[issue24983] Wrong AttributeError propagation
David Unric added the comment: Oops, this was the strict version. The lazy method call version behaves exactly like property getter. So there is probably no implementation bug, but not too well thought out decision design, making debugging AttributeError exceptions in properties difficult and quite misleading. -- ___ Python tracker <http://bugs.python.org/issue24983> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue24983] Wrong AttributeError propagation
David Unric added the comment:
This looks a bit inconsistent. See following version with "manual" getter
definition:
class MyClass(object):
def __init__(self, *args, **kwargs):
# setting access to getter by attribute
# without use of property decorator
self.__dict__['myproperty'] = self._myproperty()
def __getattr__(self, name):
print('__getattr__ <<', name)
raise AttributeError(name)
return 'need know the question'
def _myproperty(self):
print(self.missing_attribute)
return 42
my_inst = MyClass()
print(my_inst.myproperty)
# Produces (expected) output
__getattr__ << missing_attribute
Traceback (most recent call last):
File "a.py", line 55, in
my_inst = MyClass()
File "a.py", line 33, in __init__
self.__dict__['myproperty'] = self._myproperty()
File "a.py", line 48, in _myproperty
print(self.missing_attribute)
File "a.py", line 37, in __getattr__
raise AttributeError(name)
AttributeError: missing_attribute
I'd expect the original version using property decorator would behave the same
way. Possible issue in property class implementation ?
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[issue24983] Wrong AttributeError propagation
David Unric added the comment: Thanks for the comprehensive response. I did suspected the 2nd call is caused how it has been described in paragraph 4. And you are probably right. Only think exception instance raised in __getattr__ should not lead to its another call but propagated to outer level, ie. should not get continue in getter in this example. -- ___ Python tracker <http://bugs.python.org/issue24983> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue24983] Wrong AttributeError propagation
New submission from David Unric:
Hello,
it seems python interpreter improperly handles AttributeError exception raised
in __getattr__ method, after called by unresolved attribute inside a property.
Bellow is a simple Python2 example of a class which defines __getattr__ method
and a property, where is non-existing attribute accessed:
from __future__ import print_function
class MyClass(object):
def __getattr__(self, name):
print('__getattr__ <<', name)
raise AttributeError(name)
return 'need know the question'
@property
def myproperty(self):
print(self.missing_attribute)
return 42
my_inst = MyClass()
print(my_inst.myproperty)
# produces following output
__getattr__ << missing_attribute
__getattr__ << myproperty
Traceback (most recent call last):
File "a.py", line 84, in
main()
File "a.py", line 74, in main
print('==', my_inst.myproperty)
File "a.py", line 36, in __getattr__
raise AttributeError(name)
AttributeError: myproperty
By the documentation
https://docs.python.org/2/reference/datamodel.html#object.__getattr__ , if
class defines __getattr__ method, it gets called at AttributeError exception
and should return a computed value for name, or raise (new) AttributeError
exception.
The misbehavior is in 2nd call of __getattr__, with 'myproperty' as an argument.
- self.myproperty does exist, no reason to call __getattr__ for it
- AttributeError exception raised in __getattr__ should be propagated to the
outer stack, no recurrence in the same
--
components: Interpreter Core
messages: 249533
nosy: dunric
priority: normal
severity: normal
status: open
title: Wrong AttributeError propagation
type: behavior
versions: Python 2.7, Python 3.4
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