http://packages.python.org/dexml/api/dexml.fields.htmlまたはhttps://gist.github.com/485977に沿って、XMLデータの軽量クラスラッパーに取り組んでいます。クラスにはelementTree要素が含まれ、プロパティへのアクセスを提供する記述子があります（含まれているコードの量についてお詫びします。コメントのほとんどを残しておくと、より簡単に理解できると思います）
class XPropBase(object):
DEFAULT_TO = (lambda s, x: str(x))
DEFAULT_FROM = (lambda s, p: p)
def __init__(self, path, convert_from=None, convert_to = None, read_only = False, allow_missing=False):
'''
important bits here are convert_to and convert_from, which do the translation in and out of XML strings in derived classes...
'''
self.path = path
self.convert_from = convert_from or self.DEFAULT_FROM
self.convert_to = convert_to or self.DEFAULT_TO
self.read_only = read_only
self.allow_missing = allow_missing
def _get_xml(self, instance):
#only a an instance method for convenience...
#intended to be overridden for different target instances
return instance.get_element()
class XElement(XPropBase):
'''
Wraps an xml item whose content is contained in the text of
an XML tag, ie: <tag>Content</tag>. The convert_to and convert_from methods
will be applied to the text property of the corresponding Element
@note this will use the first instance of a given node path that it finds,
so it is not guaranteed if the supplied path leads to more than one xml tag.
'''
def __get__(self, instance, owner=None):
the_xml = self._get_xml(instance)
if not self.path:
return self.convert_from(the_xml.text)
try:
underlying = the_xml.find(self.path)
return self.convert_from(underlying.text)
except AttributeError:
if self.allow_missing:
return None
else:
raise XMLWrapperError, "%s has no element named %s" % (instance, self.path)
def __set__(self, instance, value, owner =None):
if self.read_only:
raise XMLWrapperError('%s is a read-only property' % self.path)
the_xml= self._get_xml(instance)
if not self.path:
the_xml.text = self.convert_to(value)
return
try:
underlying = self._get_xml(instance).find(self.path)
underlying.text = self.convert_to(value)
except AttributeError:
if self.allow_missing:
SubElement(self._get_xml(instance), self.path, text=self.convert_to(value))
else:
raise XMLWrapperError, "%s has no element named %s" % (instance, self.path)
class XAttrib(XPropBase):
'''
Wraps a property in an attribute on the containing xml tag specified by path
if the supplied attribute is not present, will raise an XMLWrapperError unless the allow_missing flag is set to True
'''
def __get__(self, instance, owner=None):
try:
res = self._get_xml(instance).attrib[self.path]
return self.convert_from(res)
except KeyError:
if self.allow_missing:
return None
raise XMLWrapperError, "%s has no attribute named %s" % (instance, self.path)
def __set__(self, instance, value, owner =None):
xml = self._get_xml(instance)
has_element = xml.get(self.path, 'NOT_FOUND')
if has_element == 'NOT_FOUND' and not self.allow_missing:
raise XMLWrapperError, "%s has no attribute named %s" % (instance, self.path)
xml.set(self.path, self.convert_to(value))
def _get_element(self):
return None
def _get_attribute(self):
return self.path
class XInstance(XPropBase):
'''
Represents an attribute which is mapped onto a class. The supplied class is specified in the constructor
@note: As with XElement, this works on the first appropriately named tag it
finds. If there are multiple path values with the same tag, it will cause
errors.
'''
def __init__(self, path, cls, convert_from=None, convert_to = None, read_only = False, allow_missing=False):
self.cls = cls
XPropBase.__init__(self, path, convert_from = convert_from , convert_to = convert_to , read_only = read_only, allow_missing=allow_missing)
def __get__(self, instance, owner=None):
sub_elem = self._get_xml(instance).find(self.path)
if not sub_elem and not self.allow_missing:
XMLWrapperError, "%s has no child named %s" % (instance, self.path)
return self.cls(sub_elem)
def __set__(self, instance, value):
my_element = self._get_xml(instance)
original_child = my_element.find(self.path)
if original_child:
my_element.remove(original_child)
my_element.append(self._get_xml(value))
class XInstanceGroup(XInstance):
'''
Represents a collection of XInstances contained below a particular tag
'''
def __get__(self, instance, owner=None):
return [self.cls(item) for item in self._get_xml(instance).findall(self.path)]
def __set__(self, instance, value):
my_element = self._get_xml(instance)
for item in my_element.findall(self.path):
my_element.remove(item)
for each_element in map(self._get_xml, value):
my_element.append(each_element)
動作しているように見えますが（徹底的なテストが行われる予定ですが）、厄介な点が1つあります。XInstanceGroup記述子は、次のようなケースを処理します。
<Object name="dummy">
<Child name="kid2" blah="dee blah"/>
<Child name="kid2" blah="zey"/>
</Object>
class Kid(XMLData):
Name = XAttribute("name")
Blah = XAttribute("blah")
class DummyWrapper(XMLData):
Name = XAttribute("name")
Kids = XInstanceGroup('Child', Kid)
したがって、子供向けのDummyWrapperの場合は、Kidオブジェクトのリストを取得します。しかし、私はそのリストを更新するプロセスに不満を持っています：
#this works
kids = Dummy_example.Kids
kids.append(Kid (name = 'marky mark', blah='funky_fresh'))
Dummy_example.Kids = kids
#this doesn't
Dummy_example.Kids.append(Kid(name = 'joey fatone', blah = 'indeed!'))
Dummy.Kidsは実際にはグループを返す関数であり、メンバーフィールドとして格納されている永続的なリストオブジェクトではないという事実によって必要になります。
さて、質問のために：記述子を使用してこれを行う方法はありますか？記述子インスタンスがデータを永続化できないことがハードルのようです。インスタンスが呼び出されたときにのみインスタンスを認識します。記述子からインスタンスにストレージを注入するというアイデアは嫌いです（他に何もない場合は、結合が不快に増加します）。明らかなグーグルは今のところ役に立っていません。