位置參數默認為None，但該參數默認是一個可迭代對象。如果不傳參的話：

In [8]: def func(iter_obj=None):
   ...:     for item in iter_obj:
   ...:         print item
   ...:

In [9]: func()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-9-08a2da4138f6> in <module>()
----> 1 func()

<ipython-input-8-f880f4d9f102> in func(iter_obj)
      1 def func(iter_obj=None):
----> 2     for item in iter_obj:
      3         print item
      4

TypeError: 'NoneType' object is not iterable

通過優先級來對準備迭代的對象進行替換：

In [10]: def func(iter_obj=None):
    ...:     for item in iter_obj or ():
    ...:         print item
    ...:

In [11]: func()

來源：werkzeug.routing.Map的__init__方法：

class Map(object):
    def __init__(self, rules=None, default_subdomain='', charset='utf-8',
                 strict_slashes=True, redirect_defaults=True,
                 converters=None, sort_parameters=False, sort_key=None,
                 encoding_errors='replace', host_matching=False):
       ...

        for rulefactory in rules or ():
            self.add(rulefactory)