?: in Python

[Andy Leszczynski]

How can do elegantly in Python:

if condition:
a=1
else:
a=2

like in C:

a=condition?1:2

[Lawrence Oluyede]

Il 2005-12-14, Andy Leszczynski <ya***@nospam.leszczynscy> ha scritto:

How can do elegantly in Python:

if condition:
a=1
else:
a=2

like in C:

a=condition?1:2

There are tons of threads on this newsgroup and in the python-dev mailing
list about a ternary operator. There's also a PEP AFAIK.

I like this:

In [1]:switch = True

In [2]:a = (1, 2)[switch]

In [3]:print a
2
--
Lawrence - http://www.oluyede.org/blog
"Anyone can freely use whatever he wants but the light at the end
of the tunnel for most of his problems is Python"

[Andy Leszczynski]

Lawrence Oluyede wrote:

Il 2005-12-14, Andy Leszczynski <ya***@nospam.leszczynscy> ha scritto:

How can do elegantly in Python:

if condition:
a=1
else:
a=2

like in C:

a=condition?1:2

There are tons of threads on this newsgroup and in the python-dev mailing
list about a ternary operator. There's also a PEP AFAIK.

I like this:

In [1]:switch = True

In [2]:a = (1, 2)[switch]

In [3]:print a
2

Like it too, thx. Switch does not have to be bool, so it is more
powerfull than ?:.

A.

[Bengt Richter]

On Wed, 14 Dec 2005 21:16:23 +0100, Lawrence Oluyede <ra***@dot.com> wrote:

Il 2005-12-14, Andy Leszczynski <ya***@nospam.leszczynscy> ha scritto:

How can do elegantly in Python:

if condition:
a=1
else:
a=2

like in C:

a=condition?1:2

There are tons of threads on this newsgroup and in the python-dev mailing
list about a ternary operator. There's also a PEP AFAIK.

I like this:

In [1]:switch = True

In [2]:a = (1, 2)[switch]

In [3]:print a
2

You won't like it in a case like
a = (2**20**20, 2)[switch]
or
a = (m/n, sys.maxint)[n==0]
the point is that if/else only evaluates
the expression in one branch, as with C ternary.

You're right, there is a PEP and a ternary expression
coming to python though.

Regards,
Bengt Richter

[gene tani]

Andy Leszczynski wrote:

How can do elegantly in Python:

if condition:
a=1
else:
a=2

like in C:

a=condition?1:2

google for ternary operator

http://www.python.org/doc/faq/progra...rnary-operator
http://www.norvig.com/python-iaq.html

http://mail.python.org/pipermail/pyt...er/310854.html
http://www.python.org/peps/pep-0308.html

http://diveintopython.org/power_of_i...r.html#d0e9975
http://aspn.activestate.com/ASPN/Pyt...k/Recipe/52310

[Peter Hansen]

Andy Leszczynski wrote:

Lawrence Oluyede wrote:

There are tons of threads on this newsgroup and in the python-dev mailing
list about a ternary operator. There's also a PEP AFAIK.

I like this:

In [1]:switch = True

In [2]:a = (1, 2)[switch]

In [3]:print a
2

Like it too, thx. Switch does not have to be bool, so it is more
powerfull than ?:.

Actually, if "switch" is neither bool nor a number that equals 0 or 1,
you'll have trouble... using bool(switch) instead would do what you
thought that did, I guess.

-Peter

[Steven D'Aprano]

On Wed, 14 Dec 2005 14:09:10 -0500, Andy Leszczynski wrote:

How can do elegantly in Python:

if condition:
a=1
else:
a=2

like in C:

a=condition?1:2

I thought you wanted to do it *elegantly*?

Your first solution is perfectly elegant to my eyes, unlike that horrible
C syntax.

--
Steven.

[bonono]

Andy Leszczynski wrote:

How can do elegantly in Python:

if condition:
a=1
else:
a=2

like in C:

a=condition?1:2

a=(condition and [1] or [2])[0]

For this simple snippet, I don't think it is better than if/else, But
you can use it in map/reduce or list comprehension/generator expression.

