Python 扩展切片与列表 : is the documentation correct?

Question

文档位于http://docs.python.org/reference/expressions.html#slicings说(摘录一些):

5.3.3. Slicings

A slicing selects a range of items in a sequence object (e.g., a string, tuple or list).

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         ::=  "..."

The semantics for a simple slicing are as follows. The primary must evaluate to a sequence object...

The semantics for an extended slicing are as follows. The primary must evaluate to a mapping object, ...

问题:

为了支持[1,2,3][a:b:c]表示法，语言引用要求列表是映射对象 (stride 仅适用于“扩展切片”，用于映射对象)。那么，语言引用是否已损坏(可能他们只是在引入 What's New in Python2.3: Extended Slices 时忘记更新它？)？

此外，切片显然不仅仅适用于序列对象(参见上面的第一句话)。

还是只有我这样？ ;)

附注

有趣的是，Python 3 文档位于 http://docs.python.org/release/3.1.3/reference/expressions.html#slicings说:

A slicing selects a range of items in a sequence object (e.g., a string, tuple or list)...

["unified" slicing definition, not differentiating between "exteneded" and "simple" here]

The semantics for a slicing are as follows. The primary must evaluate to a mapping object, ...

Answer 1

有关映射的语言已损坏；在所有情况下都应该显示“序列或映射”，以匹配解释器的行为。两种协议(protocol)都可以接受切片对象，并且解释器将在所有情况下进行转换。然而，对于内置类型，只有序列真正支持它:

 >>> a = {'a': 1, 'c': 2}
 >>> a['a':'b']
 Traceback (most recent call last):
   File "<pyshell#32>", line 1, in <module>
     a['a':'b']
 TypeError: unhashable type: 'slice'

请注意，这是字典提示切片不是有效键，而不是解释器提示您无法在映射上执行切片。这是有道理的 - 字典没有隐含的键顺序，因此不清楚切片的含义。