check the docs under Non-Backward compatible changes:
http://pymel.googlecode.com/svn/docs/index.html
this is pasted from the docs ( not sure how much google-groups will
butcher it ):
Non-Existent Objects
Previous versions of PyMEL allowed you to instantiate classes for non-
existent objects. This was common practice for testing object
existence using PyNode.exists and could also be useful in
circumstances where you wished to use name formatting methods.
Starting with this version, an exception will be raised if the passed
name does not represent an object in the scene. This has several
advantages:
you will never unknowingingly attempt to use a node or attribute that
does not exist, either due to a typo or unexpected context
it brings PyMEL's attribute handling more in line with pythonic rules,
where attributes must exist before accessing them
it prevents the awkward situation of having a python object for which
only a handful of methods will actually work
The side-effect, however, is that certain conventions for existence
testing are no longer supported, while new ones have also been added.
Compatibility Mode
We realize this is a big change so we have provided an option in the
new pymel.cfg file ( found in the root of the PyMEL directory )
called0_7_compatibility_mode. When enabled, this option causes PyMEL
to treat non-existent objects in a similar fashion to version 0.7.x:
>>> x = PyNode( 'foobar' )
>>> x
DependNodeName('foobar') # doctest: +SKIP
>>> x.exists()
False
The object returned is not a subclass of PyNode, but rather of
other.NameParser. For more information see Manipulating Names of Non-
Existent Objects. Also, keep in mind that the behavior of
"compatibility mode" is deprecated and will not be supported in PyMEL
1.0 ( unless, of course, there is strong public support to keep it ).
New Exceptions
We've added three new exceptions which can be used to test for
existence errors when creating new PyNodes: MayaObjectError,
MayaNodeError, andMayaAttributeError.
>>> for x in [ 'fooBar.spangle', 'superMonk' ] :
... try:
... PyNode( x )
... print "It Exists"
... except MayaNodeError:
... print "The Node Doesn't Exist:", x
... except MayaAttributeError:
... print "The Attribute Doesn't Exist:", x
...
The Attribute Doesn't Exist: fooBar.spangle
The Node Doesn't Exist: superMonk
Both exceptions can be caught by using the parent exception
MayaObjectError. In addition MayaAttributeError can also be caught
with the builtin exception AttributeError.
Note that you will get different exceptions depending on how you
access the attribute. This is because the shorthand notation can also
be used to access functions, in which case the MayaAttributeError does
not make sense to raise. As mentioned above, you can always use
AttributeError to catch both.
Explicit notation:
>>> x = polySphere(name='earth')[0]
>>> x.attr('myAttr')
Traceback (most recent call last):
...
MayaAttributeError: Maya Attribute does not exist: u'earth.myAttr'
Shorthand notation:
>>> x = polySphere(name='moon')[0]
>>> x.myAttr
Traceback (most recent call last):
...
AttributeError: Transform(u'moon') has no attribute or method named
'myAttr'
Testing Node Existence
No longer supported:
>>> if PyNode( 'fooBar' ).exists():
... print "It Exists"
... else:
... print "It Doesn't Exist"
It Doesn't Exist
Still supported:
>>> if objExists( 'fooBar' ):
... print "It Exists"
... else:
... print "It Doesn't Exist"
It Doesn't Exist
New construct:
>>> try:
... PyNode( 'fooBar' )
... print "It Exists"
... except MayaObjectError:
... print "It Doesn't Exist"
It Doesn't Exist
Testing Attribute Existence
No longer supported:
>>> if PyNode( 'persp.spangle' ).exists():
... print "Attribute Exists"
... else:
... print "Attribute Doesn't Exist"
Attribute Doesn't Exist
No longer supported:
>>> x = PyNode('persp')
>>> if x.spangle.exists():
... print "Attribute Exists"
... else:
... print "Attribute Doesn't Exist"
Attribute Doesn't Exist
Still supported:
>>> if objExists( 'persp.spangle' ):
... print "Attribute Exists"
... else:
... print "Attribute Doesn't Exist"
Attribute Doesn't Exist
New construct:
>>> x = PyNode('persp')
>>> if x.hasAttr('spangle'):
... print "Attribute Exists"
... else:
... print "Attribute Doesn't Exist"
Attribute Doesn't Exist
New construct:
>>> try:
... PyNode( 'persp.spangle' )
... print "Attribute Exists"
... except MayaAttributeError:
... print "Attribute Doesn't Exist"
Attribute Doesn't Exist
New construct:
>>> x = PyNode('persp')
>>> try:
... x.spangle
... print "Attribute Exists"
... except AttributeError:
... print "Attribute Doesn't Exist"
Attribute Doesn't Exist
Other PyMEL Idioms
Two other PyMEL idioms have been removed as a result of this change.
Attribute.add has been removed because, the attribute has to exist in
order to successfully get an Attribute instance. So instead you have
to use the addAttr method on the node:
No longer supported:
>>> PyNode('persp').myNewFloatAttr.add( at=float )
Still supported:
>>> PyNode('persp').addAttr( 'myNewFloatAttr', at=float )
Similarly, the force flag for setAttr functions, which creates the
attribute before setting if it does not exist, can only be safely used
from the node class and not the attribute class:
No longer supported:
>>> PyNode('persp').myNewIntAttr.set( 2, force=1 )
Still supported:
>>> PyNode('persp').setAttr( 'myNewIntAttr', 2, force=1 )
New construct:
>>> PyNode('persp').setDynamicAttr( 'myNewIntAttr', 2 )
Manipulating Names of Non-Existent Objects
One advantage of the old way of dealing with non-existent objects was
that you could use the name parsing methods of the PyNode classes to
manipulate the object's name until you found what you were looking
for. To allow for this, we've added several classes which operate on
non-existent nodes and contain only methods for string parsing and
existence testing. These nodes can be found in the other module and
are namedother.NameParser, other.AttributeName, other.DependNodeName,
and other.DagNodeName.
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http://groups.google.com/group/python_inside_maya
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