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Method(s) called by square brackets, slice objects

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  Method(s) called by square brackets, slice objects John Ladasky <john_ladasky@sbcglobal.net> - 2014-04-09 13:24 -0700
    Re: Method(s) called by square brackets, slice objects Ethan Furman <ethan@stoneleaf.us> - 2014-04-09 13:34 -0700
    Re: Method(s) called by square brackets, slice objects Steven D'Aprano <steve@pearwood.info> - 2014-04-10 02:52 +0000
    Re: Method(s) called by square brackets, slice objects John Ladasky <john_ladasky@sbcglobal.net> - 2014-04-10 11:52 -0700
      Re: Method(s) called by square brackets, slice objects Terry Reedy <tjreedy@udel.edu> - 2014-04-10 18:20 -0400

#69979 — Method(s) called by square brackets, slice objects

I would like to build a multi-dimensional array that allows numpy-style indexing and, ideally, uses Python's familiar square-bracket and slice notations.

For example, if I declare a two-dimensional array object, x, then x[4,7] retrieves the element located at the 4th row and the 7th column.  If I ask for x[3:6,1:3], I get a 3 x 2 array object consisting of the intersection of the 3rd-5th rows, and the 1st-2nd columns, of x.

In this case I'm not allowed to use numpy, I have to restrict myself to the standard library.  I thought that I might achieve the desired behavior by defining an object with specific __getitem__ and/or __getslice__ methods.  However, the documentation of these methods that I am reading suggests that the arguments are pre-parsed into certain formats which may not allow me to do things numpy's way.  Is this true?

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#69983

On 04/09/2014 01:24 PM, John Ladasky wrote:
>
> I would like to build a multi-dimensional array that allows numpy-style
>  indexing and, ideally, uses Python's familiar square-bracket and slice
>  notations.
>
> For example, if I declare a two-dimensional array object, x, then x[4,7]
>  retrieves the element located at the 4th row and the 7th column.  If I
>  ask for x[3:6,1:3], I get a 3 x 2 array object consisting of the inter-
>section of the 3rd-5th rows, and the 1st-2nd columns, of x.
>
> In this case I'm not allowed to use numpy, I have to restrict myself to
>  the standard library.  I thought that I might achieve the desired behavior
>  by defining an object with specific __getitem__ and/or __getslice__ methods.
> However, the documentation of these methods that I am reading suggests that
>  the arguments are pre-parsed into certain formats which may not allow me to
>  do things numpy's way.  Is this true?

Nope.  Whatever you put between the square brackets is what gets passed into __getitem__; the only caveat is that 
anything with : will be turned into a slice:

Python 2.7.3 (default, Sep 26 2012, 21:51:14)
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
--> class GetIndex(object):
...   def __getitem__(self, thing):
...     print thing
...     return None
...
--> test = GetIndex()

--> test[1]
1

--> test [1,2]
(1, 2)

--> test[1:3, 4:5]
(slice(1, 3, None), slice(4, 5, None))

--> test[range(3)]
[0, 1, 2]

--
~Ethan~

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#70001

On Wed, 09 Apr 2014 13:24:32 -0700, John Ladasky wrote:

> I would like to build a multi-dimensional array that allows numpy-style
> indexing and, ideally, uses Python's familiar square-bracket and slice
> notations.
> 
> For example, if I declare a two-dimensional array object, x, then x[4,7]
> retrieves the element located at the 4th row and the 7th column.  If I
> ask for x[3:6,1:3], I get a 3 x 2 array object consisting of the
> intersection of the 3rd-5th rows, and the 1st-2nd columns, of x.
> 
> In this case I'm not allowed to use numpy, I have to restrict myself to
> the standard library.  I thought that I might achieve the desired
> behavior by defining an object with specific __getitem__ and/or
> __getslice__ methods.  

Use __getitem__, __getslice__ is deprecated in Python 2 and gone in 
Python 3.

https://docs.python.org/2/reference/datamodel.html#object.__getslice__


> However, the documentation of these methods that
> I am reading suggests that the arguments are pre-parsed into certain
> formats which may not allow me to do things numpy's way.  Is this true?

Why don't you try it in the interactive interpreter and see?


py> class Test(object):
...     def __getitem__(self, thing):
...             print thing
... 
py> obj = Test()
py> obj[1]
1
py> obj[1:2]
slice(1, 2, None)
py> obj[1:2:3]
slice(1, 2, 3)
py> obj[1,5:2:3]
(1, slice(5, 2, 3))
py> obj[1:2:3,4:5:6]
(slice(1, 2, 3), slice(4, 5, 6))
py> obj[1,2,3]
(1, 2, 3)
py> obj[1,2,"spam"]
(1, 2, 'spam')
py> obj[1,2,"spam":"eggs",3]
(1, 2, slice('spam', 'eggs', None), 3)




-- 
Steven

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#70056

Thanks to both Ethan and Steven for their replies.

Steven: I was trying to use the interpreter and wasn't getting results that I understood -- because I didn't know that __getslice__ was simply gone in Python 3.  I implemented a __getslice__ method in my subclass that never got called.

Ethan: I saw that slice objects were being sent to __getitem__, but that confused me as I thought that its purpose, as implied by the method name, was to return a SINGLE item.

OK, I think that I've got all that sorted out!

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#70066

On 4/10/2014 2:52 PM, John Ladasky wrote:
> Thanks to both Ethan and Steven for their replies.
>
> Steven: I was trying to use the interpreter and wasn't getting results that I understood -- because I didn't know that __getslice__ was simply gone in Python 3.  I implemented a __getslice__ method in my subclass that never got called.
>
> Ethan: I saw that slice objects were being sent to __getitem__, but that confused me as I thought that its purpose, as implied by the method name, was to return a SINGLE item.

A slice is a single sequence object. Sequences can result from any of 
index lookup, key lookup, or slicings.

The backstory is that slicing originally supported only start and stop 
positions. The 3 __xyzslice__ had separate start and stop parameters 
instead of the 1 index/key parameter of __xyzitem__. Strides and the 
slice class were introduced for the benefit of numerical python. When 
they were 'mainstreamed' into regular python, the __xyzslice__ methods 
were deprecated (in 2.0) as a single slice object can just as well be 
passed to __xyzitem__.

-- 
Terry Jan Reedy

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