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Update ref docs on slicing.

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Thomas Wouters 2007-09-04 09:03:59 +00:00
parent aeaa546d7e
commit 53de1902e7
1 changed files with 13 additions and 30 deletions
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Doc/reference/expressions.rst
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@ -513,14 +513,10 @@ or list). Slicings may be used as expressions or as targets in assignment or
:keyword:`del` statements. The syntax for a slicing:
.. productionlist::
slicing: `simple_slicing` | `extended_slicing`
simple_slicing: `primary` "[" `short_slice` "]"
extended_slicing: `primary` "[" `slice_list` "]"
slicing: `primary` "[" `slice_list` "]"
slice_list: `slice_item` ("," `slice_item`)* [","]
slice_item: `expression` | `proper_slice`
proper_slice: `short_slice` | `long_slice`
short_slice: [`lower_bound`] ":" [`upper_bound`]
long_slice: `short_slice` ":" [`stride`]
proper_slice: [`lower_bound`] ":" [`upper_bound`] [ ":" [`stride`] ]
lower_bound: `expression`
upper_bound: `expression`
stride: `expression`
@ -530,36 +526,23 @@ expression list also looks like a slice list, so any subscription can be
interpreted as a slicing. Rather than further complicating the syntax, this is
disambiguated by defining that in this case the interpretation as a subscription
takes priority over the interpretation as a slicing (this is the case if the
slice list contains no proper slice). Similarly, when the slice list has
exactly one short slice and no trailing comma, the interpretation as a simple
slicing takes priority over that as an extended slicing.
.. XXX is the next paragraph stil correct?
The semantics for a simple slicing are as follows. The primary must evaluate to
a sequence object. The lower and upper bound expressions, if present, must
evaluate to plain integers; defaults are zero and the ``sys.maxint``,
respectively. If either bound is negative, the sequence's length is added to
it. The slicing now selects all items with index *k* such that ``i <= k < j``
where *i* and *j* are the specified lower and upper bounds. This may be an
empty sequence. It is not an error if *i* or *j* lie outside the range of valid
indexes (such items don't exist so they aren't selected).
slice list contains no proper slice).
.. index::
single: start (slice object attribute)
single: stop (slice object attribute)
single: step (slice object attribute)
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 a proper slice is a
slice object (see section :ref:`types`) whose :attr:`start`, :attr:`stop` and
:attr:`step` attributes are the values of the expressions given as lower bound,
upper bound and stride, respectively, substituting ``None`` for missing
expressions.
The semantics for a 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 a proper
slice is a slice object (see section :ref:`types`) whose :attr:`start`,
:attr:`stop` and :attr:`step` attributes are the values of the expressions
given as lower bound, upper bound and stride, respectively, substituting
``None`` for missing expressions.
.. _calls: