code: def is_none(expr):
      # py3
      if isinstance(expr, ast.Constant) and expr.value is None:
          return True
  
      # py2
      if not isinstance(expr, ast.Attribute):
          return False
  
      return expr.attr == "None"
  
file_path: catboost/contrib/python/pythran/pythran/transformations/normalize_is_none.py
imports: from pythran.passmanager import Transformation
  from pythran.analyses import Ancestors
  from pythran.syntax import PythranSyntaxError
  from functools import reduce
  import gast
label_desc: Use image node_id 1 for calling a global function with example usage: is_none(expr) and returns: unknown, True, False