provide pretty printer for xml.etree.ElementTree

[ericsnowcurrently]

BPO	17372
Nosy	@ericsnowcurrently
Superseder	bpo-14465: xml.etree.ElementTree: add feature to prettify XML output
Files	issue17372.patch: An implementation of XML pretty-printing for ElementTree

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2013-08-03.22:25:43.833>
created_at = <Date 2013-03-07.07:18:45.367>
labels = ['expert-XML', 'type-feature', 'library']
title = 'provide pretty printer for xml.etree.ElementTree'
updated_at = <Date 2013-08-05.16:45:28.327>
user = 'https://github.com/ericsnowcurrently'

bugs.python.org fields:

activity = <Date 2013-08-05.16:45:28.327>
actor = 'lpapp'
assignee = 'none'
closed = True
closed_date = <Date 2013-08-03.22:25:43.833>
closer = 'eli.bendersky'
components = ['Library (Lib)', 'XML']
creation = <Date 2013-03-07.07:18:45.367>
creator = 'eric.snow'
dependencies = []
files = ['30845']
hgrepos = []
issue_num = 17372
keywords = ['patch']
message_count = 6.0
messages = ['183635', '192558', '192562', '194062', '194312', '194491']
nosy_count = 4.0
nosy_names = ['eli.bendersky', 'eric.snow', 'alex.henderson', 'lpapp']
pr_nums = []
priority = 'low'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '14465'
type = 'enhancement'
url = 'https://bugs.python.org/issue17372'
versions = ['Python 3.4']

[ericsnowcurrently]

minidom already has something like this:

http://docs.python.org/3/library/xml.dom.minidom.html#xml.dom.minidom.Node.toprettyxml

This is something that appears to have almost made it in quite a while ago:

http://effbot.org/zone/element-lib.htm#prettyprint
http://svn.effbot.org/public/stuff/sandbox/elementlib/indent.py

A casual search found several similar implementations out there. For instance:

http://infix.se/2007/02/06/gentlemen-indent-your-xml

[alexhenderson]

I have attached a proposed patch.

This makes some design decisions which I would like someone to review:
a) To incorporate pretty-printing into the main write() method rather than adding a separate toprettyxml() method. Disadvantages: greater complexity of _serialize_xml(). Advantages: Reduced duplication of code, easy to add other pretty-printing (eg HTML) in the same way.
b) Existing whitespace on the ends of existing text is mutated. Disadvantages: existing whitespace content may get changed. Advantages: Greater readability (which is the whole point), idempotence of pretty-printing.
c) Not to add a trailing newline. I am undecided as to whether this is a bad idea or a good one, but am documenting it to ensure it gets visibility.

Of these, I think b) is the only potentially controversial one, and notably its behaviour differs from minidom's toprettyxml. I think it's the right thing to do though; and for the cases where whitespace is important, perhaps we can respect the xml:space attribute when pretty-printing?
http://www.w3.org/TR/xml/#sec-white-space

If these design choices are deemed suitable I'm happy to update the patch to support pretty-printing HTML also.

[alexhenderson]

One other design decision - currently it doesn't deal with the indentation of comments or processing instructions: it leaves them unindented. Should they be indented the same as other tags?

[elibendersky]

Thanks for the report (Eric) and the patch (Alex). There are currently some open bugs we need to handle first, so this is somewhat lower priority. I hope to get to it before the 3.4 release.

[elibendersky]

I've noticed this is a duplicate of issue bpo-14465. Closing it - let's continue the discussion there, when the time comes.

[lpapp]

This has just made me switching away from xml.etree.ElementTree today, sadly.

What a pity; it would have been all kind of cool to stick to this minimal, otherwise working parser and builder.