if in expression

[christian2.schmidt@gmx.de]

Hi,
I'm using IronPython to evaluate expressions, so I can't use the
return statement but have to say it in an one-liner. Using C/C++ I
could use "cond ? then : else" to have an expression-if, but in Python
there's no such operator. The "cond and then or else"-trick only seems
to work for non-false "then"s and is not very readable.
So is the pythonic way to def iif(cond, then, else_): if cond: return
then; else: else_; ? But actually this is not the same, because "then"
and "else_" are evaluated independently of cond...

What is the right way to have if in an expression?

Thanks,
Christian

[Diez B. Roggisch]

Re: if in expression

christian2.schm idt@gmx.de schrieb:

Hi,
I'm using IronPython to evaluate expressions, so I can't use the
return statement but have to say it in an one-liner. Using C/C++ I
could use "cond ? then : else" to have an expression-if, but in Python
there's no such operator. The "cond and then or else"-trick only seems
to work for non-false "then"s and is not very readable.
So is the pythonic way to def iif(cond, then, else_): if cond: return
then; else: else_; ? But actually this is not the same, because "then"
and "else_" are evaluated independently of cond...
>
What is the right way to have if in an expression?

Since python 2.5, it is

<then_valueif <condelse <else_value>

If you want lazy evaluation, you can use lambdas:

iif(cond, lambda: then, lambda: else_)()

Diez

[John Machin]

Re: if in expression

On Aug 18, 5:46 pm, christian2.schm ...@gmx.de wrote:

Hi,
I'm using IronPython to evaluate expressions, so I can't use the
return statement but have to say it in an one-liner.

By "evaluate expressions", do you mean using the eval built-in
function? If so, find the recent thread addressing this topic; it was
asserted that 98% of eval() use-cases were better implemented another
way. What's _your_ use-case?

Using C/C++ I
could use "cond ? then : else" to have an expression-if, but in Python
there's no such operator. The "cond and then or else"-trick only seems
to work for non-false "then"s and is not very readable.

There is an even more unreadable hack (due to Tim Peters IIRC) that
avoides the false-then problem:

(cond and [then_value] or [else_value])[0]

[Hrvoje Niksic]

Re: if in expression

"Diez B. Roggisch" <deets@nospam.w eb.dewrites:

Since python 2.5, it is
>
<then_valueif <condelse <else_value>
>
If you want lazy evaluation, you can use lambdas:
>
iif(cond, lambda: then, lambda: else_)()

Your code uses "iif" and attempts to evaluate a tuple; could you post
an example that works?

I ask because it's not clear what you mean by lazy evaluation in this
context. The ternary "if" expression introduced in Python 2.5 only
evaluates then_value or else_value depending on the outcome of cond.
How is that different than using a lambda?

[Fredrik Lundh]

Re: if in expression

John Machin wrote:

There is an even more unreadable hack (due to Tim Peters IIRC) that
avoides the false-then problem:
>
(cond and [then_value] or [else_value])[0]

The correct attribution is "due to Tim Peters (who wishes it was Steve
Majewski)."

</F>

[Diez B. Roggisch]

Re: if in expression

Hrvoje Niksic schrieb:

"Diez B. Roggisch" <deets@nospam.w eb.dewrites:
>

>Since python 2.5, it is
>>
><then_valuei f <condelse <else_value>
>>
>If you want lazy evaluation, you can use lambdas:
>>
>iif(cond, lambda: then, lambda: else_)()

>
Your code uses "iif" and attempts to evaluate a tuple; could you post
an example that works?

I ask because it's not clear what you mean by lazy evaluation in this
context. The ternary "if" expression introduced in Python 2.5 only
evaluates then_value or else_value depending on the outcome of cond.
How is that different than using a lambda?

If iif is defined as this:

def iif(cond, then, else_):
if cond:
return then
else:
return else_

you have the problem that "then" and "else_" get evaluated *before* they
get passed. So for example this factorial function will fail with too
deep recursion error:

def f(n):
return iif(n1, n * f(n-1), 1)

But if you do it like this, the then and else_ will be functions that
get returned and then they need to be called to be acutally evaluated:

def f(n):
return iif(n1, lambda: n * f(n-1), lambda: 1)()


Diez

[Fredrik Lundh]

Re: if in expression

Hrvoje Niksic wrote:

>If you want lazy evaluation, you can use lambdas:
>>
>iif(cond, lambda: then, lambda: else_)()

>
Your code uses "iif" and attempts to evaluate a tuple; could you post
an example that works?
>
I ask because it's not clear what you mean by lazy evaluation in this
context. The ternary "if" expression introduced in Python 2.5 only
evaluates then_value or else_value depending on the outcome of cond.
How is that different than using a lambda?

the second part of Diez' answer is a reply to the second part of the
OP's question.

</F>

[Hrvoje Niksic]

Re: if in expression

Fredrik Lundh <fredrik@python ware.comwrites:

Hrvoje Niksic wrote:
>

>>If you want lazy evaluation, you can use lambdas:
>>>
>>iif(cond, lambda: then, lambda: else_)()

>>
>Your code uses "iif" and attempts to evaluate a tuple; could you post
>an example that works?
>>
>I ask because it's not clear what you mean by lazy evaluation in this
>context. The ternary "if" expression introduced in Python 2.5 only
>evaluates then_value or else_value depending on the outcome of cond.
>How is that different than using a lambda?

>
the second part of Diez' answer is a reply to the second part of the
OP's question.

Ah, that explains it. Sorry about the noise!

[Christian Schmidt]

Re: if in expression

On 18 Aug., 10:22, John Machin <sjmac...@lexic on.netwrote:

On Aug 18, 5:46 pm, christian2.schm ...@gmx.de wrote:

I'm using IronPython to evaluate expressions, so I can't use the
return statement but have to say it in an one-liner.

>
By "evaluate expressions", do you mean using the eval built-in
function?Ifso, find the recent thread addressing this topic; it was
asserted that 98% of eval() use-cases were better implemented another
way. What's _your_ use-case?

User provided python expression of .net's CLR-objects (with
overloaded
operators). The result is processed again in .net.
Reading through the mentioned mail, I think I'm in the 2%. :-)

Thanks,
Christian