[Tutor] strange eval

[Doug Penny]

Thanks for the info.  Good point about using eval().  They have been
shown int(), but for some reason this student started using eval() and
I just didn't think about guiding him back to int().

Doug


On Mon, 4 Oct 2004 07:05:59 -0700 (PDT), Danny Yoo
<dyoo at hkn.eecs.berkeley.edu> wrote:
> 
> On Mon, 4 Oct 2004, Kent Johnson wrote:
> 
> > Number literals starting with 0 are interpreted as _octal_! See
> > http://docs.python.org/ref/integers.html
> >  >>> eval('010')
> > 8
> >  >>> eval('0100')
> > 64
> >
> > 8 and 9 are not valid octal digits, hence the error.
> 
> Hi Doug,
> 
> And don't use eval() to get numbers from strings: it's far too powerful a
> tool to be used casually!  *grin*
> 
> Use int() instead: it's a converter to integers:
> 
> ###
> >>> int('010')
> 10
> >>> int('0100')
> 100
> ###
> 
> If we really want an octal interpretation, we can explicitely tell int()
> what base to use:
> 
> ###
> >>> int('010', 8)
> 8
> >>> int('0100', 8)
> 64
> ###
> 
> I also think it's not a good thing to teach students to use eval()
> without showing the repercussions.  eval() has the same access to the
> Python runtime as the running program, and that's dangerous if not handled
> properly.  We can talk about the security implications of eval() if you'd
> like.
> 
> Hope this helps!
> 
>