raise None

[Mark Lawrence]

On 31/12/2015 00:09, Steven D'Aprano wrote:
> I have a lot of functions that perform the same argument checking each time:
>
> def spam(a, b):
>      if condition(a) or condition(b): raise TypeError
>      if other_condition(a) or something_else(b): raise ValueError
>      if whatever(a): raise SomethingError
>      ...
>
> def eggs(a, b):
>      if condition(a) or condition(b): raise TypeError
>      if other_condition(a) or something_else(b): raise ValueError
>      if whatever(a): raise SomethingError
>      ...
>
> Since the code is repeated, I naturally pull it out into a function:
>
> def _validate(a, b):
>      if condition(a) or condition(b): raise TypeError
>      if other_condition(a) or something_else(b): raise ValueError
>      if whatever(a): raise SomethingError
>
> def spam(a, b):
>      _validate(a, b)
>      ...
>
> def eggs(a, b):
>      _validate(a, b)
>      ...
>
> But when the argument checking fails, the traceback shows the error
> occurring in _validate, not eggs or spam. (Naturally, since that is where
> the exception is raised.) That makes the traceback more confusing than it
> need be.

I disagree.

>
> So I can change the raise to return in the _validate function:
>
> def _validate(a, b):
>      if condition(a) or condition(b): return TypeError
>      if other_condition(a) or something_else(b): return ValueError
>      if whatever(a): return SomethingError
>
> and then write spam and eggs like this:
>
> def spam(a, b):
>      ex = _validate(a, b)
>      if ex is not None: raise ex
>      ...
>
> It's not much of a gain though. I save an irrelevant level in the traceback,
> but only at the cost of an extra line of code everywhere I call the
> argument checking function.
>
> But suppose we allowed "raise None" to do nothing. Then I could rename
> _validate to _if_error and write this:
>
> def spam(a, b):
>      raise _if_error(a, b)
>      ...
>
>
> and have the benefits of "Don't Repeat Yourself" without the unnecessary,
> and misleading, extra level in the traceback.
>
> Obviously this doesn't work now, since raise None is an error, but if it did
> work, what do you think?
>

A lot of fuss over nothing.

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence