AM Gerald Britton said...
I'm looking at extended slicing and wondering when and how to use slice lists:
I think the use of the term slice_list below is simply as the content
between the encompassing brackets, eg in mylist[1:2:3] slice_list refers
to 1:2:3. So, you don't actually 'use' a slice_list - that's what the
passed in value is called.
So, here are some example of how slicing is used:
>>> a = range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[:3] # the first three
[0, 1, 2]
>>> a[-3:] # the last three
[7, 8, 9]
>>> a[3:-3] # start and stop
[3, 4, 5, 6]
>>> a[::2] # take every other
[0, 2, 4, 6, 8]
>>> a[::-2] # take every other from the end
[9, 7, 5, 3, 1]
>>> a[3:-3:2] # take every other within start stop
[3, 5]
>>> a[3:-3:-2] # I'm not sure exactly why I didn't get something here
[]
>>> a[-3:3:-2] # but apparently the polarity of stride within start
stop matters
[7, 5]
>>>
HTH
Emile
slicing ::= simple_slicing | extended_slicing
simple_slicing ::= primary "[" short_slice "]"
extended_slicing ::= primary "[" slice_list "]"
slice_list ::= slice_item ("," slice_item)* [","]
slice_item ::= expression | proper_slice | ellipsis
proper_slice ::= short_slice | long_slice
short_slice ::= [lower_bound] ":" [upper_bound]
long_slice ::= short_slice ":" [stride]
lower_bound ::= expression
upper_bound ::= expression
stride ::= expression
ellipsis
The semantics for an extended slicing are as follows. The primary must
evaluate to a mapping object, and it is indexed with a key that is
constructed from the slice list, as follows. If the slice list
contains at least one comma, the key is a tuple containing the
conversion of the slice items; otherwise, the conversion of the lone
slice item is the key. The conversion of a slice item that is an
expression is that expression. The conversion of an ellipsis slice
item is the built-in Ellipsis object. The conversion of a proper slice
is a slice object (see section The standard type hierarchy) whose
start, stop and step attributes are the values of the expressions
given as lower bound, upper bound and stride, respectively,
substituting None for missing expressions.
I'd thought that I could do this:
l = [1,2,3,4,5]
l[0:1, 3:4]
Traceback (most recent call last):
File "<stdin>", line 1, in<module>
TypeError: list indices must be integers, not tuple
but that clearly doesn't work! So, when and how can one use slice lists?
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