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Date: Mon, 04 Feb 2013 16:26:37 -0800
From: Ethan Furman <ethan@stoneleaf.us>
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To: python-list@python.org
Subject: Re: LBYL vs EAFP
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Xref: csiph.com comp.lang.python:38154

On 02/04/2013 03:16 PM, Steven D'Aprano wrote:
> The eternal conflict between "Look Before You Leap" and "Easier to Ask for
> Forgiveness than Permission" (LBYL vs EAFP) continues...
>
> I want to check that a value is a number. Let's say I don't care what sort
> of number -- float, int, complex, Fraction, Decimal, something else -- just
> that it is a number. Should I:
>
> Look Before I Leap:
>
>      from numbers import Number
>      if isinstance(x, Number):
>          ...
>      else:
>          raise TypeError
>
>
> or Ask Forgiveness:
>
>      x + 0
>      ...

As Ian mentioned, both cases are LYBL, unless of course your addition 
was just an example of some mathematical code you have further down.

Personally, I go with EAFP unless I'm trying to present friendlier error 
messages.

~Ethan~