[Cython] inspection of compile-time macro/etc. namespace

[Chuck Blake]

Hey, guys.  Thanks for the hard work on Cython.
It's been far superior to Pyrex for my needs.

Anyway, I have .pxi files that I like to
use for actual gcc-level function inlining.
I need 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 to test for a name before using
it/logically fork/etc. based on that.  Just as one
e.g., if there is any evolution of __builtin__ names
populating the environment.

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
is 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.

cb
-------------- next part --------------
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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