[Python-ideas] slice.literal notation

[Joseph Jevnik]

I was told in the thread that it might be a good idea to bring this up on
python discussions. Here is a link to the proposed patch and some existing
comments: http://bugs.python.org/issue24379

I often find that when working with pandas and numpy I want to store slice
objects in variables to pass around and re-use; however, the syntax for
constructing a slice literal outside of an indexer is very different from
the syntax used inside of a subscript. This patch proposes the following
change:

    slice.literal

This would be a singleton instance of a class that looks like:

class sliceliteral(object):
    def __getitem__(self, key):
        return key


The basic idea is to provide an alternative constructor to 'slice' that
uses the subscript syntax. This allows people to write more understandable
code.

Consider the following examples:

reverse = slice(None, None, -1)
reverse = slice.literal[::-1]

all_rows_first_col = slice(None), slice(0)
all_rows_first_col = slice.literal[:, 0]

first_row_all_cols_but_last = slice(0), slice(None, -1)
first_row_all_cols_but_last = slice.literal[0, :-1]


Again, this is not intended to make the code shorter, instead, it is
designed to make it more clear what the slice object your are constructing
looks like.

Another feature of the new `literal` object is that it is not limited to
just the creation of `slice` instances; instead, it is designed to mix
slices and other types together. For example:

>>> slice.literal[0]
0
>>> slice.literal[0, 1]
(0, 1)
>>> slice.literal[0, 1:]
(0, slice(1, None, None)
>>> slice.literal[:, ..., ::-1]
(slice(None, None, None), Ellipsis, slice(None, None, -1)

These examples show that sometimes the subscript notation is much more
clear that the non-subscript notation.
I believe that while this is trivial, it is very convinient to have on the
slice type itself so that it is quickly available. This also prevents
everyone from rolling their own version that is accesible in different ways
(think Py_RETURN_NONE).
Another reason that chose this aproach is that it requires no change to the
syntax to support.

There is a second change proposed here and that is to 'slice.__repr__'.
This change makes the repr of a slice object match the new literal syntax
to make it easier to read.

>>> slice.literal[:]
slice.literal[:]
>>> slice.literal[1:]
slice.literal[1:]
>>> slice.literal[1:-1]
slice.literal[1:-1]
>>> slice.literal[:-1]
slice.literal[:-1]
>>> slice.literal[::-1]
slice.literal[::-1]

This change actually affects old behaviour so I am going to upload it as a
seperate patch. I understand that the change to repr much be less desirable
than the addition of 'slice.literal'
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