[Python-Dev] Evil isinstance()

[Guido van Rossum]

> On Sat, Mar 30, 2002, Guido van Rossum wrote:
> > David Abrahams:
> >> 
> >> Given that Bool is immutable, I have no cases other than examples of
> >> type introspection which can be written to account for the fact that
> >> Bool is-a Int. However, it does make certain things trickier to get
> >> right:
> >> 
> >> numeric_types = [ Int, Long, Bool, Float, Complex ]
> >> for t in numeric_types:
> >>     if isinstance(x, t):
> >>         # Do something...
> >> 
> >> This sort of thing could depend on getting the order of the list right
> >> (I didn't).
> > 
> > Using isinstance() this way is usually bad.  And who knows that long
> > doesn't inherit from int?  (At some point in the future it may, or
> > they may become the same thing -- see PEP 237.
> 
> The problem is that in some cases the __init__ for a class needs to
> dispatch based on the type of the operand.  For example, int() takes
> both numbers and strings.  What should we recommend as "standard
> practice" for this issue if isinstance() is disrecommended?

Ah, bollocks.  David was just being annoying, and I was being annoying
back.  You can use isinstance() when it makes sense, like your
example.

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