Null Object Design Pattern (Python recipe)

Null Object 个人感觉非常有用。也是在review公司其他同事写代码的时候看到。

当时使用了flask的request全局请求变量g，然后使用了g.x保存了一个东西。

当时在view代码读取g.x的时候震惊了，因为这一段代码并没有保存g.x，按道理来说应该是一个空值，当我拿着空值去调用其属性的时候应该会报AttributeError。

但是什么也没有发生，既没有报错，也没有发生什么，而且对其判断还是False,于是查看其实现才发现了这个。以下全部转自http://code.activestate.com/recipes/68205/

#!/user/bin/env python
# coding: utf-8
# {{{ http://code.activestate.com/recipes/68205/ (r1)

"""null.py

This is a sample implementation of the 'Null Object' design pattern.

Roughly, the goal with Null objects is to provide an 'intelligent'
replacement for the often used primitive data type None in Python or
Null (or Null pointers) in other languages. These are used for many
purposes including the important case where one member of some group
of otherwise similar elements is special for whatever reason. Most
often this results in conditional statements to distinguish between
ordinary elements and the primitive Null value.

Among the advantages of using Null objects are the following:

- Superfluous conditional statements can be avoided
by providing a first class object alternative for
the primitive value None.

- Code readability is improved.

- Null objects can act as a placeholder for objects
with behaviour that is not yet implemented.

- Null objects can be replaced for any other class.

- Null objects are very predictable at what they do.

To cope with the disadvantage of creating large numbers of passive
objects that do nothing but occupy memory space Null objects are
often combined with the Singleton pattern.

For more information use any internet search engine and look for
combinations of these words: Null, object, design and pattern.

Dinu C. Gherman,
August 2001
"""

class Null(object):

"""A class for implementing Null objects.

This class ignores all parameters passed when constructing or
calling instances and traps all attribute and method requests.
Instances of it always (and reliably) do 'nothing'.

The code might benefit from implementing some further special
Python methods depending on the context in which its instances
are used. Especially when comparing and coercing Null objects
the respective methods' implementation will depend very much
on the environment and, hence, these special methods are not
provided here.
"""

# object constructing

def __init__(self, *args, **kwargs):
"Ignore parameters."
return None

# object calling

def __call__(self, *args, **kwargs):
"Ignore method calls."
return self

def next(self):
raise StopIteration

def __len__(self):
return 0

def __eq__(self, other):
return self is other

# misc.

def __repr__(self):
"Return a string representation."
return "<Null>"

def __str__(self):
"Convert to a string and return it."
return ""

def default(self):
return ""

def __nonzero__(self):
return False

def for_json(self):
return ''

__sub__ = __div__ = __mul__ = __floordiv__ = __mod__ = __and__ = __or__ = \
__xor__ = __rsub__ = __rdiv__ = __rmul__ = __rfloordiv__ = __rmod__ = \
__rand__ = __rxor__ = __ror__ = __radd__ = __pow__ = __rpow__ = \
__rshift__ = __lshift__ = __rrshift__ = __rlshift__ = __truediv__ = \
__rtruediv__ = __add__ = __getitem__ = __neg__ = __pos__ = __abs__ = \
__invert__ = __setattr__ = __getattr__ = __delattr__ = __delitem__ = __setitem__ = \
__iter__ = __call__

Null = Null()

这里我们在其他地方引用的时候可以直接从这个文件里引入Null，在需要的时候返回这个Null对象实例就可以了。

Reference:
http://code.activestate.com/recipes/68205/ ---Null Object Design Pattern (Python recipe)