"0 in [True,False]" returns True

[Steve Holden]

Pierre Quentel wrote:
> Hi all,
> 
> In some program I was testing if a variable was a boolean, with this 
> test : if v in [True,False]
> 
> My script didn't work in some cases and I eventually found that for v = 
> 0 the test returned True
> 
> So I changed my test for the obvious "if type(v) is bool", but I still 
> find it confusing that "0 in [True,False]" returns True
> 
> By the way, I searched in the documentation what "obj in list" meant and 
> couldn't find a precise definition (does it test for equality or 
> identity with one of the values in list ? equality, it seems) ; did I 
> miss something ?
> 
It actually uses the __contains__() method of the right-hand operand, 
and in the case of a list that will test for equality of the left-hand 
operand to one of the list elements. Since False == 0 that's why you see 
what you do.

The really interesting question your post raises, though, is "Why do you 
feel it's necessary to test to see whether a variable is a Boolean?".

regards
  Steve
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