Style question - defining immutable class data members

R. David Murray rdmurray at bitdance.com
Sun Mar 15 22:42:26 CET 2009


M R A Barnett <pcw at mrabarnett.plus.com> wrote:
> Aaron Brady wrote:
> [snip]
> > However, in my (opined) interpretation, 'list.append(...) is an in-
> > place operation' is a factual error.  In-place operations -also-
> > rebind their 'argument' (FLOBW for lack of better words).  'append' is
> > a by-side-effect operation.  However colloquially it's mostly
> > accurate.
> > 
> [snip]
> All the augmented assignments rebind, even for objects which support
> in-place operations. For example:
> 
>      my_list = []
>      my_list += [0]
> 
> rebinds, but the equivalent:
>
>      my_list = []
>      my_list.extend([0])
> 
> doesn't.

Lest someone be mislead by this discussion, how about an example:

    >>> a = b = []
    >>> a.extend([1])
    >>> a
    [1]
    >>> b
    [1]
    >>> a += [3]
    >>> a
    [1, 3]
    >>> b
    [1, 3]
    >>> a = a + [5]
    >>> a
    [1, 3, 5]
    >>> b
    [1, 3]

It is technically correct that '+=' bebinds 'a', but what it
rebinds it to is the same object that was just mutated.  Unlike
the '+' case where 'a' is bound to a newly created list object.

Personally I think += and kin were a bad idea and should have been
removed in Python 3.0 :)  Even though I occasionally use them.  (Maybe
I'll stop.)

> Augmented assignments which don't support in-place operations behave
> like normal assignments (binding). For example:
> 
>      my_int = 0
>      my_int += 1
> 
> behaves like:
> 
>      my_int = 0
>      my_int = my_int + 1

--
R. David Murray           http://www.bitdance.com




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