if does not evaluate

[Jim Newton]

A question that has bothered me ever since looking at python
the first time is that not everything evaluates to something.
I am wondering what is the reason for this. Would making
everying evaluate have caused the langugage to be less
efficient execution-wise? or was it a choice to enforce some
sort of standard?

I've read a few discussions about the fact that there is
no if/then/else operartor.

Wouldn't this problem be easily solved by making the built
in keywords actually be evaluatable.

I.e., x = if something:
expr1
else:
expr2

parentheses would of course be optional as they are for
all expressions.

[bruno modulix]

Re: if does not evaluate

Jim Newton a écrit :[color=blue]
> A question that has bothered me ever since looking at python
> the first time is that not everything evaluates to something.
> I am wondering what is the reason for this. Would making
> everying evaluate have caused the langugage to be less
> efficient execution-wise? or was it a choice to enforce some
> sort of standard?
>
> I've read a few discussions about the fact that there is
> no if/then/else operartor.
>
> Wouldn't this problem be easily solved by making the built
> in keywords actually be evaluatable.
>
> I.e., x = if something:
> expr1
> else:
> expr2
>
> parentheses would of course be optional as they are for
> all expressions.
>[/color]
Python is, by design, an 'instruction-based' language, not an
'expression-based' language. Search this ng for discussions on the
inclusion of ternary operator if you want to learn more about this point...

Now if you're looking for a language pretty similar to Python
(very-hi-level, highly dynamic, interpreted, clean and readable syntax,
friendly and enthusiast communauty etc) where everything's an
expression, you may want to try Ruby. It's not as easy to learn, it as
its own warts too, and it has not yet reached such a 'critical mass' as
Python in terms of libs and communauty, but it's a really nice and
usable language too.

Bruno (who likes both languages...)

[Tor Iver Wilhelmsen]

Re: if does not evaluate

Jim Newton <jimka@rdrop.co m> writes:
[color=blue]
> I.e., x = if something:
> expr1
> else:
> expr2[/color]

Try exploiting that a boolean expression evaluates to 0 or 1:

x = (expr1, expr2)[something];

Keep in mind that this might break on some later release of Python if
they decide to make boolean its own type, but at least it works for
2.3.3.

[Tor Iver Wilhelmsen]

Re: if does not evaluate

Tor Iver Wilhelmsen <tor.iver.wilhe lmsen@broadpark .no> writes:
[color=blue]
> x = (expr1, expr2)[something];[/color]

Um, other way around of course :) :

x = (expr2, expr1)[something];

[Matteo Dell'Amico]

Re: if does not evaluate

Tor Iver Wilhelmsen wrote:[color=blue]
> x = (expr1, expr2)[something];
>
> Keep in mind that this might break on some later release of Python if
> they decide to make boolean its own type, but at least it works for
> 2.3.3.[/color]

I think (and I hope) it will never be so, since bool is already its own
type and, as the documentation states,

"In numeric contexts (for example when used as the argument to an
arithmetic operator), they behave like the integers 0 and 1, respectively."

So, even if this a little "perlish" :-), I think it will continue to
work in future versions of python, since otherwise it would break too
much existing code.

--
Ciao,
Matteo

[Holger Türk]

Re: if does not evaluate



Tor Iver Wilhelmsen wrote:[color=blue]
> Jim Newton <jimka@rdrop.co m> writes:
>
>[color=green]
>>I.e., x = if something:
>> expr1
>> else:
>> expr2[/color]
>
>
> Try exploiting that a boolean expression evaluates to 0 or 1:
>
> x = (expr1, expr2)[something];[/color]

This is eagerly evaluated. If only one expression is meant to
be evaluated, the whole thing should look like this:

x = (lambda: expr1, lambda: expr2)[bool(something)]()

I added bool() to allow tests returning other values than
0 or 1.

Greetings,

Holger

[Tor Iver Wilhelmsen]

Re: if does not evaluate

Matteo Dell'Amico <della@toglimi. linux.it> writes:
[color=blue]
> So, even if this a little "perlish" :-), I think it will continue to
> work in future versions of python, since otherwise it would break too
> much existing code.[/color]

Yes, and at least one would hope it would work anyway with a cast,
that is "int(something) ".

[Jim Newton]

Re: if does not evaluate

this suggestion does not work because
expr2 evaluates even if expr1 is TRUE.


Tor Iver Wilhelmsen wrote:[color=blue]
> Jim Newton <jimka@rdrop.co m> writes:
>
>[color=green]
>>I.e., x = if something:
>> expr1
>> else:
>> expr2[/color]
>
>
> Try exploiting that a boolean expression evaluates to 0 or 1:
>
> x = (expr1, expr2)[something];
>
> Keep in mind that this might break on some later release of Python if
> they decide to make boolean its own type, but at least it works for
> 2.3.3.[/color]

[Tor Iver Wilhelmsen]

Re: if does not evaluate

Jim Newton <jimka@rdrop.co m> writes:
[color=blue]
> this suggestion does not work because
> expr2 evaluates even if expr1 is TRUE.[/color]

Yes, someone pointed that out already :) - the solution was to use
lambda expressions, which is reasonable.