[Python-bugs-list] [ python-Bugs-417930 ] += not assigning to same var it reads

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Bugs item #417930, was updated on 2001-04-21 16:25
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Category: Documentation
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Edward Loper (edloper)
Assigned to: Fred L. Drake, Jr. (fdrake)
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: Chris Cogdon (chriscog)
Date: 2001-05-03 18:00

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 19:46

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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