[Numpy-discussion] PEP 209: Multi-dimensional Arrays
Rob W. W. Hooft
rob at hooft.net
Wed Feb 14 02:42:36 EST 2001
Some random PEP talk.
>>>>> "PB" == Paul Barrett <Barrett at stsci.edu> writes:
PB> 2. Additional array types
PB> Numeric 1 has 11 defined types: char, ubyte, sbyte, short, int,
PB> long, float, double, cfloat, cdouble, and object. There are no
PB> ushort, uint, or ulong types, nor are there more complex types
PB> such as a bit type which is of use to some fields of science and
PB> possibly for implementing masked-arrays.
True: I would have had a much easier life with a ushort type.
PB> Its relation to the other types is defined when the C-extension
PB> module for that type is imported. The corresponding Python code
>> Int32.astype[Real64] = Real64
I understand this is to be done by the Int32 C extension module.
But how does it know about Real64?
PB> .name: e.g. "Int32", "Float64", etc.
PB> .typecode: e.g. 'i', 'f', etc.
PB> (for backward compatibility)
.typecode() is a method now.
PB> .size (in bytes): e.g. 4, 8, etc.
>> add.register('add', (Int32, Int32, Int32), cfunc-add)
Typo: cfunc-add is an expression, not an identifier.
An implementation of a (Int32, Float32, Float32) add is possible and
desirable as mentioned earlier in the document. Which C module is
going to declare such a combination?
PB> asstring(): create string from array
Not "tostring" like now?
PB> 4. ArrayView
PB> This class is similar to the Array class except that the reshape
PB> and flat methods will raise exceptions, since non-contiguous
PB> arrays cannot be reshaped or flattened using just pointer and
PB> step-size information.
This was completely unclear to me until here. I must say I find this a
strange way of handling things. I haven't looked into implementation
details, but wouldn't it feel more natural if an Array would just be
the "data", and an ArrayView would contain the dimensions and
strides. Completely separated. One would always need a pair, but more
than one ArrayView could use the same Array.
PB> a. _ufunc:
PB> 1. Does slicing syntax default to copy or view behavior?
Numeric 1 uses slicing for view, and a method for copy. "Feeling"
compatible with core python would require copy on rhs, and view on lhs
of an assignment. Is that distinction possible?
If copy is the default for slicing, how would one make a view?
PB> 2. Does item syntax default to copy or view behavior?
PB> Yet, c[i] can be considered just a shorthand for c[i,:] which
PB> would imply copy behavior assuming slicing syntax returns a copy.
If you reason that way, then c is just a shorthand for c[...] too.
PB> 3. How is scalar coercion implemented?
PB> Python has fewer numeric types than Numeric which can cause
PB> coercion problems. For example when multiplying a Python scalar
PB> of type float and a Numeric array of type float, the Numeric array
PB> is converted to a double, since the Python float type is actually
PB> a double. This is often not the desired behavior, since the
PB> Numeric array will be doubled in size which is likely to be
PB> annoying, particularly for very large arrays.
Sure. That is handled reasonably well by the current Numeric 1.
To extend this, I'd like to comment that I have never really understood
the philosophy of taking the largest type for coercion in all languages.
Being a scientist, I have learned that when you multiply a very accurate
number with a very approximate number, your result is going to be very
approximate, not very accurate! It would thus be more logical to have
Float32*Float64 return a Float32!
PB> In a future version of Python, the behavior of integer division
PB> will change. The operands will be converted to floats, so the
PB> result will be a float. If we implement the proposed scalar
PB> coercion rules where arrays have precedence over Python scalars,
PB> then dividing an array by an integer will return an integer array
PB> and will not be consistent with a future version of Python which
PB> would return an array of type double. Scientific programmers are
PB> familiar with the distinction between integer and float-point
PB> division, so should Numeric 2 continue with this behavior?
Numeric 2 should be as compatible as reasonably possible with core python.
But my question is: how would we do integer division of arrays? A ufunc
for which no operator shortcut exists?
PB> 7. How are numerical errors handled (IEEE floating-point errors in
I am developing my code on Linux and IRIX. I have seen that where
Numeric code on Linux runs fine, the same code on IRIX may "core dump"
on a FPE (e.g. arctan2(0,0)). That difference should be avoided.
PB> a. Print a message of the most severe error, leaving it to
PB> the user to locate the errors.
What is the most severe error?
PB> c. Minimall UFunc class:
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