[Python-bugs-list] [ python-Bugs-417930 ] += not assigning to same var it reads
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Tue, 25 Jun 2002 06:40:27 -0700
Bugs item #417930, was opened at 2001-04-21 18:25
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Category: Documentation
Group: None
>Status: Closed
Resolution: Accepted
Priority: 6
Submitted By: Edward Loper (edloper)
Assigned to: Raymond Hettinger (rhettinger)
Summary: += not assigning to same var it reads
Initial Comment:
My understanding of the augmented assignment
statements is that they should always assign to the
variable that they read from. However, consider the
following Python session:
>>> class A: x = 1
...
>>> a=A()
>>> a.x += 1
>>> a.x, A.x
(2, 1)
Here, the expression "a.x += 1" read from a class
variable, and wrote to an instance variable. A
similar effect can occur within a member function:
>>> class A:
... x = 1
... def f(s): s.x += 1
...
>>> a = A()
>>> a.f()
>>> a.x, A.x
(2, 1)
I have elicited this behavior in Python 2.0 and 2.1
under Linux (Redhat 6.0), and Python 2.0 on sunos5.
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>Comment By: Raymond Hettinger (rhettinger)
Date: 2002-06-25 08:40
Message:
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Committed as ref6.tex 1.55 and 1.47.4.2.
Closing bug.
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Comment By: Fred L. Drake, Jr. (fdrake)
Date: 2002-06-25 08:23
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Thanks! Check in as patched. Does the tutorial need
changes as well?
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Comment By: Raymond Hettinger (rhettinger)
Date: 2002-06-24 23:34
Message:
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See attached documentation patch.
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Comment By: Chris Cogdon (chriscog)
Date: 2001-05-03 20:00
Message:
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IMHO, this is not a bug. Assignments are always made to the
class instance, not the class type. X+=1 should be
equivalent to going X=X+1, and is just so in the above
cases.
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Comment By: Tim Peters (tim_one)
Date: 2001-04-21 21:46
Message:
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Sorry, but your understanding is flawed. Changed the
category to Documentation and assigned to Fred, because I
agree the Ref Man isn't clear enough here: "The target is
evaluated only once" is certainly how Guido *thinks* of it,
but it really needs a by-cases explanation:
For an attributeref target, the primary expression in the
reference is evaluated only once, but a getattr is done on
the RHS and a setattr on the LHS. That is,
e1.attr op= e2
acts like
temp1 = e1
temp2 = e2
temp1.attr = temp1.attr op temp2
Etc. It's going to take some real work to write this up so
they're comprehensible! I recommend skipping most words
and presenting "workalike" pseudo-code sequences instead.
That will take some sessions with the disassembler to be
sure the pseudo-code is 100% accurate.
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