With one interface defined, and most of the module design and DOM-compliance
properties and methods figured out, today's post will continue with some concrete
implementation and more interface definition. I'm going to get as far through all
of the non-concrete implementations as I can, since the foundations they provide,
while critically important in the long run, aren't as demonstrable as the concrete
markup implementations.
Defining the BaseNode Abstract Class
BaseNode is the first abstract class I'm going to tackle in the
markup module, and the first definition of any concrete functionality
there. Since it's intended to provide some concrete implementation while just
passing
some of the abstraction from IsNode on to the
concrete classes that will derive from it, there isn't a lot of concrete
implementation, though.
Implementing and Testing the Concrete Properties
Since I've noted in some detail in my
coding standards exactly how I prefer to implement instance properties,
I'll be focusing more on how the properties get their jobs done than what
the code underneath the public interface really looks like. BaseNode
will provide concrete implementations for six properties that are required by the
IsNode interface:
nextElementSibling;nextSibling;parentElement;parentNode;previousElementSibling; andpreviousSibling
next* and previous* items,
rely on their corresponding parent* property — if the instance
in question doesn't have a parent of the appropriate type in the parent*
property, then there can't be a next* or previous*
value. Those parent* properties, then, need to be worked out first.
What Constitutes a parent Anyway?
The short and obvious answer is probably best expressed as an
That's something that I haven't really addressed in any detail yet.
A fairly complete list of the properties and methods that involve child nodes,
taken from the big list presented in the last post, would include:
IsNode
instance that has the various *child* properties and *Child
methods.
markup Module Equivalent Class |
|||
|---|---|---|---|
| Member Name | Comment | Tag | Text |
| Property Members | |||
| childElementCount | n/a | number | n/a |
| childNodes | object | object | object |
| children | n/a | object | n/a |
| firstChild | null | object | null |
| firstElementChild | n/a | null | n/a |
| lastChild | null | object | null |
| lastElementChild | n/a | null | n/a |
| Method Members | |||
| appendChild | function | function | function |
| contains | function | function | function |
| getElementsByClassName | n/a | function | n/a |
| getElementsByTagName | n/a | function | n/a |
| hasChildNodes | function | function | function |
| insertBefore | function | function | function |
| replaceChild | function | function | function |
At present, in the current class-relationships diagram, there really isn't any single interface, abstract class or class that feels to me like the right place to set those up:
Tag — although down the line, any
concrete document-classes that derive from BaseDocument (which in
turn derives from Tag) will need those as well. From an architectural
standpoint, that feels to me like a need for an interface (call it IsElement
for now) at a minimum that Tag will implement, and that ties in to
the various *child*-related members listed above.
I'll plan on working out IsElement right after I finish with
BaseNode, then.
Another item for consideration: In the JavaScript DOM functionality that I'm
trying to keep consistent with, there are two distinct parent-types: Elements (tags)
and nodes (everything else). I've already established that non-tags really
can't have children in at least one major browser-engine branch (webkit,
used by Safari, Chrome and Chromium). In Firefox (mozilla), it's not much different
— the specifics of the error-messages are different, but the fundamental DOM-object
relationship is the same: text-nodes can't have children. That, then, begs the
question: Why is there a
Particularly since running code like this:
parentElement and a parentNode
method?
tag = document.createElement( 'tag' );
text = document.createTextNode( 'this is a text-node' );
tag.appendChild( text );
console.log( 'text.parentElement ..................... ' +
text.parentElement );
console.log( 'text.parentElement == tag .............. ' +
( text.parentElement == tag ) );
console.log( 'text.parentElement.isSameNode( tag ) ... ' +
text.parentElement.isSameNode( tag ) );
console.log( 'text.parentNode ........................ ' +
text.parentNode );
console.log( 'text.parentNode == tag ................. ' +
( text.parentNode == tag ) );
console.log( 'text.parentNode.isSameNode( tag ) ...... ' +
text.parentNode.isSameNode( tag ) );parentNode and parentElement
return the same tag-element:
text.parentElement ..................... [object HTMLUnknownElement] text.parentElement == tag .............. true text.parentElement.isSameNode( tag ) ... true text.parentNode ........................ [object HTMLUnknownElement] text.parentNode == tag ................. true text.parentNode.isSameNode( tag ) ...... trueIt just seems... weird, I guess. I hope that it's some sort of concession made for backwards compatibility, but I can't be sure that's the case. On top of that, I can't think of a use-case at all where
parentNode
wouldn't return the same thing as parentElement. Non-element nodes
can't have children, and thus can't be parents, bu the naming convention in the
JavaScript DOM-methods seems to be pretty consistent in *Element*
methods returning elements (tags) only, while *Node*
methods can return any node-type, including elements.
