"James Harris" <james.harri...@googlemail.com> wrote in message
news:e8b46ea8-8d1e-4db9-91ba-501fd1a44...@g18g2000yqk.googlegroups.com...
On 29 Sep, 18:20, Seebs <usenet-nos...@seebs.net> wrote:
On 2010-09-29, Tracubik <affdfsdfds...@b.com> wrote:
> Hi all,
> I'm studying PyGTK tutorial and i've found this strange form:
> button = gtk.Button(("False,", "True,")[fill==True])
> the label of button is True if fill==True, is False otherwise.
> i have googled for this form but i haven't found nothing, so can any of
> you pass me any reference/link to this particular if/then/else form?
Oh, what a nasty idiom.
Here's the gimmick.
("False,", "True,")
is a tuple. That means you can index it. For instance:
("False,", "True,")[0]
is the string "False,".
So, what is the numeric value of "fill == True"? Apparently, at least
at the time this was written, it was 0 if fill was not equal to True,
and 1 if fill was equal to True.
Let me say, though, that I'm a C programmer, so I'm coming from a
language
where the result of 0-or-1 for test operators is guaranteed, and I still
wouldn't use this in live code. It's insufficiently legible.
I'm surprised you don't like this construct. I hadn't seen it until I
read the OP's question just now. However, it's meaning was immediately
apparent.
I should say where I'm coming from. Contrast the following C and
Python:
text = fill == true ? "True," : "False,"; (C)
text = ("False,", "True,")[fill == true] (Python)
Surely the C should be: fill ? "True," : "False,"; ?
I never liked C's ?: construct partly because it doesn't scale well.
To accept more than two options it requires the programmer to build a
small hierarchy which can be awful to read and may be best expressed
as a separate function. I'd rather have a language change a predicate
to a small integer and use that to index a set of results - and this
is exactly what the OP's tutorial does.
I use this syntax where there are two possibilities chosen according to
condition 'a':
(a | b | c)
similar to C's ?: operator. Where there are N possibilities chosen from a
linear set, I use:
(i | a, b, c, ... |z) # indexed from 1, default to z
(I think from Algol68 originally.)
The difference from what the Python is doing above, is that only one of the
possibilities is ever evaluated. Extrapolating the syntax a little, Python I
think will evaluate all N expressions (maybe even construct the tuple),
before choosing one.
And I'm not sure there is provision for a default value either, without
having a far more complex expression:
x = ("One","Two","Three") [i-1]
While this works for i = 1,2,3, it goes funny for i=0,-1,-2, and generates
an error for the rest (I'm sure Python has myriad ways of achieving this
succinctly, but this isn't it...)
As another hypothetical example where sgn() returns -1, 0 or +1
position = ("less", "equal", "greater")[sgn(a - b) + 1]
Though where the list gets much longer it would be good to be able to
label the cases for legibility.
You can do, until you want to insert an item in the middle and have to
re-label everything...
For more complex cases I'd just use a conventional case or switch expression
which (in my syntax at least), also evaluates just one expression, and
returns that value. But then you can also start using if-else chains, so
this is no longer a compact construction useful in an expression.
--
Bartc
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