New submission from Eugene Toder <elto...@gmail.com>:
Since the time 'x in set' optimization was added to peephole it has the
following bug with set unpacking:
>>> def foo(x,y):
a,b = {x,y}
return a,b
>>> dis(foo)
2 0 LOAD_FAST 0 (x)
3 LOAD_FAST 1 (y)
6 ROT_TWO
7 STORE_FAST 2 (a)
10 STORE_FAST 3 (b)
3 13 LOAD_FAST 2 (a)
16 LOAD_FAST 3 (b)
19 BUILD_TUPLE 2
22 RETURN_VALUE
That is, unpacking of literal set of sizes 2 and 3 is changed to ROT
instructions. This, however, changes the semantics, as construction of set
would eliminate duplicates.
The difference can be demonstrated like this:
Python 3.1
>>> foo(1,1)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in foo
ValueError: need more than 1 value to unpack
Python 3.2
>>> foo(1,1)
(1, 1)
----------
components: Interpreter Core
messages: 130917
nosy: eltoder
priority: normal
severity: normal
status: open
title: Peephole breaks set unpacking
type: compile error
versions: Python 3.2
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue11510>
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