More clever (ab)use of
In a comment on the last entry I mumbled yet again how I was annoyed that Python doesn't have an equivalent of C's '? :' conditional expression operator. Then some neurons in the back of my mind woke up and asked 'didn't Guido van Rossum say he was going to add this to Python 2.5?'
In the process of trying to figure out if this
was still happening (it is), I read this message
and found out about a trick (used in the standard library no less) that
gets most of what I want. For '
A ? B : C', we can write:
A and B or C
(Python precedence rules make this equivalent to '
(A and B) or C'.
It works because both
or return the value of the last
expression they evaluated.)
This isn't quite a full emulation of
?:, because it gives you
A is true and
B is something that Python will consider false,
but it is close enough for many uses. Often my code is such that the
A expression isn't true unless
B has a non-empty, non-zero value.
And if your
C is guaranteed to be true, you can always invert the
(This doesn't help the code in the last entry, though, since we
can't guarantee that
blst will be considered true is the caller
supplied it, and our
C default value will be considered false.)
Another day, another useful trick, and it's all because of a comment. (Which makes me very glad I bothered to add comments to DWiki.)
Sidebar: Python 2.5 conditional expressions
From Guido van Rossum's message
, the syntax will be '
B if A else C' (to use my ordering). I'm not
really sure I like this; in general, I don't like out of order syntaxes
because they make me backtrack on expressions when I read them. (I
touched on this before back here.)
According to the Python 2.5 release schedule, Python 2.5 is supposed to come out around 30 September 2006, so we have a while to wait for this.
Comments on this page:Written on 25 February 2006.