Design-Patterns-in-Python/null.py at master · coder-zh/Design-Patterns-in-Python · GitHub

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#!/user/bin/env python

"""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:

"""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

# Attribute handling

def __getattr__(self, mname):

"Ignore attribute requests."

return self

def __setattr__(self, name, value):

"Ignore attribute setting."

return self

def __delattr__(self, name):

"Ignore deleting attributes."

return self

# Misc.

def __repr__(self):

"Return a string representation."

return "<Null>"

def __str__(self):

"Convert to a string and return it."

return "Null"

def test():

"Perform some decent tests, or rather: demos."

# Constructing and calling

n = Null()

n = Null('value')

n = Null('value', param='value')

n()

n('value')

n('value', param='value')

# Attribute handling

n.attr1

n.attr1.attr2

n.method1()

n.method1().method2()

n.method('value')

n.method(param='value')

n.method('value', param='value')

n.attr1.method1()

n.method1().attr1

n.attr1 = 'value'

n.attr1.attr2 = 'value'

del n.attr1

del n.attr1.attr2.attr3

# Representation and conversion to a string

assert repr(n) == '<Null>'

assert str(n) == 'Null'

if __name__ == '__main__':

test()

Design-Patterns-in-Python/null.py at master · coder-zh/Design-Patterns-in-Python · GitHub

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