[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Stefan Behnel]

Stefan Behnel  added the comment:

Florent, what you describe is exactly the definition of a new feature.
Users even have to change their code in order to make use of it.

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Florent Xicluna]

Florent Xicluna  added the comment:

Well, it fixes the behavior of ElementTree in some multi-threaded cases, 
provided you pass the namespace map as an argument of the serializer call.

The fix implements an optional argument for this use case.
As a side effect, it makes it easier to work with custom namespaces.

If the consensus is to wait for next version, I'm fine with that.

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Stefan Behnel]

Stefan Behnel  added the comment:

Looks like a new feature to me.

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Eli Bendersky]

Eli Bendersky  added the comment:

Can this be honestly classified as a bugfix though? If it's a feature it will 
have to be postponed to 3.4

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Florent Xicluna]

Florent Xicluna  added the comment:

Do we merge the patch for 3.3?
I'm +1 on this (patch submitted 8 months ago, backward compatible and reviewed).

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Florent Xicluna]

Florent Xicluna  added the comment:

Of course it's better to have someone else to review the patch.
However in this case, I'm not sure it is a major feature.

BTW, I noticed that effbot is currently marked as *inactive* maintainer
http://docs.python.org/devguide/experts.html#stdlib

If it is not an oversight, it means that this issue might wait "an extended 
period" before receiving a response.

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Stefan Behnel]

Stefan Behnel  added the comment:

Given that this is a major new feature for the serialiser in ElementTree, I 
think it's worth asking Fredrik for any comments.

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Stefan Behnel]

Changes by Stefan Behnel :


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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Florent Xicluna]

Florent Xicluna  added the comment:

Updated with documentation.
Thank you for the review.

I know this does not cover different namespaces in subtree.
But this use case seems very specific. The user could find other means to 
achieve it.

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Added file: 
http://bugs.python.org/file23895/issue13378_non_global_namespaces_v3.diff

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Florent Xicluna]

Florent Xicluna  added the comment:

Thank you Stefan for the comments.
I've added the prefix collision detection, and removed the **kw argument.
(+ tests)

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Stefan Behnel]

Stefan Behnel  added the comment:

Reading the proposed patch, I must agree that it makes more sense in 
ElementTree to support this as a serialiser feature. ET's tree model doesn't 
have a notion of prefixes, whereas it's native to lxml.etree.

Two major advantages of putting this into the serialiser are: 1) cET doesn't 
have to be modified, and 2) it does not require additional memory to store the 
nsmap reference on each Element. The latter by itself is a very valuable 
property, given that cET aims specifically at a low memory overhead.

I see a couple of drawbacks:

1) it only supports the case that namespaces are globally defined. The 
implementation cannot handle the case that local namespaces should only be 
defined in subtrees, or that prefixes are being reused. This is no real 
restriction because globally defined namespaces are usually just fine. It's 
more of an inconvenience in some cases, such as multi-namespace languages like 
SOAP or WSDL+XSD, where namespaces are commonly declared on the subtree where 
they start being used.

2) lxml.etree cannot support this because it keeps the prefixes in the tree 
nodes and uses them on serialisation. This cannot easily be overridden because 
the serialiser is part of libxml2.

I didn't see in the patch how (or if?) the prefix redefinition case is handled. 
Given that prefixes are always defined globally, it would be nice if this only 
resulted in an error if two namespaces that are really used in the document map 
to the same prefix, not always when the namespace dict is redundant by itself.

Also note that it's good to be explicit about the keyword arguments that a 
function accepts. It aids when help(tostring) tells you directly what you can 
pass in, instead of just printing "**kw".

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Stefan Behnel]

Stefan Behnel  added the comment:

Florent, thanks for the notification.

Nekmo, note that you are misusing this feature. The _namespace_map is meant to 
provide "well known namespace prefixes" only, so that common namespaces end up 
using the "expected" prefix. This is also the reason why it maps namespaces to 
prefixes and not the other way round. It is not meant to temporarily assign 
arbitrary prefix to namespaces. That is the reason for it being a global option.

