Wrapping C Constants

$Header: /cvs/projects/SWIG/Examples/python/constants/index.html,v 1.1 2000/06/17 21:40:59 beazley Exp $

When SWIG encounters C preprocessor macros and C declarations that look like constants, it creates Python variables with an identical value. Click here to see a SWIG interface with some constant declarations in it.

Accessing Constants from Python

Click here to see a script that prints out the values of the constants contained in the above file.

Key points

  • The values of preprocessor macros are converted into Python constants.
  • Types are inferred by syntax (e.g., "3" is an integer and "3.5" is a float).
  • Character constants such as 'x' are converted into Python strings.
  • C string literals such as "Hello World" are converted into Python strings.
  • Macros that are not fully defined are simply ignored. For example:
    #define EXTERN extern
    is ignored because SWIG has no idea what type of variable this would be.

  • Expressions are allowed provided that all of their components are defined. Otherwise, the constant is ignored.
  • Certain C declarations involving 'const' are also turned into Python constants.
  • The Python variables that SWIG creates are not protected from modification. For example, even if you had this:
    #define FOO 73
    a user could come along in a script and type
    example.FOO = 13
    Unfortunately, there's no easy way to prevent this.

  • The constants that appear in a SWIG interface file do not have to appear in any sort of matching C source file since the creation of a constant does not require linkage to a stored value (i.e., a value held in a C global variable or memory location).