insert method in ElementTree

Discussion in 'Python' started by mirandacascade@yahoo.com, Jul 16, 2006.

  1. Guest

    O/S: Win2K
    Vsn of Python: 2.4

    Example:

    <a>
    <b createAnotherWhenCondition="x">
    <c>text for c</c>
    <d>text for d</d>
    </b>
    <e>
    <f>text for f</f>
    <g>text for g</g>>
    </e>
    <h>
    <i>text for i</i>
    <j createAnotherWhenCondition="y">
    <k>text for k</k>
    <l>text for l</l>
    </j>
    <m>text for m</m>
    </h>
    </a>

    Python script reads XML document above into ElementTree. The script
    outputs an XML document that is based on the XML document above. The
    output XML will contain all of the elements in the XML document above.
    If there is a parent element that does not have the
    createAnotherWhencondition attribute, then that element and its
    descendants will be part of the output XML. If there is a parent
    element with atribute createAnotherWhenCondition and that condition is
    true, then the output XML should contain another instance of the parent
    element below the original instance. For example if x were true when
    the script ran, the output XML document would be:

    <a>
    <b>
    <c>text for c</c>
    <d>text for d</d>
    </b>
    <b>
    <c>text for c</c>
    <d>text for d</d>
    </b>
    <e>
    <f>text for f</f>
    <g>text for g</g>>
    </e>
    <h>
    <i>text for i</i>
    <j>
    <k>text for k</k>
    <l>text for l</l>
    </j>
    <m>text for m></m>
    </h>
    </a>

    The example attempts to illustrate that the createAnotherWhenCondition
    attribute may appear in parent elements that are at different levels in
    the hierarchy; the <b> element is at the 2nd level in the hierarchy,
    and the <j> element is at the 3rd level. There will never be
    'nesting', i.e. a parent element with the
    createAnotherWhenCondition attribute that is a descendant of a parent
    element with a createAnotherWhenCondition attribute.

    I'm pretty sure I can figure out how to create the output XML by
    creating a second XML document. It would be created by iterating
    through the input XML. When a new element is encountered, an element
    or subelement would be created in the output XML. When a parent
    element with the createAnotherWhenCondition is encountered and that
    condition is true, I think I can figure out how to propagate another
    instance of that parent element and all its descendants.

    My request for advice is this: instead of creating a second XML
    document that represents the output, would it be possible to expand the
    input XML document as needed? I was thinking that the program could
    iterate through all the elements. As it is iterating, it would check
    for the createAnotherWhenCondition attribute. If encountered and if
    the condition were true, the program would:
    - make a copy of the parent element (perhaps with copy.copy)
    - use the insert method to insert the just-created copy
    Where I'm struggling is figuring out what the index argument should
    be in the insert method. Using the example above

    # assume rootElement is the root of the input XML
    xList = rootElement.getiterator()
    idx = 0
    for x in xList:
    # mix of pseudo-code and Python code
    if (this element has createAnotherWhenCondition attribute)
    and
    (y is true):
    jcopy = copy.copy(x)
    ??.insert(??, jcopy)
    idx = idx + 1

    If the program were run when y was true, then I believe it would
    encounter the <j> element when idx has a value of 9. Conceptually, I
    think that jcopy should be inserted after the <j> element and before
    the <m> element. But I'm not sure that 9 (the index in the list
    created from rootElement.getiterator()) has any relevance for this
    insert task. Assuming that I want to insert jcopy after the <j>
    element and before the <m> element:
    a) would the insert need to be done relative to the <h> element, which
    is the parent of <j>
    b) if so, would the index argument in the insert method be relative
    index within the <h> element?
    , Jul 16, 2006
    #1
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  2. wrote:
    > Where I'm struggling is figuring out what the index argument should
    > be in the insert method. Using the example above
    >
    > # assume rootElement is the root of the input XML
    > xList = rootElement.getiterator()
    > idx = 0
    > for x in xList:
    > # mix of pseudo-code and Python code
    > if (this element has createAnotherWhenCondition attribute)
    > and
    > (y is true):
    > jcopy = copy.copy(x)
    > ??.insert(??, jcopy)
    > idx = idx + 1


    ElementTree does not have parent pointers. You have to look one element level
    ahead to add the child. You can use a parent stack for this, although
    getiterator() is not very helpful in that case as it does not tell you when it
    backtracks.

    If you want to use lxml instead, you can either
    * access the parent directly (element.getparent()) and add the child there
    or
    * use the iterwalk() function to add the child when backtracking up the tree or
    * implement the whole thing in XSLT (with a custom evaluator function in Python).

    There may also be other ways to do it. Just give it a try:
    http://codespeak.net/lxml/
    http://cheeseshop.python.org/pypi/lxml/1.1alpha

    Stefan
    Stefan Behnel, Jul 16, 2006
    #2
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  3. Carl Banks Guest

    wrote:

    > My request for advice is this: instead of creating a second XML
    > document that represents the output, would it be possible to expand the
    > input XML document as needed? I was thinking that the program could
    > iterate through all the elements. As it is iterating, it would check
    > for the createAnotherWhenCondition attribute. If encountered and if
    > the condition were true, the program would:
    > - make a copy of the parent element (perhaps with copy.copy)
    > - use the insert method to insert the just-created copy
    > Where I'm struggling is figuring out what the index argument should
    > be in the insert method.


    I recommend not using the insert method; instead, build a new list of
    elements and set the parent to the new list. Another problem is that
    you have to have the parent node around, since children don't indicate
    their parents. Using getiterator won't work. I recommend a recursive
    function instead. Something like this should do it:

    def expand_children(parent):
    if len(parent) == 0:
    return
    newchildren = []
    for child in parent:
    expand_children(child) # recursively process child nodes
    if <you're supposed to create another child>:
    <remove createAnotherWhenCondition attr>
    newchildren.append(child) # or copy.copy(child)
    newchildren.append(child)
    parent[:] = newchildren # slice assign

    So, basically, for each node, you build a list of children in parallel,
    making duplicates as necessary, then use slice assignment to set the
    parent's children to be the new list. No more mucking around with
    indexes.


    Carl Banks
    Carl Banks, Jul 16, 2006
    #3
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