__getattr__ and __setattr__ from custom python objects.

[mcot]

I want to be able to use __getattr__ and __setattr__ from my Python
code for dynamic property access. I am noticing some strange behavior
with PyV8.

import PyV8

class Global(PyV8.JSClass):

def __init__(self):
self.x = 10
self.y = self

def write(self, val):
print val

def __setattr__(self, name, value):

print "Set Attribute: %s %s" % (name, value)
object.__setattr__(self, name, value)

def __getattr__(self, name):

print "Get Attribute: %s" % name

with PyV8.JSContext(Global()) as ctx:
ctx.eval("""

write(x);
y.z = 10;
write(z);
""")

With the above code sample I get an output of:


Set Attribute: x 10
Set Attribute: y <__main__.Global object at 0x7faf8f597d50>
10
Get Attribute: z
Set Attribute: z 10
10

I am wondering why __getattr__ gets called when I am setting a
property. Standard python would usually yield this:

class Global(object):

def __init__(self):
self.x = 10
self.y = self

def write(self, val):
print val

def __setattr__(self, name, value):

print "Set Attribute: %s %s" % (name, value)
object.__setattr__(self, name, value)

def __getattr__(self, name):

print "Get Attribute: %s" % name

#return PyV8.JSClass.__getattr__(self, name)

x = Global()
x.y.z = 10


Output:


Set Attribute: x 10
Set Attribute: y <__main__.Global object at 0x7f5f134da790>
Set Attribute: z 10



I am also wondering about __getattr__ in PyV8.py

def __getattr__(self, name):
if name == 'constructor':
return JSClassConstructor(self.__class__)

raise AttributeError(name)

I used the code from the first example but added "return
PyV8.JSClass.__getattr__(self, name)" to my own __getattr__. An
AttributeError exception from the "Get Attribute: z" is being raised
as I would expect, but then it is being caught somewhere. I am trying
to figure out where and why and how the exception gets handled.

Thanks in advance.

[Flier Lu]

The first question was caused by a design problem. Because PyV8
support to use a Python map object as a Javascript object. It means,
if you pass a map object to Javascript, and get/set its property, if
the property doesn't exists, the value will be read from or add to the
map. To implement it, pyv8 must use PyObject_HasAttrString to check
whether the property exists, which will call the __getattr__. So, if
the behavior real broke your code, I think we could remove the check
code, and force to get/set value when the object is a python map
object.

The second question was caused by the Javascript standard. When you
try to get a property that doesn't exists, Javascript should return
undefined. It means, even you raise a exception from Python part, pyv8
will catch the exception and just return a undefined to Javascript
code. You could try to comment the "y.z=10" line

def __getattr__(self, name):
print "Get Attribute: %s" % name

if name == 'constructor':
return JSClassConstructor(self.__class__)

print "here"
raise AttributeError(name)

with PyV8.JSContext(Global()) as ctx:
ctx.eval("""
write(x);

//y.z = 10;
write("z="+z);
""")

Output:

Get Attribute: z
z=undefined


http://www.ecma-international.org/publications/standards/Ecma-262.htm

8.6.2.1 [[Get]] (P)
When the [[Get]] method of O is called with property name P, the
following steps are taken:
1. If O doesn’t have a property with name P, go to step 4.
2. Get the value of the property.
3. Return Result(2).
4. If the [[Prototype]] of O is null, return undefined.
5. Call the [[Get]] method of [[Prototype]] with property name P.
6. Return Result(5).

[mcot]

I am having a little trouble understanding the output of this:

import PyV8

class Global(PyV8.JSClass):
def __init__(self):

pass

with PyV8.JSContext(Global()) as ctx:
ctx.eval("""var x = z""")


with PyV8.JSContext() as ctx:
ctx.eval("""var x = z""")

Only the second one produces a reference error. Shouldn't they both
produce a reference error?

[Flier Lu]

I think it is a bug, the global object should throw a reference error
when access a nonexists field.

I have submitted a bug to trace it, and it should be fixed soon,
thanks :)

http://code.google.com/p/pyv8/issues/detail?id=61

[Flier Lu]

done, now pyv8 will throw ReferenceError when access nonexists
property of the global object

please verify the issue with SVN trunk r304 or later, thanks