[Python-Dev] trunc()

[Guido van Rossum]

On Jan 24, 2008 10:36 AM, Raymond Hettinger <python at rcn.com> wrote:
> Can anyone explain to me why we need both trunc() and int()?

trunc() has well-defined semantics -- it takes a Real instance and
converts it to an Integer instance using round-towards-zero semantics.

int() has undefined semantics -- it takes any object and converts it
to an int (a concrete type!) using whatever rules it likes -- the
definition of __int__ is up to whatever the source type likes to do.
For float this has been defined the same as trunc() above, but for
other types, who knows! int() of a string does something completely
different.

Perhaps worse is that sometimes int() is lossy (e.g. with a float
input) and sometimes it is not (e.g. with a string input, or with a
non-standard representation of integers). There are still some places
where a float is accepted incorrectly (silently truncating) due to the
use of the __int__ slot. If trunc() had been part of the language from
the beginning we wouldn't have needed to introduce __index__.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)