On Fri, Sep 02, 2022 at 12:53:47AM -0000, Steve Jorgensen wrote:
> I didn't say that I was talking about a file. In fact, today, I'm
> talking about an object that manages a subprocess. If a caller tries
> to call a method of the manager to interact with the subprocess when
> the subprocess has not yet been started or after it has been
> terminated, then I want to raise an appropriate exception.
Your API shouldn't allow the object to be created and returned until it
has been started.
Long ago I read a great blog post describing this idiom as an
anti-pattern:
obj = SomeObject(args)
obj.make_it_work()
obj.do_something()
where constructing the object isn't sufficient to start using it, you
have to call a second, magical method (sometimes called "init()"!) to
make it actually usable. Unfortunately I lost the URL and have never
been able to find it again, but it made a big impression on me.
You will notice that we don't do that with Python builtins:
# we don't do this!
f = open('filename', 'w')
f.open() # Actually open it for writing.
f.write('stuff')
# Or this:
d = dict(key=value)
d.init() # Can't use the dict until we make it work.
d.update(something)
Returning a dict in an initialiated but invalid state, which needs a
second method call to make it valid, would be an obvious antipattern.
Why would we do such a thing? Its a bad design! Just have the dict
constuctor return the dict in the fully valid state.
Which of course we do: for most (all?) objects in the stdlib, creating
the object *fully* initialises it, so it is ready to use, immediately,
without needing a special "make it work" method call.
Of course there may be some exceptions to that rule, but they are
uncommon.
> I am
> raising a custom exception, and it annoys me that it has to simply
> inherit from Exception when I think that an invalid state condition is
> a common enough kind of issue that it should have a standard exception
> class in the hierarchy.
Perhaps you should take your lead from file objects and inherit
from ValueError. Trying to interact with a terminated process is
analogous to writing to a closed file.
What sort of features or APIs do you expect to inherit from this
InvalidStateError class?
--
Steve
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