Most compact "X if X else Y" idiom

[jbperez808]

I find myself having to do the following:

x = (some complex expression)
y = x if x else "blah"

and I was wondering if there is any built-in idiom that
can remove the need to put (some complex expression)
in the temporary variable x.

e.g. something like the below:

y= foobar ((some complex expression), "blah")

I realized foobar() can be easily coded as:
def foobar(a,b):
if a: return a
else: return b

But I was wondering if there was a built-in function or syntax
that already does this.

[Aaron \Castironpi\ Brady]

On Oct 12, 12:01*am, [e-mail] wrote:

I find myself having to do the following:

* x = (some complex expression)
* y = x if x else "blah"

and I was wondering if there is any built-in idiom that
can remove the need to put (some complex expression)
in the temporary variable x.

e.g. something like the below:

*y= foobar ((some complex expression), "blah")

I realized foobar() can be easily coded as:
* def foobar(a,b):
* * if a: return a
* * else: return b

But I was wondering if there was a built-in function or syntax
that already does this.

You could take your chances on 'or', as follows:

>>(6+ (3<< 1) ) or 'blah'

12

>>(6- (3<< 1) ) or 'blah'

'blah'

You don't need to use the ternary statement:

y = (some complex expression)
if not y:
y = "blah"

If you find yourself using it a lot, why not add it to your site's
utilities modules? Take your time, and if you find numerous uses,
present them and make the case Python should have a built-in to do it,
something like 'ditto' marks:

(6- (3<< 1) ) if ditto else 'blah'

[Terry Reedy]

jb********@yahoo.com wrote:

I find myself having to do the following:

x = (some complex expression)
y = x if x else "blah"

and I was wondering if there is any built-in idiom that
can remove the need to put (some complex expression)
in the temporary variable x.

A common idiom for this particular case where the if-expression is also
the conditional or the basic of the conditional expression is

y = <some complex expression>
if not y: y = "blah"

[Steven D'Aprano]

On Sat, 11 Oct 2008 22:01:46 -0700, jbperez808 wrote:

I find myself having to do the following:

x = (some complex expression)
y = x if x else "blah"

and I was wondering if there is any built-in idiom that can remove the
need to put (some complex expression) in the temporary variable x.

Use short-circuit Booleans:

y = x or "blah"

If x is any true value (non-zero number, non-empty string etc.) then y
will be set to x; but if x is any false value (zero, empty string, None,
empty list, etc.) then y will be set to "blah".
However, this technique doesn't work for arbitrary tests. For example,
you can't simplify the following:

x = (some complex expression)
y = x if 100<=x<250 else "blah"

(at least I can't think of any way).

--
Steven

[Steven D'Aprano]

On Sun, 12 Oct 2008 05:30:33 +0000, Steven D'Aprano wrote:

Use short-circuit Booleans:

y = x or "blah"

Except of course you don't use x, you use the complex expression.

y = (some complex expression) or "blah"

Sorry for the itchy posting finger.

--
Steven

[jbperez808]

Thanks, folks.

Short-circuit boolean was the syntax I had in mind which
momentarily escaped me, but the "if not x: x='blah'" idiom
was instructive as well.