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Slices
A slice selects a range of elements in a sequence object (e.g., a string, tuple, or list). Slices can be used as expressions or as targets in assignments or statements. The syntax for a slice is as follows:
slicing ::= primary "[" slice_list "]"
slice_list ::= slice_item ("," slice_item)* [","]
slice_item ::= expression | proper_slice
proper_slice ::= [lower_bound] ":" [upper_bound] [ ":" [stride] ]
lower_bound ::= expression
upper_bound ::= expression
stride ::= expression
There is ambiguity in the formal syntax here: anything that looks like a list expression also looks like a list slice, so any subscription might be interpreted as a slice. Instead of complicating the syntax further, this is disambiguated by defining that in this case, the interpretation as a subscription takes precedence over the interpretation as a slice (this is the case if the list slice does not contain a proper slice).
The semantics for a slice are as follows. The primary is indexed (using the same __getitem__() method as in a normal subscription) with a key constructed from the slice list, as follows. If the slice list contains at least one comma, the key is a tuple that contains the conversion of the slice elements; otherwise, the conversion of the single slice element is the key. The conversion of a slice element that is an expression is that expression. The conversion of a proper slice is a slice object whose start, stop, and step attributes are the values of the expressions given as the lower bound, upper bound, and step, respectively, substituting None for missing expressions.