to add this code to Python, compile and link into an object file; the Cygwin makefile
in Example 20-9 builds the C source code for dynamic binding on imports.

Example 20-9. PP4E\Integrate\Extend\Cenviron\makefile.cenviron

##################################################################

Compile cenviron.c into cenviron.dll--a shareable object file

on Cygwin, which is loaded dynamically when first imported.

##################################################################

PYLIB = /usr/local/bin
PYINC = /usr/local/include/python3.1

cenviron.dll: cenviron.c
gcc cenviron.c -g -I$(PYINC) -shared -L$(PYLIB) -lpython3.1 -o $@

clean:
rm -f *.pyc cenviron.dll

To build, type make -f makefile.cenviron at your shell. To run, make sure the result-
ing .dll file is in a directory on Python’s module path (the current working directory
works too):

.../PP4E/Integrate/Extend/Cenviron$ python

>>> import cenviron

>>> cenviron.getenv('USER') # like os.environ[key] but refetched

'mark'

>>> cenviron.putenv('USER', 'gilligan') # like os.environ[key]=value

>>> cenviron.getenv('USER') # C sees the changes too

'gilligan'

As before, cenviron is a bona fide Python module object after it is imported, with all
the usual attached information, and errors are raised and reported correctly on errors:

>>> dir(cenviron)

['__doc__', '__file__', '__name__', '__packge__', 'getenv', 'putenv']

>>> cenviron.__file__

'cenviron.dll'

>>> cenviron.__name__

'cenviron'

>>> cenviron.getenv

<built-in function getenv>

>>> cenviron

<module 'cenviron' from 'cenviron.dll'>

>>> cenviron.getenv('HOME')

'/home/mark'

>>> cenviron.getenv('NONESUCH')

SystemError: Error calling getenv

1498 | Chapter 20: Python/C Integration