C[omp]ute

From: andrew cooke <andrew@...>

Date: Fri, 23 Jul 2010 10:07:27 -0400

There's not much love for Python 2.4 and SVG (which, I admit is something of
an odd combination - who would be so conservative they would use Python 2.4
and then require SVG?), particularly if you need a non-GPL solution.  So I
ended up wiring a simple wrapper around DOM that helps generate the XML.
Here's a summary of the approach (there's nothing really hard here, just
fiddly DOM details):

class SvgBase(object):
    '''
    Support class.  Contains a reference to the DOM document and
    the element corresponding to this node.  Note that we allow any
    node to be extended with any child node; we also allow any 
    attribute to be added.
    '''
    
    SVG = 'http://www.w3.org/2000/svg'
    
    def __init__(self, doc, element, **attrs):
        self._doc = doc
        self._element = element
        for name in attrs:
            value = attrs[name]
            if value is not None:
                self._element.setAttributeNS(self.SVG, name, str(value))
        
    def line(self, (x1, y1), (x2, y2), **attrs):
        '''
        Add a child line
        '''
        return Line(self, (x1, y1), (x2, y2), **attrs)
       
    # more child metods here.....


class Svg(SvgBase):
    '''
    The root svg element.  This creates the document, allows a 
    stylesheet to be used, etc.
    '''

    def __init__(self, version='1.1', width=None, height=None):
        implementation = getDOMImplementation('')
        doctype = implementation.createDocumentType('svg', 
            '-//W3C//DTD SVG 1.1//EN', 
            'http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd')
        document = implementation.createDocument(self.SVG, 'svg', doctype)
        for node in document.childNodes:
            if node.nodeType == Node.ELEMENT_NODE:
                element = node
                break
        # xlink lets "a" element work correctly (use xlink:href)
        ns = {'xmlns': self.SVG,
              'xmlns:xlink': 'http://www.w3.org/1999/xlink'}
        super(Svg, self).__init__(document, element, 
                                  version=version, width=width, height=height,
                                  **ns)
        
    def add_stylesheet(self, url):
        style = self._doc.createProcessingInstruction('xml-stylesheet',
                        'href="%s" type="text/css"' % url)
        for node in self._doc.childNodes:
            if node.nodeType == Node.DOCUMENT_TYPE_NODE:
                self._doc.insertBefore(style, node)
                break

    def __str__(self):
        '''
        The XML of the entire document.
        '''
        return self._doc.toxml()
    

class Line(SvgBase):
    '''
    An example child node.
    '''

    def __init__(self, parent, (x1, y1), (x2, y2), **attrs):
        element = parent._doc.createElementNS(self.SVG, 'line')
        super(Line, self).__init__(parent._doc, element,
                                   x1=self._1dp(x1), y1=self._1dp(y1), 
                                   x2=self._1dp(x2), y2=self._1dp(y2), 
                                   **attrs)
        parent._element.appendChild(self._element)


With that, it's pretty easy to do things like:

    >>> svg = Svg()
    >>> svg.line((1,2), (3,4), stroke='blue')
    >>> svg.add_stylesheet('http://example.com/foo')
    >>> str(svg)
    '''<?xml version="1.0" ?>
       <?xml-stylesheet href="http://example.com/foo" type="text/css"?>
       <!DOCTYPE svg  PUBLIC '-//W3C//DTD SVG 1.1//EN'  
                 'http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd'>
       <svg version="1.1" xmlns="http://www.w3.org/2000/svg"
            xmlns:xlink="http://www.w3.org/1999/xlink">
         <line x1="1.0" x2="3.0" y1="2.0" y2="4.0" stroke="blue"/>
       </svg>'''

Andrew