5.3.3 Slicings
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:
-
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.
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