code: def __build_python_module_cache(self):
          
          cache = {}
          for importer, mname, ispkg in pkgutil.walk_packages(b2.__path__, prefix='b2.'):
              basename = mname.split('.')[-1]
              # since the jam code is only going to have "import toolset ;"
              # it doesn't matter if there are separately named "b2.build.toolset" and
              # "b2.contrib.toolset" as it is impossible to know which the user is
              # referring to.
              if basename in cache:
                  self.manager.errors()('duplicate module name "{0}" '
                                        'found in boost-build path'.format(basename))
              cache[basename] = mname
          self.__python_module_cache = cache
docs: Recursively walks through the b2/src subdirectories and
          creates an index of base module name to package name. The
          index is stored within self.__python_module_cache and allows
          for an O(1) module lookup.
  
          For example, given the base module name `toolset`,
          self.__python_module_cache['toolset'] will return
          'b2.build.toolset'
  
          pkgutil.walk_packages() will find any python package
          provided a directory contains an __init__.py. This has the
          added benefit of allowing libraries to be installed and
          automatically avaiable within the contrib directory.
  
          *Note*: pkgutil.walk_packages() will import any subpackage
          in order to access its __path__variable. Meaning:
          any initialization code will be run if the package hasn't
          already been imported.