code: def push_ctx(app=None):
      
      if app is not None:
          ctx = app.test_request_context()
          ctx.fixtures_request_context = True
          ctx.push()
          if _app_ctx_stack is not None:
              _app_ctx_stack.top.fixtures_app_context = True
  
      # Make sure that we have an application in the current context
      if (_app_ctx_stack is None or _app_ctx_stack.top is None) and _request_ctx_stack.top is None:
          raise AssertionError('A Flask application must be specified for Fixtures to work.')
docs: Creates new test context(s) for the given app
  
      If the app is not None, it overrides any existing app and/or request
      context. In other words, we will use the app that was passed in to create
      a new test request context on the top of the stack. If, however, nothing
      was passed in, we will assume that another app and/or  request context is
      already in place and use that to run the test suite. If no app or request
      context can be found, an AssertionError is emitted to let the user know
      that they must somehow specify an application for testing.