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Python Reference Manual |  |
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A slicing selects a range of items in a sequence object (e.g., a string, tuple or list). Slicings may be used as expressions or as targets in assignment or del statements. The syntax for a slicing:
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: "..."
There is ambiguity in the formal syntax here: anything that looks like an 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 nor ellipses). 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.
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).
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 3.2) 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.