Proposed PEP for a Conditional Expression

[Bernhard Herzog]

Michael Chermside <mcherm at destiny.com> writes:

Good summary of the arguments usually made for or agains conditional
operators!

>          WORKAROUND_1: (this fails)
>              > def cond(c, a, b):
>              >     if c: return a
>              >    else: return b
>              > x = cond(c, a, b)
>              This fails, because it does not short-circuit...

It also fails because of inconsistent indentation :)

>      ARG_23: Lots of people ask for it.
>          Lots of people ask for a conditional expression. Even if the
>          other pro arguments don't convince you, what harm is there in
>          settling this one? For a LONG time, we used to hear
>          complaints about how Python lacked "+=", "*=" and such; now
>          that it has them, the complaints have died to a trickle.
>          What's wrong with just satisfying people?

Lots of people asking for it is a reason to write the PEP, but not
necessarily a reason to actually adopt the PEP. 


>      SPEC_1: Use the "C syntax".
>          The following production would be added to Python syntax:
>              expression:   expression ?  exprression : expression
>          Since "?" is not currently used in Python, this would never
>          be syntactically ambiguous. Sample: a = b ? c : d
> 
>          PRO - [familiar to programmers]

But only to programmers coming from "C-like" languages.

>      SPEC_2: Use "if-then-else".
[...] 
>      SPEC_3: Use "if-then-else" without keyword.
[...]

These are the best spellings for this, I think.


Another way that come to mind:

        <expr> if <cond> else <expr>

The order in which the condition and the braches occur is unusual for
python, but this spelling has the advantage of not needing an additional
keyword or colons.

>      SPEC_4: Use "if c: a else: b".
>          The following production would be added to Python syntax:
>              expression: "if" expression ":" expression "else" ":"
>                          expression
>          The famous "dangling-else" problem would NOT arise because
>          the "else:" part would be mandatory.

I think this would look better without the colon after the else. I'm not
sure why though.

   Bernhard

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