A property variation that allows property access using an index or key.
Python, 124 lines
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Copy to clipboard1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 | class itemproperty(object):
def __init__(self, fget=None, fset=None, fdel=None, doc=None):
if doc is None and fget is not None and hasattr(fget, "__doc__"):
doc = fget.__doc__
self._get = fget
self._set = fset
self._del = fdel
self.__doc__ = doc
def __get__(self, instance, owner):
if instance is None:
return self
else:
return bounditemproperty(self, instance)
def __set__(self, instance, value):
raise AttributeError("can't set attribute")
def __delete__(self, instance):
raise AttributeError("can't delete attribute")
def getter(self, fget):
return itemproperty(fget, self._set, self._del, self.__doc__)
def setter(self, fset):
return itemproperty(self._get, fset, self._del, self.__doc__)
def deleter(self, fdel):
return itemproperty(self._get, self._set, fdel, self.__doc__)
class bounditemproperty(object):
def __init__(self, item_property, instance):
self.__item_property = item_property
self.__instance = instance
def __getitem__(self, key):
fget = self.__item_property._get
if fget is None:
raise AttributeError("unreadable attribute item")
return fget(self.__instance, key)
def __setitem__(self, key, value):
fset = self.__item_property._set
if fset is None:
raise AttributeError("can't set attribute item")
fset(self.__instance, key, value)
def __delitem__(self, key):
fdel = self.__item_property._del
if fdel is None:
raise AttributeError("can't delete attribute item")
fdel(self.__instance, key)
if __name__ == "__main__":
class Element(object):
def __init__(self, tag, value=None):
self.tag = tag
self.value = value
self.children = {}
@itemproperty
def xpath(self, path):
"""Get or set the value at a relative path."""
path = path.split('/')
element = self
for tag in path:
if tag in element.children:
element = element.children[tag]
else:
raise KeyError('path does not exist')
return element.value
@xpath.setter
def xpath(self, path, value):
path = path.split('/')
element = self
for tag in path:
element = element.children.setdefault(tag, Element(tag))
element.value = value
@xpath.deleter
def xpath(self, path):
path = path.split('/')
element = self
for tag in path[:-1]:
if tag in element.children:
element = element.children[tag]
else:
raise KeyError('path does not exist')
tag = path[-1]
if tag in element.children:
del element.children[tag]
else:
raise KeyError('path does not exist')
tree = Element('root')
tree.xpath['unladen/swallow'] = 'african'
assert tree.xpath['unladen/swallow'] == 'african'
assert tree.children['unladen'].xpath['swallow'] == 'african'
assert tree.children['unladen'].children['swallow'].value == 'african'
tree.xpath['unladen/swallow'] = 'european'
assert tree.xpath['unladen/swallow'] == 'european'
assert len(tree.children) == 1
assert len(tree.children['unladen'].children) == 1
tree.xpath['unladen/swallow/airspeed'] = 42
assert tree.xpath['unladen/swallow'] == 'european'
assert tree.xpath['unladen/swallow/airspeed'] == 42
del tree.xpath['unladen/swallow']
assert 'swallow' not in tree.children['unladen'].children
try:
tree.xpath['unladen/swallow/airspeed']
except KeyError:
pass
else:
assert False
|
This recipe demonstrates using descriptors to implement random access of a virtualized collection in a property-like fashion. This is akin to parameterized properties from Visual Basic.
Note that non-random access methods like __iter__ and __len__ are not implemented here. In cases where these are desired it is better to create a class representing the collection and assign it to an instance attribute.
