2007/4/19, Martijn Faassen <[EMAIL PROTECTED]>:
Hi there,
I just tracked down what I believe is a bug in the Zope 3 page template
engine. I didn't know it was a bug in the Zope 3 page template engine as
I was tracking down a bug in Silva with Zope 2.10, which of course uses
the Zope 3 page template engine. For the proposed solution, see the
bottom of the mail.
Let's consider the following code fragment:
<img tal:replace="structure python:'foo'" tal:attributes="title
python:'bar'" />
In Zope 2 before Zope 2.10, this line works without error. The
tal:attributes line of course is pretty useless, as the whole img tag
gets replaced anyway, but it doesn't bail out with an error.
But those attributes *do* work. If you call that image without the
attributes, it won't have a title attribute. If you keep the tag as it
is, it will have a title attribute.
In fact, you can override an attribute that is provided by the
replace, since the attributes are executed later. For instance you can
override the alt attribute, which in the replace might be taken from
the image's title, with some other string that's more informative.
In Zope 2.10 and Zope 3.3, this gives us the following error [taken from
Zope 3]:
File
"/home/faassen/buildout/z331/lib/python/zope/tal/talinterpreter.py",
line 271, in __call__
self.interpret(self.program)
File
"/home/faassen/buildout/z331/lib/python/zope/tal/talinterpreter.py",
line 346, in interpret
handlers[opcode](self, args)
File
"/home/faassen/buildout/z331/lib/python/zope/tal/talinterpreter.py",
line 534, in do_optTag_tal
self.no_tag(stuff[-2], stuff[-1])
File
"/home/faassen/buildout/z331/lib/python/zope/tal/talinterpreter.py",
line 516, in no_tag
self.interpret(program)
File
"/home/faassen/buildout/z331/lib/python/zope/tal/talinterpreter.py",
line 346, in interpret
handlers[opcode](self, args)
File
"/home/faassen/buildout/z331/lib/python/zope/tal/talinterpreter.py",
line 760, in do_insertStructure_tal
self.insertHTMLStructure(text, repldict)
File
"/home/faassen/buildout/z331/lib/python/zope/tal/talinterpreter.py",
line 784, in insertHTMLStructure
gen = AltTALGenerator(repldict, self.engine, 0)
File
"/home/faassen/buildout/z331/lib/python/zope/tal/talinterpreter.py",
line 65, in __init__
TALGenerator.__init__(self, expressionCompiler, xml)
File
"/home/faassen/buildout/z331/lib/python/zope/tal/talgenerator.py", line
41, in __init__
self.CompilerError = expressionCompiler.getCompilerError()
AttributeError: 'ZopeContext' object has no attribute 'getCompilerError'
The difference between Zope 2 and Zope 3 is that ZopeContext is defined
somewhere else, but that is irrelevant to this error.
Let's take a look at talgenerator.py, line 41:
self.CompilerError = expressionCompiler.getCompilerError()
An AttributeError occurs here as getCompilerError does not exist. That's
correct, as ZopeContext doesn't define one, nor should it. We should've
gotten a ZopeEngine instance here, and in other code paths, that's what
we get. This provides the interface ITALExpressionCompiler, which has
getCompilerError.
What is calling talgenerator with the wrong argument for
expressionCompiler? Let's go to talinterpreter, around line 784:
def insertHTMLStructure(self, text, repldict):
from zope.tal.htmltalparser import HTMLTALParser
gen = AltTALGenerator(repldict, self.engine, 0)
That looks innocent enough. 'self.engine' is passed into AltTALGenerator
(which in turns ends up in __init__ of TALGenerator). Unfortunately
self.engine is very very badly named and is in fact a ZopeContext instance!
Why does this bug not happen more often? Because insertHTMLStructure is
*only* called if tal:attributes there. Let's go to the relevant section,
do_insertStructure_tal in talinterpreter.py:
def do_insertStructure_tal(self, (expr, repldict, block)):
...
if not (repldict or self.strictinsert):
# Take a shortcut, no error checking
self.stream_write(text)
return
if self.html:
self.insertHTMLStructure(text, repldict)
else:
self.insertXMLStructure(text, repldict)
What is going on here is that normally, if no repldict exists (this is
generated by tal:attributes), stream_write(text) is called. This works
just fine.
In the case where there *is* a repldict, the broken version of
insertHTMLStructure is called, leading to this error.
Now what's the hacky fix? It turns out that self.engine has an attribute
_engine, that *is* the expression compiler. We can therefore fix the
broken insertHTMLStructure like this:
def insertHTMLStructure(self, text, repldict):
from zope.tal.htmltalparser import HTMLTALParser
gen = AltTALGenerator(repldict, self.engine._engine, 0)
That's ugly however. I looked at the old Zope 2 implementation, and that
looks prettier:
def insertHTMLStructure(self, text, repldict):
from zope.tal.htmltalparser import HTMLTALParser
gen = AltTALGenerator(repldict, self.engine.getCompiler(), 0)
Unfortunately, 'getCompiler' doesn't exist in Zope 3, so that won't work
unless we add it again to the appropriate interface.
I imagine adding this back would be the best way to go. Anyone have any
objections? I will go and fix this in Zope 3.3 and trunk. I will add a
test. I will need to make sure the interface also has getCompiler() added.
This might also be a good occasion for a new Zope 2.10 bugfix release.
Silva is running into this currently, though of course now that we know
what the cause is we can work around it.
Just please keep the attributes working with replace :-)
Kit
--
Kit BLAKE · Infrae · http://infrae.com/ + 31 10 243 7051
Hoevestraat 10 · 3033 GC · Rotterdam + The Netherlands
OpenPGP 0xE67AD0F2 · Contact = http://xri.net/=kitblake
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