xml - python - lxml: enforcing a specific order for attributes

Question

I have an XML writing script that outputs XML for a specific 3rd party tool.

I've used the original XML as a template to make sure that I'm building all the correct elements, but the final XML does not appear like the original.

I write the attributes in the same order, but lxml is writing them in its own order.

I'm not sure, but I suspect that the 3rd part tool expects attributes to appear in a specific order, and I'd like to resolve this issue so I can see if its the attrib order that making it fail, or something else.

Source element:

<FileFormat ID="1" Name="Development Signature" PUID="dev/1" Version="1.0" MIMEType="text/x-test-signature"> 

My source script:

sig.fileformat = etree.SubElement(sig.fileformats, "FileFormat", ID = str(db.ID), Name = db.name, PUID="fileSig/{}".format(str(db.ID)), Version = "", MIMEType = "")

My resultant XML:

<FileFormat MIMEType="" PUID="fileSig/19" Version="" Name="Printer Info File" ID="19">

Is there a way of constraining the order they are written?

Answer 1

It looks like lxml serializes attributes in the order you set them:

>>> from lxml import etree as ET
>>> x = ET.Element("x")
>>> x.set('a', '1')
>>> x.set('b', '2')
>>> ET.tostring(x)
'<x a="1" b="2"/>'
>>> y= ET.Element("y")
>>> y.set('b', '2')
>>> y.set('a', '1')
>>> ET.tostring(y)
'<y b="2" a="1"/>'

Note that when you pass attributes using the ET.SubElement() constructor, Python constructs a dictionary of keyword arguments and passes that dictionary to lxml. This loses any ordering you had in the source file, since Python's dictionaries are unordered (or, rather, their order is determined by string hash values, which may differ from platform to platform or, in fact, from execution to execution).