[Python-Dev] Array Enhancements (original) (raw)

Scott Gilbert xscottg@yahoo.com
Fri, 5 Apr 2002 12:30:29 -0800 (PST)

I noticed that in the newsgroup that someone suggested adding the new boolean type to the array module. I was pleased to see that an appropriate patch adding this feature would be accepted. I would find this useful, and since I'm on the verge of writing (yet another) array module, I'm curious if any of the following would be accepted enhancements.

Translation: Since none of the existing array modules that I'm aware of (array, Numeric, Numarray) meet all of my needs, I'd be happy to submit a patch for arraymodule.c if I can make enough changes to meet all of my needs. Otherwise, I'll have to write an internal 'xarray' module for my company to get stuff done with.

I doubt this warrants a PEP, so I humbly propose:

*** Adding a new typecode 't' to implement a bit array. Implementation would be an array of bytes, but it would be 1 bit per element. 't' is for 'truth value' since 'b' for 'boolean' was already taken by 'b' for 'byte'. Accepted values would be 0, 1, True, False. Looking at the arraymodule.c, this seems like the most work out of any of theses suggestions because all of the other types are easily addressable. Slicing operations are going to be tricky to do quickly and correctly. It was already acknowledged by GvR that this would be desirable.

*** Adding pickle support. This is a no brainer I think. Currently we have to call tostring() if we want to serialize the data.

*** Changing the 'l' and 'L' typecodes to use LONG_LONG. There isn't a consistent way to get an 8 byte integer out of the array module. About half of our machines at work are Alphas where long is 8 bytes, and the rest are Sparcs and x86's where long is 4 bytes.

*** I'd really like it if the array module gave a "hard commitment" to the sizes of the elements instead of just sayings "at least n bytes". None of the other array modules do this either. I know Python has been ported to a bazillion platforms, but what are the exceptions to 'char' being 8 bits, 'short' being a 16 bits, 'int' being 32 bits, 'long long' or __int64 being 64 bits, 'float' being 32 bits, and 'double' being 64 bits? I know that an int is 16 bits on Win16, but does Python live there any more? Even so, there is a 32 bit int type on Win16 as well.

I guess changing the docs to give a "hard commitment" to this isn't such a big deal to me personally, because the above are true for every platform I think I'll need this for (alpha, x86, sparc, mips).

*** In the absence of fixing the 'l' and 'L' types, adding new typecodes ('n' and 'N' perhaps) that do use LONG_LONG. This seems more backwards compatible, but all it really does is make the 'l' and 'L' typecodes duplicate either 'i' or 'n' depending on the platform specific sizeof(long). In otherwords, if an 'n' typecode was added, who would want to use the 'l' one? I suppose someone who knew they wanted a platform specific long.

*** I really need complex types. And more than the functionality provided by Numeric/Numarray, I need complex integer types. We frequently read hardware that gives us complex 16 or 32 bit integers, and there are times when we would use 32 or 64 bit fixed point complex numbers. Sometimes we scale our "soft decision" data so that it would fit fine in a complex 8 bit integer. This could be easily added in one of two ways: either adding a 'z' prefix to the existing typecodes, or by creating new typecodes like such:

'u' - complex bytes (8 bit) 'v' - complex shorts (16 bit) 'w' - complex ints (32 bit) 'x' - complex LONG_LONGs (64 bit) 'y' - complex floats (32 bits) 'z' - complex doubles (64 bits)

The downside to a 'z' prefix is that typecodes could now be 2 characters 'zi', 'zb', and that would be a bigger change to the implementation. It's also silly to have complex unsigned types (who wants complex numbers that are only in one quadrant?).

The downside to adding 'u', 'v', 'w', 'x', 'y', 'z' is that they aren't very mnemonic, and the namespace for typecodes is getting pretty big.

Also, I'm unsure how to get the elements in and out of the typecode for 'x' above (a 2-tuple of PyInt/PyLongs?). Python's complex type is sufficient to hold the others without losing precision.

*** The ability to construct an array object from an existing C pointer. We get our memory in all kinds of ways (valloc for page aligned DMA transfers, shmem etc...), and it would be nice not to copy in and copy out in some cases.

*** Adding an additional signature to the array constructor that specifies a length instead of initial values.

a = array.array('d', [1, 2, 3])

would work as it currently does, but

a = array.array('d', 30000)

would create an array of 30000 doubles. My current hack to accomplish this is to create a small array and use the Sequence operation * to create an array of the size I really want:

a = array.array('d', [0])*300000

Besides creating a (small) temporary array, this calls memcpy 300000 times. Yuk.

*** In the absence of the last one, adding a new constructor:

a = array.xarray('d', 30000)

would create an array of 30000 doubles.

*** If a signature for creating large arrays is accepted, an optional keyword parameter to specify the value of each element:

a = array.xarray('d', 30000, val=1.0)

The default val would be None indicating not to waste time initializing the array.

*** Multi-dimensional arrays would be valuable too, but this might be treading too much in the territory of the Numarray guys. (I really wish there was a completed one size fits all of my needs array module.) I would propose the following for multi-dimensional arrays:

a = array.array('d', 20000, 20000)

or:

a = array.xarray('d', 20000, 20000)

Well if someone authoritative tells me that all of the above is a great idea, I'll start working on a patch and scratch my plans to create a "not in house" xarray module.

Cheers, -Scott Gilbert

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