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Diffstat (limited to 'Doc/reference/expressions.rst')
| -rw-r--r-- | Doc/reference/expressions.rst | 43 |
1 files changed, 13 insertions, 30 deletions
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst index 0994ea8..a5bfd43 100644 --- a/Doc/reference/expressions.rst +++ b/Doc/reference/expressions.rst @@ -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: |