minidom + wxPython woes

[Lonnie Princehouse]

Hi all,

I'm getting a seg fault when I try to use minidom to parse some XML
inside a wxPython app.

I was wondering if someone else could run the simple code below on
Linux and, if it doesn't crash horribly, post which versions of
(Python, wxPython) they are using? I can't find other messages related
to this, so I suspect it is something broken with my installation.
I'm using Python 2.4.2 and wx 2.6.1.0.

Incidentally, writing XML with minidom works fine, and this is all
happening in wxPython's main thread.


import wx
from xml.dom import minidom

app = wx.PySimpleApp( )
frame = wx.Frame(None, -1, "Hello World")
frame.Show(True )
button = wx.Button(frame , -1, "Click me")

testxml = 'xml version="1.0" ?><foo></foo>'

def click(event):
doc = minidom.parseSt ring(testxml) # seg fault?!
print "Success!"

frame.Bind(wx.E VT_BUTTON, click, button)

app.MainLoop()

[Lonnie Princehouse]

Re: minidom + wxPython woes

Oops, I missed a bracket... that should read:

testxml = '<xml version="1.0" ?><foo></foo>'

But it still crashes ;-)

[Peter Hansen]

Re: minidom + wxPython woes

Lonnie Princehouse wrote:[color=blue]
> Oops, I missed a bracket... that should read:
>
> testxml = '<xml version="1.0" ?><foo></foo>'
>
> But it still crashes ;-)[/color]

Maybe missing a question mark still too?

<?xml version="1.0"?>

(It's like a processing instruction, not an element.)

-Peter

[Frank Millman]

Re: minidom + wxPython woes


Lonnie Princehouse wrote:[color=blue]
> Hi all,
>
> I'm getting a seg fault when I try to use minidom to parse some XML
> inside a wxPython app.
>
> I was wondering if someone else could run the simple code below on
> Linux and, if it doesn't crash horribly, post which versions of
> (Python, wxPython) they are using? I can't find other messages related
> to this, so I suspect it is something broken with my installation.
> I'm using Python 2.4.2 and wx 2.6.1.0.
>
> Incidentally, writing XML with minidom works fine, and this is all
> happening in wxPython's main thread.
>
>
> import wx
> from xml.dom import minidom
>
> app = wx.PySimpleApp( )
> frame = wx.Frame(None, -1, "Hello World")
> frame.Show(True )
> button = wx.Button(frame , -1, "Click me")
>
> testxml = 'xml version="1.0" ?><foo></foo>'
>
> def click(event):
> doc = minidom.parseSt ring(testxml) # seg fault?!
> print "Success!"
>
> frame.Bind(wx.E VT_BUTTON, click, button)
>
> app.MainLoop()[/color]

This sounds similar to a problem I reported a few months ago. This is
the link.

http://groups.google.com/group/comp.lang.python/browse_frm/thread/6fc1097d26083e43/5fbdf493f3c38942?q=&rnum=1&hl=en#5fbdf493f3c38942

In my case, it turned out to be a bug in the pyexpat module - it is
known about, but for some reason difficult to fix, so it is still
there.

I found a workaround, which is documented in the above thread.

HTH

Frank Millman

[Fredrik Lundh]

Re: minidom + wxPython woes

Frank Millman wrote:
[color=blue]
> This sounds similar to a problem I reported a few months ago. This is
> the link.
>[/color]
http://groups.google.com/group/comp....bdf493f3c38942[color=blue]
>
> In my case, it turned out to be a bug in the pyexpat module - it is
> known about, but for some reason difficult to fix, so it is still
> there.[/color]

no, it's not a bug in the pyexpat module -- the problem is that
wxPython uses it's own incompatible version of the expat library,
and loads it in a way that causes problems for any library that's
tries to use its own statically linked version.

see MvL's comments in the sourceforge tracker for more info.