However, in the interests of preserving the DOM-object consistency that I want to preserve, I guess I'll have to keep both those properties. That doesn't mean, though, that I need to have separate property-getters for each, though!
The parent, parentElement and
parentNode Properties
It may be simpler to just show the code in this case, then note the differences from my usual patterns:
#-----------------------------------#
# Instance property-getter methods #
#-----------------------------------#
@describe.AttachDocumentation()
@describe.returns( 'IsElement instance or None' )
def _GetParent( self ):
"""
Returns the IsElement object that the instance is a child of, or None if there is
no parent-child relationship available for the instance."""
return self._parent
#-----------------------------------#
# Instance property-setter methods #
#-----------------------------------#
@describe.AttachDocumentation()
@describe.argument( 'value',
'the object to set as the parent of the instance',
IsElement
)
def _SetParent( self, value ):
"""
Sets the instance's parent to the supplied IsElement object."""
if not isinstance( IsElement, value ):
raise TypeError( '%s.parent expects an instance of a class '
'derived from IsElement, but was passed "%s" (%s), '
'which is not one' % (
self.__class__.__name__, value.__repr__(),
type( value ).__name__
)
)
self._parent = value
#-----------------------------------#
# Instance property-deleter methods #
#-----------------------------------#
def _DelParent( self ):
"""
Deletes the instance's parent relationship by setting it to None"""
self._parent = None
#-----------------------------------#
# Instance Properties (abstract OR #
# concrete!) #
#-----------------------------------#
parent = describe.makeProperty(
_GetParent, None, None,
'the IsElement object that the instance is a child of',
IsElement, None
)
parentElement = describe.makeProperty(
_GetParent, None, None,
'the IsElement object that the instance is a child of',
IsElement, None
)
parentNode = describe.makeProperty(
_GetParent, None, None,
'the IsElement object that the instance is a child of',
IsElement, None
)parent,
parentElement and parentNode) that are all pointed at the
same property-getter method (_GetParent). If, in the future, there's
a demonstrable need to separate those out for some reason, it should be a relatively
simple matter of creating a new property-getter, -setter and -deleter method-set,
then re-assigning the methods in whichever property-declaration need to point to
the new method(s). The one concern that I think would come up in that
sort of scenario is what would have to happen to the parent property.
Right now, the three are completely interchangeable, but if an actual
difference between parentElement and parentNode ever
surfaces, the parent property may well need to be deprecated or even
removed, rather than linger there being confusing.
The nextElementSibling, nextSibling,
previousElementSibling and previousSibling
Properties
There is a common theme that runs across these four properties, all based
around the idea that if the instance has a parent, then that parent has
children and childNodes properties that are a
sequence of IsElement- and IsNode-derived objects,
respectively. Given that, all of these methods need to look at all of the
instance's parent's childNodes (which will always
include all IsNode-derived types), find the position of the
instance whose sibling is being sought in that sequence, then return the
previous or next node or element before or after that position, respectively.
The first step, finding the position of the instance in its parent's
childNodes is common across all four methods.
The _GetnextElementSibling and _GetnextSibling
getter-methods show the determination of the index of the instance in its parent's
collection of children (parentIndex in parent.childNodes)
and the slicing of those childNodes to retrieve everything
after the instance in that collection. _GetnextElementSibling
also shows filtering of that slice, so that only IsElement
items will be considered as candidates for the return value.
@describe.AttachDocumentation()
@describe.returns( 'IsElement object or None' )
def _GetnextElementSibling( self ):
"""
Gets the next IsElement element in the instance's parent's children after the
instance's position in that sequence of objects"""
if self.parent:
# Get the index of the instance in its parent's collection
# of children. If this fails, there's an issue with adding
# children somewhere else...
parentIndex = self.parent.childNodes.index( self )
# Get a slice of the parent's children that captures all the
# children *after* the index
nodesAfter = self.parent.childNodes[ parentIndex + 1: ]
# Since this is an "element" property, filter those down to just
# IsElement members
nodesAfter = [
node for node in nodesAfter
if isinstance( IsElement, node )
]
if len( nodesAfter ) > 0:
# If there's at least two items, return the first one in
# the list
return nodesAfter[ 0 ]
else:
# Otherwise, there aren't any *elements* after the instance,
# so return None
return None
else:
