parser for a tiny subset of the C preprocessor lang

parseheader.py

def parseheader(headerstream, defined_constants):
    depth = 0
    ignoring = False

    for line in headerstream:
        line = line.strip()

        if not line.startswith('#'):
            continue

        if line.startswith('#endif'):
            depth -= 1
            ignoring = False
        elif line.startswith('#else'):
            ignoring = not ignoring
        elif not ignoring:
            splitted = line.split(' ')
            if line.startswith('#define'):
                constant = splitted[1]
                if len(splitted) > 2:
                    value = splitted[2]
                else:
                    value = True
                defined_constants[constant] = value
            elif line.startswith('#ifdef'):
                depth += 1
                constant = line.split(' ')[1]
                if constant in defined_constants:
                    ignoring = False
                else:
                    ignoring = True
            elif line.startswith('#ifndef'):
                depth += 1
                constant = line.split(' ')[1]
                if constant in defined_constants:
                    ignoring = True
                else:
                    ignoring = False
            elif line.startswith('#undef'):
                constant = splitted[1]
                del defined_constants[constant]
            else:
                raise Exception('Unknown preprocessor command: %s' % line)

    return defined_constants

if __name__ == "__main__":
    testdoc = """
#ifndef PROJECT_H
#define PROJECT_H

#define PROJECT_NAME "project"

#ifdef BASELINE
#define REVISION 0
#else
#define REVISION 7
#endif

#endif
"""
    from cStringIO import StringIO
    print(repr(parseheader(StringIO(testdoc), {})))