Greg Wilson wrote:
>
> If a mechanism for providing meta-information about code is going to be
> added to Python, then I would like it to be flexible enough for developers
> to define/add their own. It's just like allowing developers to extend the
> type system with new classes, rather than handing them a fixed set of
> built-in types and saying, "Good luck". (Most commercial Fortran
> compilers take the second approach, by providing a bunch of inflexible,
> vendor-specific pragmas. It's a nightmare...)
I don't think that Python will move in that direction. pragmas are
really only meant to add some form of meta-information to a Python
source file which would otherwise have to be passed to the compiler
in order to produce correct output. It's merely a way of defining
compile time flags for Python modules which allow more flexible
compilation.
Other tools might also make use of these pragmas, e.g. freeze,
to allow inspection of a module without having to execute it.
> I think that pragmas are essentially callbacks into the interpreter. When
> I put:
>
> pragma encoding = "UTF-16"
>
> I am telling the interpreter to execute its 'setEncoding()' method right
> away.
pragmas have a different target: they tell the compiler (or some
other non-executing tool) to make a certain assumption about the
code it is currently busy compiling.
The compiler is not expected to execute any Python code when it
sees a pragma, it will only set a few internal variables according
to the values stated in the pragma or simply ignore it if the
pragma uses an unknown key and then proceed with compiling.
> So, why not present pragmas in that way? I.e., why not expose the Python
> interpreter as a callable object while the source is being parsed and
> compiled? I think that:
>
> __python__.setEncoding("UTF-16")
>
> is readable, and can be extended in lots of well-structured ways by
> exposing exactly as much of the interpreter as is deemed safe. Arguments
> could be restricted to constants, or built-in operations on constants, to
> start with, without compromising future extensibility.
The natural place for these APIs would be the sys module...
no need for an extra __python__ module or object.
I'd rather not add complicated semantics to pragmas -- they
should be able to set flags, but not much more.
--
Marc-Andre Lemburg
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