New submission from Terry J. Reedy <tjre...@udel.edu>:
Example (which can serve as testcase with buggy output corrected).
class C(object):
def __iter__(self):
yield 'yes!'
def __radd__(self, other):
other.append('bug!')
return other
def __rmul__(self, other):
other *= 2
return other
def __index__(self):
return 3
class L(list):
def __iadd__(self, other):
list.__iadd__(self,other)
return self
def __mul__(self, other):
return L(list.__imul__(self,other))
z1, z2, c = [], L(), C()
z1 += c
z2 += c
print(z1, z2, [1]*c, L([1])*c)
>>>
['bug!'] ['yes!'] [1, 1] [1, 1, 1] # PyPy prints ['yes!'], [1, 1, 1]
Cause was diagnosed by Greg Price in
http://codespeak.net/pipermail/pypy-dev/2011q1/006958.html
as checking forward and reverse numeric slots before checking sequence slots
for C-coded classes. Such a difference does not exist in Python itself.
In "About raising NotPortableWarning for CPython specific code"
pydev thread starting at
http://codespeak.net/pipermail/pypy-dev/2011q1/006958.html
Nick Coghlin identified the fix as "When a given operation has multiple C level
slots, shouldn't we be checking all the LHS slots before checking any of the
RHS slots?"
Guido replied "Yeah, indeed, on everything you said. The code dispatching based
on internal slots is horribly ad-hoc and likely wrong in subtle ways."
I personally think fix should be backported to 2.7 and 3.2, but did not select
them because that may be controversial.
----------
components: Interpreter Core
messages: 130698
nosy: terry.reedy
priority: normal
severity: normal
stage: needs patch
status: open
title: Bug in code dispatching based on internal slots
type: behavior
versions: Python 3.3
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue11477>
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