New submission from nother :
Not sure where to file this suggestion properly and how.
Currently when mixing attributes and properties it can happen that an
AttributeError not raised intentionally by @property getter can cause @property
to vanish from from classinstance. This happens when on the class or any of its
super classes __getattr__ method is defined and has no clue on how to handle
attribute with name of property. In this case the latter is reported as not
existent.
The understood purpose of this behavior is that this is the only means to
enable @property getters to indicate the availability of the property for a
specific class instance dependent upon local state of instance object. Another
purpose is to implement canonical load/creation on deemand schema for
@properties the same way as for attributes.
The down of this is that in case inside a more complex getter method an
AttribteError is triggered unintentionally this also triggers call to
__getattr__ whithout any means for the latter to figure whether the @property
getter needs assistance or the error should be passed on unhandled.
Therefore i do suggest the following little change to slot_tp_getattr_hook
method in file typeobject.c (github python/cpython master) on lines 6604 - 6607
from
`
if (res == NULL && PyErr_ExceptionMatches(PyExc_AttributeError)) {
PyErr_Clear();
res = call_attribute(self, getattr, name);
}
`
to
`
if (res == NULL && PyErr_ExceptionMatches(PyExc_AttributeError)) {
res = call_attribute(self, getattr, name);
if ( res != NULL) {
PyErr_Clear();
}
}
`
this little change would allow __getattr__ special method to
1) use plain raise (see pythoncode example attached) to reraise exception
which caused the call to it
2) allow @property getters convey by adding custom attribute to
AttributteExcpetion the reason for an intentionally raised AttributeError over
to __getattr__ allowing for distinction between AttributeError raised to hide
@property from class instance versus triggering load/creation on demmand for
class instance vs AttributeError accidentially triggered by
getter/setter/deleter code.
Still not possible would be for getattr and hasattr methods to figure whether
the AttributeError is raised cause class instance does not have the requested
attirbute or whether accessing the attribute or @property respekctive caused
some there downstream an AttributeError. In case that would be reliably
feasible at all.
--
components: Interpreter Core
files: test6.py
messages: 346323
nosy: nother
priority: normal
severity: normal
status: open
title: Provide triggering AttributeError exception to __getattr__ for reraising
type: enhancement
Added file: https://bugs.python.org/file48432/test6.py
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<https://bugs.python.org/issue37379>
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