inspection of compilation namespace for IF/ELIF/ELSE/DEF

Reported by cb on 19 Jul 2008 15:43 UTC I tried to post this to cython-dev twice and got one awaiting moderator approval and one silent no-op. So, I'll try it here.

It would be nice to have conditional compilation directives that make multiple inclusion idempotent. The usual guards in the C/C++ world would be:

#ifndef FOO_H
#define FOO_H
...
#endif

In Cython, I have been doing DEF FOO_1 = 1 DEF FOO_2 = 1 IF FOO_1 == FOO_2: DEF FOO_2 = 2 ...

That seems kind of ugly, though. Beyond aesthetics, it is probably nice in other contexts besides include idempotency to test for a name before using it.

So, I propose a tiny addition to add a new compile- time builtin called DEFINED() that just evaluates to a boolean based on whether its string argument exists in the compile-time environment.

This allows the above construct to be written both more simply and more clearly:

IF not DEFINED("FOO"):
    DEF FOO = 1
    ...

Name choice follows the principle of least surprise since the lowercase cpp name has similar functionality, just like the other conditional compilation features.

Here is a very simple patch that implements this, attached as well.

diff -ruw Cython-0.9.8.orig/Cython/Compiler/Scanning.py Cython-0.9.8/Cython/Compiler/Scanning.py --- Cython-0.9.8.orig/Cython/Compiler/Scanning.py 2008-06-11 14:25:34.000000000 -0400 +++ Cython-0.9.8/Cython/Compiler/Scanning.py 2008-07-17 16:39:27.691882798 -0400 @@ -186,6 +186,9 @@ else: raise

def defined(self, name):
return name in self.entries + def initial_compile_time_env(): benv = CompileTimeScope() names = ('UNAME_SYSNAME', 'UNAME_NODENAME', 'UNAME_RELEASE', @@ -201,6 +204,7 @@ for name in names: benv.declare(name, getattr(builtin, name)) denv = CompileTimeScope(benv)
denv.declare('DEFINED', denv.defined) return denv
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Migrated-From: http://trac.cython.org/ticket/27

robertwb / issues-import-test

inspection of compilation namespace for IF/ELIF/ELSE/DEF #53

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