[Python-Dev] Pre-PEP: Allow Empty Subscript List Without Parentheses

Sat Jun 10 05:41:00 CEST 2006

Alex Martelli wrote:
> On Jun 9, 2006, at 4:55 PM, Greg Ewing wrote:
> ...
>>>Think about how you get from an N dimensional array to
>>an N-1 dimensional array: you index it, e.g.
>>>> A2 = [[1, 2], [3, 4]] # a 2D array
>>>> A1 = A2[1] # a 1D array
>>>> A0 = A1[1] # a 0D array???
>>>> print A0
>>>>What do you think this will print?
>>> Don't confuse arrays with lists...:
>> >>> A2 = Numeric.array([[1, 2], [3, 4]], Numeric.Float32)
> >>> A1 = A2[1]
> >>> A0 = A1[1]
> >>> type(A0)
> <type 'array'>
> >>>
>> It doesn't work the same if you specify Numeric.Float64 instead -- an 
> ancient wart of Numeric, of course. Still, Numeric and its 
> descendants are "the" way in Python to get multi-dimensional arrays, 
> since the stdlib's array module only supports one-dimensional ones, 
> and lists are not arrays.

Note that this wart has been pretty much killed in numpy by supplying a 
full complement of scalar types:
 >>> import numpy
 >>> A2 = numpy.array([[1,2], [3,4]], numpy.float32)
 >>> A1 = A2[1]
 >>> A0 = A1[1]
 >>> A0
4.0
 >>> type(A0)
<type 'float32scalar'>
The same excercise with float64 will give you a float64 scalar. The 
behaviour in this area is overall much more consistent now. You can 
still get a 0-D array by doing array(4.0) and possibly a few other ways, 
but there much less common. These scalar objects are immutable, but have 
all (or at least most) of the the array methods and attributes. For example:
 >>> A0.dtype
dtype('<f4')
dtype is more or less equivalent to Numeric's typecode().
-tim