code: def check_package(self, package, package_dir):
          
          try:
              return self.packages_checked[package]
          except KeyError:
              pass
  
          init_py = orig.build_py.check_package(self, package, package_dir)
          self.packages_checked[package] = init_py
  
          if not init_py or not self.distribution.namespace_packages:
              return init_py
  
          for pkg in self.distribution.namespace_packages:
              if pkg == package or pkg.startswith(package + '.'):
                  break
          else:
              return init_py
  
          with io.open(init_py, 'rb') as f:
              contents = f.read()
          if b'declare_namespace' not in contents:
              raise distutils.errors.DistutilsError(
                  "Namespace package problem: %s is a namespace package, but "
                  "its\n__init__.py does not call declare_namespace()! Please "
                  'fix it.\n(See the setuptools manual under '
                  '"Namespace Packages" for details.)\n"' % (package,)
              )
          return init_py
docs: Check namespace packages' __init__ for declare_namespace