new_contents: from importlib import import_module, reload
  from os.path import join as join_path, dirname
  from os import walk
  from sys import modules
  
  from yunity.utils.tests.abc import BaseTestCase
  import yunity
  
  
  def _path_to_module(path, root_module_path, pysuffix='.py'):
      path = path[len(dirname(root_module_path)) + 1:-len(pysuffix)]
      path = path.replace('/', '.')
      return path
  
  
  def iter_modules(root_module_path, excludes=None, pysuffix='.py'):
      def is_module(_):
          return _.endswith(pysuffix) and not _.startswith('__init__')
  
      def is_blacklisted(_):
          return excludes and any(_.startswith(exclude) for exclude in excludes)
  
      for root, _, leaves in walk(root_module_path):
          for leaf in filter(is_module, leaves):
              module = _path_to_module(join_path(root, leaf), root_module_path)
              if not is_blacklisted(module):
                  yield module
  
  
  def import_or_reload(resource):
      module = modules.get(resource)
      if module:
          return reload(module)
      else:
          return import_module(resource)
  
  
  class PytonIsValidTestCase(BaseTestCase):
      def test_all_modules_import_cleanly(self):
          self.given_data(root_module_path=yunity.__path__[0])
          self.given_data(excludes={
              'yunity.resources',                               # intgration test data files have side-effects
              'yunity.tests.integration.test_integration',      # integration test runner has side-effects
              'yunity.management.commands.create_sample_data',  # sample data command has side-effects
          })
          self.when_importing_modules()
          self.then_all_modules_import_cleanly()
  
      def when_importing_modules(self):
          self.exception = []
          for module in iter_modules(*self.args, **self.kwargs):
              try:
                  import_or_reload(module)
              except Exception as e:
                  self.exception.append((module, e))
  
      def then_all_modules_import_cleanly(self):
          for module, exception in self.exception:
              self.fail('{} did not import cleanly: {}'.format(module, exception.args[0]))
  
subject: Add expectation that all Python code is correct