CODEWITHSUNDEEP
pythonswig
user3263972
user3263972

Reputation: 33

SWIG/python - defined constants in C not accessible

I'm using Swig 3.0.7 to create python 2.7-callable versions of C functions that define constants in this manner: 

#define MYCONST 5.0

In previous versions of swig these would be available to python transparently:

import mymodule

x = 3. * mymodule.MYCONST

But now this generates a message

AttributeError: 'module' object has no attribute 'MYCONST'

Functions in 'mymodule' that use the constant internally work as expected.

Interestingly, if I include this line in the Swig directive file mymodule.i,

#define MYCONST 5.0

then doing dir(mymodule) returns a list that includes

['MYCONST_swigconstant', 'SWIG_PyInstanceMethodNew',  (etc.) .... ]

typing to the python interpreter

 mymodule.MYCONST_swigconstant

gives

<built-in function MYCONST_swigconstant>

which offers no obvious way to get at the value.

So my question is, can one make the previous syntax work so that mymodule.MYCONST evaluates correctly

If not, is there a workaround?

Upvotes: 1

Views: 468

Answers (2)

Mark Tolonen
Mark Tolonen

Reputation: 177675

I can't reproduce your issue. If the below demonstration doesn't help, post your build process:

test.i:

%module test
%inline %{
#define MYCONST 5.0
%}

Swig 3.0.8 and compiled with VS2012 into a C extension:

swig -python test.i
cl /MD /LD /Fe_test.pyd /Ic:\python33\include test_wrap.c -link /libpath:c:\python33\libs

Use:

Python 3.3.5 (v3.3.5:62cf4e77f785, Mar  9 2014, 10:35:05) [MSC v.1600 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import test
>>> test.MYCONST
5.0

Upvotes: 1

Jens Munk
Jens Munk

Reputation: 4725

Definitions are not exported to SWIG through cvar. You can do the following

#define MYCONST 5.0
const float gMYCONST = MYCONST;

In this way, you add type information to your constant and it can be accessed through

import mymodule
mymodule.cvar.gMYCONST

I have SWIG 3.0.2 and using this version, the definition can be accessed using mymodule.MYCONST, but for other reasons, I usually convert macro definitions to typed definitions.

Upvotes: 1

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