checking if a list is empty

[Steven D'Aprano]

On Wed, 11 May 2011 11:48:16 +0200, Laurent Claessens wrote:

> Once I wrote something like:
> 
> def f(x=None):
>     if x:
>        print x
>     else:
>        print "I have no value"
> 
> 
> The caller of that function was something like f(cos(2*theta)) where
> theta come from some computations.
> 
> Well. When it turned out that theta was equal to pi/4, I got "I have no
> value". I spent a while to figure out the problem :)

I believe you are grossly oversimplifying whatever code you had. Using 
the definition of f from above:

>>> theta = math.pi/4
>>> f(math.cos(2*theta))
6.12303176911e-17

But even if you rounded the result of cos(2*theta) to zero, you will get 
the same result regardless of whether you test for "if x" or "if x != 0".


> Conclusion: the boolean value of an object is to be used with care in
> order to tests if an optional parameter is given or not (when default
> value is None).

Or, to put it another way: if you want to test for an object being None, 
test for the object being None.


-- 
Steven