Good code patterns in Python

[Erik Max Francis]

Kirk Job-Sluder wrote:

> Will Stuyvesant <hwlgw at hotmail.com> wrote:
>
> > If you know that your source code is going to be used
> > later by others, then I feel that code with the pattern:
> >
> >    if some_condition:
> >        some_name = some_value
> >    else:
> >        some_name = other_value
> >
> > is often a mistake.  Much better, safer, would be:
> >
> >    some_name = some_value
> >    if not some_condition:
> >        some_name = other_value
> >
> > Why?
> 
> My personal opinion is that it depends on context.  The first idiom
> is more clear when you are dealing with some kind of a switch.  The
> second idiom works better if you have a default value that needs to be
> overridden in some cases.

I presume the "mistake" he's referring to with the first snippet is the
potential problem that could happen if the variable is named
incorrectly:

	if condition:
	    variable = someValue
	else:
	    varaible = otherValue # [sic] note the typo!

When you really want an atomic assignment of one singular variable.  The
complete solution to this would probably be a conditional expression,
e.g.:

	variable = (if condition: someValue else: otherValue)

eliminating the duplication which can lead to errors.  (Python, of
course, doesn't have such a construct, and the silence after the
conditional expression PEP vote and the long silence thereafter suggests
that it never will.)

I still don't think this significant of a risk to warrant widespread
conversion of statements to the form Will suggests, especially when you
have things like PyChecker that can check for (probable) typos.  It's a
slightly unfortunate wart in dynamic languages without conditional
operators, but I don't think it rises to the level of something that
should be corrected via such (what seems to me) a heavy-handed style.

-- 
   Erik Max Francis && max at alcyone.com && http://www.alcyone.com/max/
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