benrg added the comment:
This is bizarre:
Python 3.3.0 (v3.3.0:bd8afb90ebf2, Sep 29 2012, 10:55:48) [MSC v.1600 32 bit
(Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> x = y = [1, 2]
>>> x += [3]
>>> y
[1, 2, 3]
>>> x = y = {1, 2}
>>> x -= {2}
>>> y
{1}
>>>
Since when has this been standard behavior? The documentation says:
"An augmented assignment expression like x += 1 can be rewritten as x = x + 1
to achieve a similar, but not exactly equal effect. In the augmented version, x
is only evaluated once. Also, when possible, the actual operation is performed
in-place, meaning that rather than creating a new object and assigning that to
the target, the old object is modified instead."
What is "when possible" supposed to mean here? I always thought it meant "when
there are known to be no other references to the object". If op= is always
destructive on lists and sets, then "where possible" needs to be changed to
"always" and a prominent warning added, like "WARNING: X OP= EXPR DOES NOT
BEHAVE EVEN REMOTELY LIKE X = X OP EXPR IN PYTHON WHEN X IS A MUTABLE OBJECT,
IN STARK CONTRAST TO EVERY OTHER LANGUAGE WITH A SIMILAR SYNTAX."
----------
nosy: +benrg
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Python tracker <[email protected]>
<http://bugs.python.org/issue16701>
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