</F>

[Frank Millman]

Re: minidom + wxPython woes


Fredrik Lundh wrote:[color=blue]
> Frank Millman wrote:
>[color=green]
> > This sounds similar to a problem I reported a few months ago. This is
> > the link.
> >[/color]
> http://groups.google.com/group/comp....bdf493f3c38942[color=green]
> >
> > In my case, it turned out to be a bug in the pyexpat module - it is
> > known about, but for some reason difficult to fix, so it is still
> > there.[/color]
>
> no, it's not a bug in the pyexpat module -- the problem is that
> wxPython uses it's own incompatible version of the expat library,
> and loads it in a way that causes problems for any library that's
> tries to use its own statically linked version.
>
> see MvL's comments in the sourceforge tracker for more info.
>
> </F>[/color]

I had a look at the sourceforge tracker. I did not understand much of
it - rather too technical for me. There are two points worth noting.

Firstly, it seems from various posts to the tracker item that the same
problem has been reported with pygtk, Qt, and VTK.

Secondly, in tracker item 1295808 (which, according to the notes, is
actually the same bug), there is talk of submitting a patch in 2.5 to
address the issue.

It seems to me (FWIW - as I said, I do not really understand much of
what I read) that it may not technically be a bug in pyexpat, but there
is a real issue there, and the decision has been taken to make a change
to pyexpat so that the problem will not arise in the future.

Frank

[Paul Boddie]

Re: minidom + wxPython woes

Frank Millman wrote:[color=blue]
> Fredrik Lundh wrote:[color=green]
> >
> > no, it's not a bug in the pyexpat module -- the problem is that
> > wxPython uses it's own incompatible version of the expat library,
> > and loads it in a way that causes problems for any library that's
> > tries to use its own statically linked version.[/color][/color]

[...]
[color=blue]
> Firstly, it seems from various posts to the tracker item that the same
> problem has been reported with pygtk, Qt, and VTK.[/color]

There used to be issues with Expat, PyXML and mod_python which may be
vaguely related to this, mostly because there was some usage of Expat
within some Apache component which conflicted with PyXML's Expat
configuration. In the end, I just dropped PyXML and started using other
libraries not affected by such issues, and I'm not totally sure that
anyone really resolved the problem definitively (although this was
possibly four or five years ago, so a lot can have happened since).

Paul

[Lonnie Princehouse]

Re: minidom + wxPython woes

I'm hesitant to resort to tricks like "import pyexpat before wx, so
that symbols are loaded from the right library".

Luckily, I stumbled onto pxdom. It's a pure-python DOM implementation,
and switching to it was as easy as this:

# import xml.dom.minidom as dom
import pxdom as dom

[grahamd@dscpl.com.au]

Re: minidom + wxPython woes

Paul Boddie wrote:[color=blue]
> Frank Millman wrote:[color=green]
> > Fredrik Lundh wrote:[color=darkred]
> > >
> > > no, it's not a bug in the pyexpat module -- the problem is that
> > > wxPython uses it's own incompatible version of the expat library,
> > > and loads it in a way that causes problems for any library that's
> > > tries to use its own statically linked version.[/color][/color]
>
> [...]
>[color=green]
> > Firstly, it seems from various posts to the tracker item that the same
> > problem has been reported with pygtk, Qt, and VTK.[/color]
>
> There used to be issues with Expat, PyXML and mod_python which may be
> vaguely related to this, mostly because there was some usage of Expat
> within some Apache component which conflicted with PyXML's Expat
> configuration.[/color]

FYI, the incompatibility issues that arise with pyexpat in mod_python
are well documented at:

http://www.dscpl.com.au/articles/modpython-006.html

Graham

[Paul Boddie]

Re: minidom + wxPython woes

grah...@dscpl.c om.au wrote:[color=blue]
>
> FYI, the incompatibility issues that arise with pyexpat in mod_python
> are well documented at:
>
> http://www.dscpl.com.au/articles/modpython-006.html[/color]

Nice document! Is there some possibly-similar explanation of character
encoding issues with mod_python and Expat somewhere, too, or is that
problem long forgotten?

Paul