import sys

PY3 = sys.version_info == 3

if PY3:

import builtins

else:

import __builtin__ as builtins

if PY3:

exec_ = getattr(builtins, 'exec')

else:

def exec_(_code_, _globals_=None, _locals_=None):
  """Execute code in a namespace."""
  if _globals_ is None:
    frame = sys._getframe(1)
    _globals_ = frame.f_globals
    if _locals_ is None:
      _locals_ = frame.f_locals
    del frame
  elif _locals_ is None:
    _locals_ = _globals_
  exec("""exec _code_ in _globals_, _locals_""")