[myhdl-list] Re: range object

[Beni C. <cb...@us...>]

Haitao Zhang wrote:
> On Thu, 03 Mar 2005 11:49:08 +0100, Jan Decaluwe <ja...@ja...> wrote:
> 
>>Haitao Zhang wrote:
>>
>>> [...] However in Python the native slicing is limited to integer
>>> arguments (?), so one has to define two named constants instead of
>>> passing an indexing object.
>>
Before Python 2.3, builtin sequences didn't support slice objects (which also 
means they didn't support slicing with a step - ``a[start:stop:step]``).   In 
2.3 and later it works.  But behavior of builtin sequence types doesn't say 
anything about the behavior of `intbv`.

>>This is not correct. Consider:
>>
>>>>>from myhdl import intbv
>>>>>a = intbv(0xffff)
>>>>>CTRL_FIELD = slice(4, 2)
>>>>>type(CTRL_FIELD)
>>
>><type 'slice'>
>>
>>>>>a[CTRL_FIELD]
>>
>>intbv(3L)
>>
>>>>>a[CTRL_FIELD] = 0
>>>>>hex(a)
>>
>>'0xFFF3L'
>>
This works in MyHDL 0.4.1 (0.4 used ``__getslice__`` methods).  Incidentally, 
0.4.1 requires Python 2.3 for several reasons.

>>The slice object is not often used explicitly in Python code but
>>probably deserves more attention. (Warning: toVerilog doesn't
>>support his - as always, patches are welcome!).
>>
Also note that before Python 2.3, `slice` was a constuctor function distinct 
from the <type 'slice'> object (accessible as `types.SliceType`) which.  In 
Python 2.3 they were merged so now `slice` is the type and is callable (like 
`int` and other builtin types).

> 
> Thanks for the tip! This is exactly what I want for now. (Matlab
> behaviour is probably an over kill unless one has no-contigous field,
> which for hardware designer only happens under special circumstances).
> I will see what I can do about toVerilog when I get that far.
> 
> Haitao
> 
>>>In Matlab one can index any vector with another vector. I don't see
>>>why this shouldn't be doable in Python as well. Maybe just no need for
>>>it, until the hardware designers come along? Is the simplest way to
>>>overload __getslice__ to accept a tuple (defining range)?
>>
You can have the matlab behavior with Numeric arrays.  `intbv` could easily be 
extended to support it if there is a real use for it.

BTW, `intbv` presently silently ignores the step of the slice.  To prevent 
bugs going unnoticed, it should raise an exception if ``key.step is not None`` 
(or properly support it).