# SWIG:Examples:python:constants

`SWIG/Examples/python/constants/`

## Wrapping C Constants

`$Header$`

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).