[Andy Leszczynski]

Steven D'Aprano wrote:

On Wed, 14 Dec 2005 14:09:10 -0500, Andy Leszczynski wrote:

How can do elegantly in Python:

if condition:
a=1
else:
a=2

like in C:

a=condition?1:2

I thought you wanted to do it *elegantly*?

Your first solution is perfectly elegant to my eyes, unlike that horrible
C syntax.

I can tell you what is not elegant in the if else: approach. It is
logically a one operation while you are forced to use varaible "a"
twice. Fundamental flaw IMO.

A.

[Steven Bethard]

Andy Leszczynski wrote:

How can do elegantly in Python:

if condition:
a=1
else:
a=2

like in C:

a=condition?1:2

Step (1): Wait for Python 2.5[1]
Step (2): Write the code::

a = 1 if condition else 2

STeVe

[1]http://www.python.org/peps/pep-0308.html

[Dave Hansen]

On Wed, 14 Dec 2005 21:16:23 +0100 in comp.lang.python, Lawrence
Oluyede <ra***@dot.com> wrote:

Il 2005-12-14, Andy Leszczynski <ya***@nospam.leszczynscy> ha scritto:

How can do elegantly in Python:

if condition:
a=1
else:
a=2

like in C:

a=condition?1:2

There are tons of threads on this newsgroup and in the python-dev mailing
list about a ternary operator. There's also a PEP AFAIK.

I like this:

In [1]:switch = True

In [2]:a = (1, 2)[switch]

In [3]:print a
2

Note, however, you have the logic backwards. To duplicate the
functionality of the OP's example, you need

a = (2,1)[condition]

or

a = (1,2)[not condition]

Regards,
-=Dave

--
Change is inevitable, progress is not.

[Steven D'Aprano]

On Wed, 14 Dec 2005 20:17:28 -0500, Andy Leszczynski wrote:

I can tell you what is not elegant in the if else: approach. It is
logically a one operation while you are forced to use varaible "a"
twice. Fundamental flaw IMO.

"Logically" one operation?

def twenty_countries_in_seven_days_bus_tour():
...
if today() == Monday:
write_postcode_to_mother("We must be in Belgium.")
else:
get_back_on_the_bus("Not again!")
...
if...else expressions with a single operation are just a special case.
Perhaps a common special case, but still a special case.
--
Steven.

[Andrew Koenig]

"Andy Leszczynski" <ya***@nospam.leszczynscy> wrote in message
news:pa******************************@comcast.com. ..

How can do elegantly in Python:

if condition:
a=1
else:
a=2

I believe that before long Python will support

a=1 if condition else 2

[Kent Johnson]

Andy Leszczynski wrote:

How can do elegantly in Python:

if condition:
a=1
else:
a=2

like in C:

a=condition?1:2

a = condition and A or B
is concise but will fail if A can evaluate as false, e.g.
a = condition and None or 2 # won't do what you want

I tend to use 'condition and A or B' if I'm sure A won't be false, otherwise just write
out the if / else.

Kent

[Andy Leszczynski]

Steven D'Aprano wrote:

On Wed, 14 Dec 2005 20:17:28 -0500, Andy Leszczynski wrote:

I can tell you what is not elegant in the if else: approach. It is
logically a one operation while you are forced to use varaible "a"
twice. Fundamental flaw IMO.

"Logically" one operation?

def twenty_countries_in_seven_days_bus_tour():
...
if today() == Monday:
write_postcode_to_mother("We must be in Belgium.")
else:
get_back_on_the_bus("Not again!")
...
if...else expressions with a single operation are just a special case.
Perhaps a common special case, but still a special case.

First:
"Special cases aren't special enough to break the rules.
Although practicality beats purity."
Second, let's look at again:if condition:
a=1
else:
a=2

The primer meaning behind that is that I want to assign something to a.
What I want to assign is secondary issue. I do not like C syntax of ?:
either but I think it is just practical and self-explanatory.

A.