Wrapping C Constants

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

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).