[Python-3000] PEP 3100 Comments

[Steven Bethard]

On 5/8/06, Guido van Rossum <guido at python.org> wrote:
> On 5/8/06, Steven Bethard <steven.bethard at gmail.com> wrote:
> > It'd certainly be nice to be able to tell the difference between
> > the following two TypeErrors:
> >
> >     >>> def s():
> >     ...     raise TypeError()
> >     ...
> >     >>> 's'()
> >     Traceback (most recent call last):
> >       File "<interactive input>", line 1, in ?
> >     TypeError: 'str' object is not callable
> >     >>> s()
> >     Traceback (most recent call last):
> >       File "<interactive input>", line 1, in ?
> >       File "<interactive input>", line 2, in s
> >     TypeError
>
> You're kidding yourself. Consider these two:
>
> def s():
>   raise NotCallable("ha ha, fooled you!")

This one does exactly what I would hope it to.  The writer of the
callable has indicated that this function is not to be called by
raising NotCallableError.  This would be especially appropriate in a
subclass that wants to break substitutability and remove the __call__
method.  Sure, people could abuse it, but we're all supposed to be
adults here, right?

> or more likely any variant of this:
>
> def s():
>   's'()

Sorry if I came across as suggesting that adding NotCallableError
would solve all the world's problems ;-) If you caught a
NotCallableError, you would know that either the object you called was
not callable, or it called some other object that was not callable and
didn't trap the exception.  That's a fair bit more informative than
the current situation with TypeError, since a lot more things raise
TypeErrors:

    >>> iter(1)
    Traceback (most recent call last):
      File "<interactive input>", line 1, in ?
    TypeError: iteration over non-sequence
    >>> 'abc'['1']
    Traceback (most recent call last):
      File "<interactive input>", line 1, in ?
    TypeError: string indices must be integers
    >>> str(1, 16)
    Traceback (most recent call last):
      File "<interactive input>", line 1, in ?
    TypeError: str() takes at most 1 argument (2 given)
    >>> 42 + '56'
    Traceback (most recent call last):
      File "<interactive input>", line 1, in ?
    TypeError: unsupported operand type(s) for +: 'int' and 'str'

So if I catch a TypeError, it could be because the object I called was
not callable, or it could be because when calling that object, any one
of the above TypeErrors was raised.

Still, like you say, NotCallableError's not a silver bullet.

STeVe
--
Grammar am for people who can't think for myself.
        --- Bucky Katt, Get Fuzzy