That being said, lxml.etree's Element factory takes an "nsmap" parameter that 
implements the feature you want. It's documented here:

http://lxml.de/tutorial.html#namespaces

Note that it maps prefixes to namespaces and not the other way round. This is 
because there is a corresponding "nsmap" property on Elements that provides the 
currently defined prefixes in the context of an Element. ElementTree itself 
does not (and cannot) support this property because it drops the prefixes 
during parsing. However, I would still request that an implementation of the 
parameter to the Element() factory should be compatible for both libraries.

Also look for "nsmap" in the compatibility docs (appears in two sections):

http://lxml.de/compatibility.html

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Florent Xicluna]

Changes by Florent Xicluna :


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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Florent Xicluna]

Florent Xicluna  added the comment:

This patch proposes an implementation of the feature.


>>> from xml.etree import ElementTree as ET
>>> ET.tostring(ET.Element('{http://localhost/house}iq'), encoding="unicode", 
>>> namespaces={'http://localhost/house': 'home'})
'http://localhost/house"; />'

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Added file: 
http://bugs.python.org/file23655/issue13378_non_global_namespaces.diff

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Nekmo]

Nekmo  added the comment:

In my case, I have several clients, and they define the namespaces. I am 
interested in return the same namespace that they gave me, for example, the 
client "A" gives me this:

http://localhost/house"; />

To name the namespace, I set it at nsmap:

>>> import xml.etree.ElementTree as etree
>>> etree.register_namespace('house', 'http://localhost/house')
>>> etree._namespace_map
{'http://localhost/house': 'house',
 'http://purl.org/dc/elements/1.1/': 'dc',
 'http://schemas.xmlsoap.org/wsdl/': 'wsdl',
 'http://www.w3.org/1999/02/22-rdf-syntax-ns#': 'rdf',
 'http://www.w3.org/1999/xhtml': 'html',
 'http://www.w3.org/2001/XMLSchema': 'xs',
 'http://www.w3.org/2001/XMLSchema-instance': 'xsi',
 'http://www.w3.org/XML/1998/namespace': 'xml'}

Thus, keeping the name of the namespace:
>>> etree.tostring(etree.Element('{http://localhost/house}iq'))
b'http://localhost/house"; />'

But if I have a client "B", which uses a different name, and run in parallel, 
problems can occur:

http://localhost/house"; />

>>> import xml.etree.ElementTree as etree
>>> etree.register_namespace('home', 'http://localhost/house')
>>> etree._namespace_map
{'http://localhost/house': 'home',
 'http://purl.org/dc/elements/1.1/': 'dc',
 'http://schemas.xmlsoap.org/wsdl/': 'wsdl',
 'http://www.w3.org/1999/02/22-rdf-syntax-ns#': 'rdf',
 'http://www.w3.org/1999/xhtml': 'html',
 'http://www.w3.org/2001/XMLSchema': 'xs',
 'http://www.w3.org/2001/XMLSchema-instance': 'xsi',
 'http://www.w3.org/XML/1998/namespace': 'xml'}

Therefore, I ask that _namespace_map is within etree._Element instance, and not 
global

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Florent Xicluna]

Changes by Florent Xicluna :


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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Jesús Cea Avión]

Jesús Cea Avión  added the comment:

Tagging this as targeting 3.3.

Nekmo, could you possibly poste some code showing the problem?

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[issue13378] Change the variable "nsmap" from global to instance (xml.etree.ElementTree)

[Nekmo]

New submission from Nekmo :

Currently, the mapping of namespaces is global and can cause failures if 
multiple instances are used or in multithreading. The variable is in 
xml.etree.ElementTree._namespace_map. I ask it to be switched to 
xml.etree._Element instance.

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components: XML
messages: 147378
nosy: Nekmo
priority: normal
severity: normal
status: open
title: Change the variable "nsmap" from global to instance 
(xml.etree.ElementTree)
type: feature request
versions: Python 3.2

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