# The instance has no parent, and thus there are no siblings.
return None
@describe.AttachDocumentation()
@describe.returns( 'IsNode object or None' )
def _GetnextSibling( self ):
"""
Gets the next IsNode element in the instance's parent's children after the
instance's position in that sequence of objects"""
if self.parent:
# Get the index of the instance in its parent's collection
# of children. If this fails, there's an issue with adding
# children somewhere else...
parentIndex = self.parent.childNodes.index( self )
# Get a slice of the parent's children that captures all the
# children *after* the index
nodesAfter = self.parent.childNodes[ parentIndex + 1: ]
if len( nodesAfter ) > 0:
# If there's at least two items, return the first one in
# the list
return nodesAfter[ 0 ]
else:
# Otherwise, there aren't any *elements* after the instance,
# so return None
return None
else:
# The instance has no parent, and thus there are no siblings.
return None_GetnextElementSibling
and _GetnextSibling is whether the intermediate list
(nodesAfter) is filtered.
The same basic pattern is used in _GetpreviousElementSibling
and _GetpreviousSibling, including the filtering or non-filtering
of the intermediate results (nodesBefore). The major difference
between either _Getprevious* method and its _Getnext*
counterpart is the initial slice of the instance's parent.childNodes:
# Get a slice of the parent's children that captures all the
# children *before* the index
nodesBefore = self.parent.childNodes[ 0:parentIndex - 1 ]
filteringaspect in
_GetpreviousElementSibling is
identical to the code above for _GetnextElementSibling, and
doesn't exist at all in _GetpreviousSibling.
Implementing and Testing the Concrete Methods
I'd originally expected to implement only two of the abstract methods of
IsNode in BaseNode: IsEqualNode and
isSameNode. With the addition of the IsElement
interface to the markup class-zoo, though, any concrete
implementation of isEqualNode will, I think, have to be moved
out to the concrete classes — since those are the most-shallow points in
the inheritance structure where all of the various properties
that the method needs will actually exist.
That leaves isSameNode as the only concrete method-implementation
of BaseNode:
@describe.AttachDocumentation()
@describe.argument( 'node',
'the node-object to compare to the instance to '
'see if they are the same',
IsNode
)
@describe.raises( TypeError,
'if passed a node value that is not an IsNode instance'
)
@describe.returns(
'True if the supplied node is the same node-object as '
'the instance, False otherwise'
)
def isSameNode( self, node ):
"""
Determines if a supplied node is the same node-object as
the instance"""
if not isinstance( node, IsNode ):
raise TypeError( '%s.isSameNode expects an instance '
'of IsNode for comparison, but was passed '
'"%s" (%s)' % (
self.__class__.__name__,
node, type( node ).__name__
)
)
# If the node is the same object, it will
# have the same id, so:
return id( self ) == id( node )Normally, I'd also be looking to implement unit-tests of BaseNode,
now that all of its concrete implementation is complete. In this case, because all
of the *Sibling properties require participation in a node-tree
structure that won't be available until I have both IsElement
and a concrete class that derives from it implemented (Tag
in this case), I only went as far as getting the test-method requirements stubbed
out, along the lines of:
def testpreviousSibling(self):
"""Unit-tests the previousSibling property of a BaseNode instance."""
self.fail( 'testpreviousSibling is not implemented' )def testisSameNode(self):
"""Unit-tests the isSameNode method of a BaseNode instance."""
self.fail( 'testisSameNode is not implemented' )######################################## Unit-test results ######################################## Tests were successful ... False Number of tests run ..... 37 + Tests ran in ......... 0.01 seconds Number of errors ........ 0 Number of failures ...... 15 ########################################I could implement a
dummyclass in the test-module that derives from
BaseNode, and test against that class, and if there
weren't a concrete class expected that would serve that purpose, that's
exactly what I'd do. Since I will have one, eventually,
that just feels... wasteful, I guess, so I'd rather get Tag
operational and then come back to these tests. Until then, I'll just have to
live with these test-failures.
Defining the IsElement Interface
Between the previous post
and the breakdown of members needed in IsElement above,
there's really not much discussion needed, I think, nor a whole lot of code to show
and explain.
