My own preference would be:

def foo(x): if x >= 0: return math.sqrt(x) return None

Kind of getting into the weeds here, but I would always invert this to "return errors early, and keep the normal flow at the main indentation level". Depends a little on what foo() means, but it seems to me the "return None" case is the exceptional/error case, so this would be: def foo(x): if x < 0: return None return math.sqrt(x) The handling of errors is done first, and the normal case stays at the main indentation level. Is this discussed in style guides at all? I don't see anything directly in PEP 8, but I might be missing something. Oh wait, I just noticed this is exactly how Guido has it in his PEP addition with the definition of bar(). :-| -Ben

On Tue, 07 Apr 2015 03:11:30 +0100 Rob Cliffe wrote:

On 07/04/2015 02:08, Guido van Rossum wrote:

...

I've taken the liberty of adding the following old but good rule to PEP 8 (I was surprised to find it wasn't already there since I've lived by this for ages):

*

Be consistent in return statements. Either all return statements in a function should return an expression, or none of them should. If any return statement returns an expression, any return statements where no value is returned should explicitly state this asreturn None, and an explicit return statement should be present at the end of the function (if reachable).

Yes:

def foo(x): if x >= 0: return math.sqrt(x) else: return None

That would seem to be good style and common sense.

As a matter of interest, how far away from mainstream am I in preferring, *in this particular example* (obviously it might be different for more complicated computation),

def foo(x): return math.sqrt(x) if x >= 0 else None

I agree with you on this. Regards Antoine.

Talk about hijacking an unrelated thread... :-( Though there's maybe a style rule for the ternary operator to be devised here? :-) On Apr 7, 2015 11:36 AM, "Stefan Behnel" wrote:

Antoine Pitrou schrieb am 07.04.2015 um 14:26:

...

On Tue, 07 Apr 2015 03:11:30 +0100 Rob Cliffe wrote:

...

On 07/04/2015 02:08, Guido van Rossum wrote:

...

I've taken the liberty of adding the following old but good rule to PEP 8 (I was surprised to find it wasn't already there since I've lived by this for ages):

*

Be consistent in return statements. Either all return statements in a function should return an expression, or none of them should. If any return statement returns an expression, any return statements where no value is returned should explicitly state this asreturn None, and an explicit return statement should be present at the end of the function (if reachable).

Yes:

def foo(x): if x >= 0: return math.sqrt(x) else: return None

That would seem to be good style and common sense.

As a matter of interest, how far away from mainstream am I in preferring, *in this particular example* (obviously it might be different for more complicated computation),

def foo(x): return math.sqrt(x) if x >= 0 else None

I agree with you on this.

+1, the ternary operator reads best when there is a "normal" case that can go first and a "special" or "unusual" case that can go last and does not apply under the (given) normal conditions.

Stefan

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