Type checking in python?

[Eric Hopper]

In article <397296BC.415C97E1 at prescod.net>, Paul Prescod
<paul at prescod.net> wrote:
> Matthew Cline wrote:
>> 
>>   def my_func(foo, bar, quux):
>>      if not isinstance(quux, Quux):
>>         raise RuntimeError("param 'quux' must be of class 'Quux'")
> 
> Let me also point out that in many cases, this style of programming
> would be frowned upon by serious Python programmers. For instance, if
> you check that something is a string, your code will complain when it is
> handed a Unicode string, even though it would probably work fine. If you
> check that it is an open file, then your code probably will complain
> about stringIO file-like objects, even though it would probably work
> fine. If you check that it is an integer, your code will complain about
> integers, even though ti would probably work fine.
> 
> In general, the Python philosophy is to "just try it" and see if it
> works. Sometimes type checks are appropriate but if you do it on the
> entry to every function, you are probably doing too much work and making
> your own life (or someone else's) harder later on.

Oh, yes.  Inheritance by mysteriously coiniciding method names.  How fun.

Actually, I'm going to write a type checking function that throws a
coherent exception when the type of the argument fails to be a particular
type.  I'll probably put in some specialized checks for the built in types
so it'll accept things that are objects of those types, or derived from
the User<class> version.  It'd be nice if Python had lisp-like macros, but
they aren't necessary.

My problem is this:

Just accepting any old type, and letting it error out when it tries to
access a non-existent attribute is very confusing for people trying to use
your code.  Heck, maybe you store away their object, and then call the
offending method at some random later point.  I bet it'd be a heck of a
lot of fun to try to trace down why the exception happened then.

Type checking and the scoping rules are the only two problems I have with
Python so far.  Other than that, it's a wondefrul language.

--
Eric Hopper  (hopper at omnifarious.mn.org, http://www.omnifarious.org/~hopper)