The Abstract Properties of IsElement
On basic principle, I did do another run through the members of an
element listed at the w3schools
site, just to ensure that I didn't miss any. What I netted out with for
properties in IsElement was:
childElementCount = abc.abstractproperty()
childNodes = abc.abstractproperty()
children = abc.abstractproperty()
firstChild = abc.abstractproperty()
firstElementChild = abc.abstractproperty()
lastChild = abc.abstractproperty()
lastElementChild = abc.abstractproperty()There were other properties that I had to think about too, though —
accessKey (which may well be globally available at the
implementation-level of a Tag), and attributes
(which absolutely is a Tag property). When push came to
shove, though I opted to implement those as concrete members of Tag
rather than drop them into the IsElement interface. The rationale
for that decision was mostly based on the realization that the only other
elements that I'm expecting to be concerned with are documents, and my
current plan is to derive a BaseDocument abstract class from
Tag anyway. In that scenario, all of the other properties and
methods would be implemented by Tag, and available to documents
through their derivation from BaseDocument anyway. Those
members include all tag-level properties that are also attributes in
markup, as well as any properties that aren't specifically related in some way
to having, working with, or altering a parent-child relationship between a
Tag and any other IsNode instance.
The Abstract Methods of IsElement
The same criteria noted above for properties was also used to cull
down the list of methods that would be required by IsElement,
for pretty much te same reasons. The resulting abstract methods are:
@abc.abstractmethod
def appendChild( self, child ):
raise NotImplementedError( '%s.appendChild is not implemented as '
'required by IsElement' % self.__class__.__name__ )
@abc.abstractmethod
def hasChildNodes( arg1, arg2=None, *args, **kwargs ):
raise NotImplementedError( '%s.hasChildNodes is not implemented as '
'required by IsElement' % self.__class__.__name__ )
@abc.abstractmethod
def insertBefore( self, newChild, existingChild ):
raise NotImplementedError( '%s.insertBefore is not implemented as '
'required by IsElement' % self.__class__.__name__ )
@abc.abstractmethod
def insertChildAt( self, newChild, index ):
raise NotImplementedError( '%s.insertChildAt is not implemented as '
'required by IsElement' % self.__class__.__name__ )
@abc.abstractmethod
def removeChild( self, child ):
raise NotImplementedError( '%s.removeChild is not implemented as '
'required by IsElement' % self.__class__.__name__ )
@abc.abstractmethod
def removeChildAt( self, index ):
raise NotImplementedError( '%s.removeChildAt is not implemented as '
'required by IsElement' % self.__class__.__name__ )
@abc.abstractmethod
def removeSelf( self ):
raise NotImplementedError( '%s.removeSelf is not implemented as '
'required by IsElement' % self.__class__.__name__ )
@abc.abstractmethod
def replaceChild( self, newChild, existingChild ):
raise NotImplementedError( '%s.RequiredMethod is not implemented as '
'required by IsElement' % self.__class__.__name__ )Testing the Abstract Members of IsElement
The unit-testing of the abstract members of IsElement follows
the pattern established by the testing of IsNode members shown in
my previous post, with the
hopefully-obvious change of class-name being tested:
def testPROPERTYNAME(self):
"""Unit-tests the PROPERTYNAME property of an IsElement instance."""
try:
testInstance = markup.IsElement()
except TypeError, error:
actual = 'PROPERTYNAME' in str( error )
self.assertTrue( actual, 'The TypeError raised by trying to '
'instantiate IsElement should include the "PROPERTYNAME" '
'abstract method-name' )
except Exception, error:
self.fail( 'testPROPERTYNAME expected a TypeError, '
'but %s was raised instead:\n - %s' % (
error.__class__.__name__, error
)
)def testMETHODNAME(self):
"""Unit-tests the METHODNAME method of an IsElement instance."""
try:
testInstance = markup.IsElement()
except TypeError, error:
actual = 'METHODNAME' in str( error )
self.assertTrue( actual, 'The TypeError raised by trying to '
'instantiate IsElement should include the "METHODNAME" '
'abstract method-name' )
except Exception, error:
self.fail( 'testMETHODNAME expected a TypeError, '
'but %s was raised instead:\n - %s' % (
error.__class__.__name__, error
)
)BaseNode, those same failures should still
appear, but the number of tests run and passed should increase:
######################################## Unit-test results ######################################## Tests were successful ... False Number of tests run ..... 56 + Tests ran in ......... 0.01 seconds Number of errors ........ 0 Number of failures ...... 15 ########################################
With the change made to the markup module's class-zoo (the
addition of IsElement in the upper right of the diagram), I
didn't get quite as far long in this post as I'd hoped before hitting my
post-length cut-off, but I feel like I made solid progress.
HasTextData. With
that done, I'll be able to knock out three concrete classes pretty
quickly, I think: CDATA, Comment and Text.
The completion of those will also require me to give some thought to exactly how I plan for rendered markup to be issued back out to a browser, so there will be at least some discussion around that as well.
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