weird behaviour of "0 in [] is False"
Duncan Booth
duncan.booth at invalid.invalid
Tue Nov 30 14:24:42 CET 2004
Sylvain Thenault wrote:
> Hi there !
>
> Can someone explain me the following behaviour ?
>
>>>> l = []
>>>> 0 in (l is False)
> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> TypeError: iterable argument required
>>>> (0 in l) is False
> True
>>>> 0 in l is False
> False
>
>
> This is really obscur to me...
>
>From the language reference (5.9 Comparisons):
> comparison ::= or_expr ( comp_operator or_expr )*
> comp_operator ::= "<" | ">" | "==" | ">=" | "<=" | "<>" | "!="
> | "is" ["not"] | ["not"] "in"
>
> ...snip...
>
> Formally, if a, b, c, ..., y, z are expressions and opa, opb, ..., opy
> are comparison operators, then a opa b opb c ...y opy z is equivalent
> to a opa b and b opb c and ... y opy z, except that each expression is
> evaluated at most once.
In other words '0 in l is False' is equivalent to '0 in l and l is False'.
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