question about True values

["Martin v. Löwis"]

skip at pobox.com schrieb:
> the string class's "nil" value.  Each of the builtin types has such an
> "empty" or "nil" value:
> 
>     string              ""
>     list                []
>     tuple               ()
>     dict                {}
>     int                 0
>     float               0.0
>     complex             0j
>     set                 set()
> 
> Any other value besides the above will compare as "not false".

This list of values that are considered false is incomplete,
though. Four obvious omissions are

long       0L
unicode    u""
bool       False
NoneType   None

Not-so-obviously, arbitrary user-defined values can also be
treated as false: If they implement __nonzero__, they are
false if False is returned from __nonzero__; otherwise,
if they implement __len__, they are false if 0 is returned
from __len__. Under these rules, array.array objects can
also be false, as can UserList and UserDict objects.

Regards,
Martin