I would like to make diagrams with arrow heads. Using Inkscape, with the Fill and Stroke tool I can generate XML that looks like

  <defs id="defs11454">
    <marker style="overflow:visible;"  id="Arrow1Lend" ..

Programatically I can also read defs in Python with

def effect(self):

        doc = self.get_template(width=w, height=h)
        svg = doc.getroot()
        defs = svg.defs

In order to set a new definition I think I should pass something into get_template(). If someone has done this before and can share an example .. well that would be marvellous :-)

Thanks

I've not made <defs> before:

You shouldn't need to create a <defs> element if the file is open in Inkscape, as it will be automatically added ( you can see that in the Edit>XML Editor panel ) 

If you write a simple extension which deletes the <defs> element - Inkscape automatically regenerates it as soon as the extension exits.

Maybe this will do what you want: 

import inkex
from lxml import etree

def add_to_defs(self, my_object):

    my_object_id = my_object.get_id()

    svg_defs = self.svg.xpath('//svg:defs')
    svg_defs[0].append(my_object)

    parent = self.svg
    my_use_object = etree.SubElement(parent, inkex.addNS('use', 'svg'))
    my_use_object.set('xlink:href', '#' + str(my_object_id))

class DefsMan(inkex.EffectExtension):

    def effect(self):

        my_selected = self.svg.selected
        # Make sure something is selected
        if len(my_selected) > 0:
            for my_object in my_selected:
                add_to_defs(self, my_object)

if __name__ == '__main__':
    DefsMan().run()

Sorry it took a while to reply .. I got seriously distracted with another issue :-)

And thanks for sharing your code. It worked first time! But then I tried to extend it and Inkscape started crashing. Looking at the XML that is dumped prior to the crash, I can see that <defs /> contains the marker. Here is my amended version. Any idea what causes the crash?

import inkex
from lxml import etree
from inkex import Marker

def marker():
  m = Marker()
  m.set('style', "overflow:visible")
  m.set_id("Arrow1Lend")
  m.set('refX', "0.0")
  m.set('refY', "0.0")
  m.set('orient', "auto")
  m.set('inkscape:stockid', "Arrow1Lend")
  m.set('inkscape:isstock', "true")
  return m

def add_to_defs(self, my_object):

    my_object_id = my_object.get_id()

    svg_defs = self.svg.xpath('//svg:defs')
    svg_defs[0].append(my_object)

    parent = self.svg
    my_use_object = etree.SubElement(parent, inkex.addNS('use', 'svg'))
    my_use_object.set('xlink:href', '#' + str(my_object_id))

class DefsMan(inkex.EffectExtension):

    def effect(self):

        marker_to_add = marker()
        add_to_defs(self, marker_to_add)

if __name__ == '__main__':
    DefsMan().run()

I think it might be because the <marker> you are adding is empty.

from inkex import Circle

I added this in the marker block

my_circle = Circle()
my_circle.set("cx", "20")
my_circle.set("cy", "30")
my_circle.set("r", "10")

m.append(my_circle)

It's working now 😊

To finish the story here is a beautiful arrow and some ugly code. 

import inkex
from lxml import etree
from inkex import Marker, TextElement, Ellipse, PathElement, Transform

class DefsMan(inkex.EffectExtension):

  def add_to_defs(self, marker):
    svg_defs = self.svg.xpath('//svg:defs')
    svg_defs[0].append(marker)

  def effect(self):

    m = Marker()
    m.set_id('m')
    p = PathElement()
    p.set_path("M 0.0,0.0 L 5.0,-5.0 L -12.5,0.0 L 5.0,5.0 L 0.0,0.0 z ")
    p.transform = Transform(rotate=(180), scale=(0.5))
    p.set_id('p')
    m.append(p)
    self.add_to_defs(m)

    s = self.svg

    e1 = Ellipse(cx='45', cy='112', rx="12", ry="10")
    e1.set_id('e1')
    e1.style = { "fill": "#ccc", "stroke": "#ccc" }
    s.add(e1)

    e2 = Ellipse(cx='145', cy='112', rx="12", ry="10")
    e2.set_id('e2')
    e2.style = { "fill": "#ccc", "stroke": "#ccc" }
    s.add(e2)

    p1 = PathElement()
    p1.set_path('m 57,112 h 70')
    p1.set_id('p1')
    p1.style = { "marker-end": "url(#m)", "stroke": "#000" }
    s.add(p1)

    t1 = TextElement(x='45', y='114')
    t1.style = { 'text-anchor': 'middle', 'text-align': 'center' }
    t1.text = 'A'
    t1.set_id('t1')
    s.add(t1)

    t2 = TextElement(x='145', y='114')
    t2.style = { 'text-anchor': 'middle', 'text-align': 'center' }
    t2.text = 'B'
    t2.set_id('t2')
    s.add(t2)

if __name__ == '__main__':
  DefsMan().run()

Well the arrow was beautiful before the upload crushed it 😭

I'm glad it all worked out :)