Modifying a paragraph using python-docx and xml

Question

I want to change a paragraph of a document by changing the XML. For example I need to change the indent after a paragraph:

import docx
from docx.oxml import OxmlElement
from docx.oxml.ns import qn

doc = docx.Document('example.docx')
paragraphs = doc.paragraphs

for paragraph in doc.paragraphs:
    p = paragraph._p 
    pPr = OxmlElement('w:pPr')
    spacingp = OxmlElement('w:spacing')
    pPr.append(spacingp)
    p.append(pPr)
    afterp = OxmlElement('w:after')
    afterp.set(qn('w:after'), '0') 
    lineRulep = OxmlElement('w:lineRule')
    lineRulep.set(qn('w:lineRule'), 'auto')
    linep = OxmlElement('w:line')
    linep.set(qn('w:line'), '240')
    spacingp.append(afterp)
    spacingp.append(lineRulep)
    spacingp.append(linep)
    pPr.append(spacingp)
    p.append(pPr)
doc.save('restyled.docx')

But this code doesn't work. Please tell me why it doesn't work?