On Wednesday, April 6, 2011 11:57:32 AM UTC-4, Νικόλαος Κούρας wrote:
> >>> mail = None
> >>> mail = mail or 7
> >>> mail
> 7
Quote:
The expression ``x or y`` first evaluates *x*; if *x* is
true, its value is returned; otherwise, *y* is evaluated
and the resulting value is returned.
Since 'mail is None' and None evaluates to False, the operation returns the
right-hand operand, 7.
> >>> mail = None
> >>> mail = 7 or mail
> >>> mail
> 7
>
> Here no matter the order iam writing the comparison it always return
> the number.
In this case the number 7 evaluates to True.
> why not the same here?
>
> >>> mail = None
> >>> mail = mail or ''
> >>> mail
> ''
> >>> mail = None
> >>> mail = '' or mail
> >>> mail
> >>>
>
> Why the or operator behaves differently with numbers than from
> strings?
It's behaving the same. You're overlooking the fact that the empty string is
False:
In [1]: bool('')
Out[1]: False
Length zero sequences are normally False, but you can override this in a
subclass by implementing the __nonzero__ method:
In [2]: class mystr(str):
...: def __nonzero__(self):
...: return True
In [3]: bool(mystr(''))
Out[3]: True
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