null object python是一个设计模式,python实现如下,此模式是想减少无必要的is null逻辑判断,不过没能想到具体怎么应用在项目或者自己的代码中,希望有个应用的实例代码,thanks。
#!/usr/bin/env python
# -*- coding: utf-8 -*-
"""http://code.activestate.com/recipes/68205-null-object-design-pattern/"""
class Null:
def __init__(self, *args, **kwargs):
"""Ignore parameters."""
return None
def __call__(self, *args, **kwargs):
"""Ignore method calls."""
return self
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
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()
print(n)
n = Null('value')
print(n)
n = Null('value', param='value')
print(n)
n()
n('value')
n('value', param='value')
print(n)
# attribute handling
n.attr1
print('attr1', 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()
### OUTPUT ###
# Null
# Null
# Null
# Null
# attr1 Null