My Python annoyances

[Alex Martelli]

Ben Collver <collver at peak.org> wrote:

> Chris Mellon wrote:
> > You should "check" for the methods by calling them. If the object
> > doesn't support the method in question, you will get a runtime
> > exception. Premature inspection of an object is rarely useful and
> > often outright harmful.
> 
> That makes sense, thank you for the response.
> 
> What about the case where I have an array of objects that represent some
> particular binary file format.  If the object is a file, then I want to
> copy its contents.  If the object is a string, then I want to write the
> string.  And so forth.

"Type-switching" in this way is a rather dubious practice in any
language (it can't respect the "open-closed" principle).  Can't you have
those objects wrapped in suitable wrappers with a "copyorwrite" method
that knows what to do?  For example, StringIO.StringIO is a standard
library wrapper type that makes a string look like a file.

It's a reasonable request you can make to whatever code is putting stuff
in that container: make the container "weakly homogeneous" by having it
stuffed only with "suitably file-like" objects.  Dealing with totally
and weirdly heterogeneous containers is not a sensible task, because
there will be infinite types that COULD be in the container and about
which your code just can't be expected to guess right what to do.


Alex