In the patch below, I have replaced codegen.py's getopt based option parsing with OptionParser. There are no feature changes but some nicities added:

before:

$ python codegen.py
usage: codegen.py [-o overridesfile] [-p prefix] defsfile

after:

$ python codegen.py
usage: codegen.py [options] defsfile

options:
  -o OVERRIDE, --override=OVERRIDE
                        overrides file
  -p PREFIX, --prefix=PREFIX
                        prefix for variable names inside generated code
  -r REGISTER, --register=REGISTER
                        file with types to register
  --outfilename=OUTFILENAME
                        c file to generate
  --errorfilename=ERRORFILENAME
                        file to log errors to
  --py_ssize_t_clean    set py_ssize_t_clean
  -D DEFINES            add preprocessor definition key-value
  -I INCLUDES           add include path
  -t LOAD_TYPES, --load-types=LOAD_TYPES
                        script to load before code generation
  -h, --help            show this help message and exit

The option descriptions might not be 100% correct, I had to make educated guesses at what the undocumented options do. There is also some more exception handling so you should (in more cases) get an error message instead of a traceback when a file is missing. 

I cannot swear that the patch doesn't break something, but I've tried running it on some examples and it seems to work ok. So someone else should try it too. It incidentally solves the silent error "python codegen.py -r foobar.defs -DMOO=BOO" since OptionParser-parsing is position independent.