[Python-Dev] PEP 0484 - the Numeric Tower (original) (raw)

Chris Barker - NOAA Federal chris.barker at noaa.gov
Wed Oct 14 20:45:55 EDT 2015

Yes, but what I expect the type annotations to be used for, especially in the SciPy world, is to make things easier for Numba to generate fast code.

Well, probably not.

There are two reasons to have type declarations: performance and type safety.

But the current type annotations are designed only for the latter. A number of us brought this up in the discussion because we do, in fact, want the magic fairy dust of faster code, whether by numba or Cython(my favorite) and I had hoped that type annotations would be useful for that. But it was explicitly stated that that was not the intent. And indeed, if you want to preserve any of python's nifty dynamic typing ( and we all do ) it can't support truly static typing.

I.e. In Python, you want to say that your function works with, say, a sequence of numbers. But for performant compilation, you'd need to know the binary layout of that sequence.

Side notes:

Numba does JIT compilation, so pre specifying the types isn't needed anyway. Certainly adding these type annotations will buy you nothing.

Bokeh, on the other hand, has to pass everything off to the browser anyway, presumably with JSON, so in theory it shouldn't need Numpy arrays anyway.

(Of course, JSON doesn't support Decimal anyway -- or, strictly speaking, only supports decimal, but JavaScript conveys it to floats...)

I really hope that the SciPy world is not going to go nuts for useless annotations, but I have, alas, all too many years dealing with people whose desire to have faster code vastly outstrips their ability to understand just what is possible in that regard.

In James Barrie's novel, Peter Pan, earthly children are given the ability to fly by having Fairy Dust (supplied by the unwilling Tinkerbell) sprinkled over them. I now know that what a large number of people who want faster code, really want is magic Fairy Dust. They wanted psyco to be it, they wanted pypy to be it, and now they want Numba to be it. The notion that sprinkling type annotations all over their code will make it fly absolutely deeply resonates with how these people wish the world worked. They will believe that type annotations are magic fairy dust until they are forced to confront the fact poorly written code can not be made fly until it is rewritten. Laura

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