Understanding Python's augmented assignment (a += b)

http://stupidpythonideas.blogspot.com/2015/02/augmented-assignments-b.html

by ceronman • 11 years ago

72 15 11 years ago

stupidpythonideas.blogspot.com

Nice article, though I already understood the matter. Thanks for the link to PEP 463[1], though; I can see it cleaning up some of our code. Do you know if there have been any developments into implementing it?

[1] http://legacy.python.org/dev/peps/pep-0463/

maxerickson• 11 years ago

It died:

https://mail.python.org/pipermail/python-dev/2014-March/1331...

(the PEP author posts a couple messages in the thread too)

zrail• 11 years ago

This is a wonderful explanation of a deeply confusing situation. Thank you for posting.

aftbit2• 11 years ago

Why couldn't Python do something like:

    >>> _tmp = getitem(t, 0)
    >>> setitem(t, 0, _tmp)
    >>> try:
    >>>     _tmp2 = _tmp.__iadd__([2])
    >>> except AttributeError:
    >>>     _tmp2 = _tmp.__add__([2])
    >>> setitem(t, 0, _tmp2)

Then if the first setitem raises, the rest of the code will be skipped. Obviously this isn't perfect:

1. If setitem has strange side-effects, this will cause them to happen twice. 2. If setitem raises based on the value (rather than the key), this won't notice that.

But otherwise, it seems like a pretty straightforward improvement, no?

peterderivaz• 11 years ago

I ran into this when computing PSNR comparisons using a Numpy function that returned a numpy integer type:

    >>> import numpy
    >>> x=0
    >>> y=numpy.int32(2*10**9)
    >>> x+=y
    >>> x+=y
    >>> x
    -294967296

In later versions of Python this gives a RuntimeWarning so this is less likely to cause problems now.

PhantomGremlin• 11 years ago

Python 2.7.2 on OS X 10.8.5 behaves even more strangely. I get:

   >>> import numpy
   >>> x=0
   >>> y=numpy.int32(2*10**9)
   >>> y
   2000000000
   >>> x+=y
   >>> x
   2000000000
   >>> x+=y
   >>> x
   4000000000
   >>> x*x
   -2446744073709551616
   >>> y*y
   __main__:1: RuntimeWarning: overflow encountered in int_scalars
   -1651507200

I didn't think it was possible to get integer wraparound in Python. I guess it's because the "int32" from numpy "breaks" x, which subsequently no longer behaves like a python variable.

danbruc3• 11 years ago

What this article says about C++ is not true.

oldmanjay• 11 years ago

"Not true" sort of undersells it. They seem to make up some hypothetical language entirely and call it c++ for rhetorical convenience.

anon42• 11 years ago

It seems to me it describes pretty much what happens in C++, but with pythonized syntax, because explaining the exact syntax of all the sigils would be too much for such a short article.

danbruc3• 11 years ago

Absolutely not. Take for example the part about t = [0, 1] followed by t[0] = 2. t[0] does return nothing at all, neither 0 nor any strange complex object. It just evaluates to the memory address of the first element of the array. The idea expressed later that C++ unlike Python treats every piece of code exactly the same no matter whether it appears on the left hand side or the right hand side of an assignment operator is fundamentally wrong.

danbruc3• 11 years ago

The example look ill-conceived to begin with. If you want to extend the list that is the first element of the tuple just use t[0].extend([1]). Using an augmented assignment operator makes no sense if the left hand side can not be assigned to. I don't know Python well enough to tell why it does not check if the left hand side can be assigned to before evaluating the right hand side, i.e. why it dos not check that __setitem__ is present. This would make the behavior more similar to a compiled statically typed language where this would be a compile time error and the right hand side would never get evaluated.

aftbit2• 11 years ago

In fact, the second half of the article is about why it doesn't do that. Also, Python is not a compiled statically typed language, and doesn't have compile errors.

cagriaksay• 11 years ago

Official FAQ is easier to understand. I recommend reading that first before the article.

Understanding Python's augmented assignment (a += b)

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