问题
Green Tree Snakes gives an example of using ExtSlice
:
>>> parseprint("l[1:2, 3]")
Module(body=[
Expr(value=Subscript(value=Name(id='l', ctx=Load()), slice=ExtSlice(dims=[
Slice(lower=Num(n=1), upper=Num(n=2), step=None),
Index(value=Num(n=3)),
]), ctx=Load())),
])
However this syntax won't work in interactive python shell:
>>> foo = range(10)
>>> foo[1:2,3]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: list indices must be integers, not tuple
Anyone got an idea how to use this feature?
Related discussion:
- What syntax is represented by an ExtSlice node in Python's AST?
回答1:
The syntax works fine in the shell, it is just that list
objects don't support extended slicing. What you tried raised a TypeError
, not a SyntaxError
.
Many Numpy array types do; that project was instrumental in driving the extended slicing syntax. Numpy arrays use extended slicing to address the different dimensions of multi-dimensional arrays. See the Numpy Indexing chapter for details on how they use the syntax.
Extended slicing is explicitly documented in the Subscription section, the AST nodes encode the extended_slicing
term:
extended_slicing ::= primary "[" slice_list "]"
slice_list ::= slice_item ("," slice_item)* [","]
slice_item ::= expression | proper_slice | ellipsis
proper_slice ::= short_slice | long_slice
There are no types in the Python standard library itself that make use of extended slicing, however.
You can easily build your own class to accept an extended slice; just expect to handle a tuple in your object.__getitem__() method implementation:
>>> class Foo(object):
... def __getitem__(self, item):
... return item
...
>>> foo = Foo()
>>> foo[1, 2:3]
(1, slice(2, 3, None))
Each element of the slice_list
becomes an object in a tuple, with :
-separated slice indices passed in as slice()
instances.
来源:https://stackoverflow.com/questions/35815397/when-to-use-extslice-node-in-pythons-ast