[Python-Dev] file(fileinput.input())

[Guido van Rossum]

> >Aahz:
> >> Should file(fileinput.input()) work?  Currently it raises an
> >> exception because fileinput.input() returns neither a string nor a
> >> buffer object.

Guido:
> > What on earth would you want it to do?  It doesn't seem to make any
> > sense to me.

Aahz:
> Well, if it should work, it should return the FileInput instance (i.e.
> return self).

Hm, what analogy would lead you to that thinking?  'file' is the
concrete file type.

> On thinking further, I'm going to use a slightly
> different question divorced from my current problem:
> 
> What should list(UserList()) return?

That's perfectly well-defined already: a list consisting of the items
of the UserList.

> What about 
> 
>     class myList(list):
>         pass
>     l = list(myList())
> 
> I suppose this question is related to PEPs 245/246.  The broader
> question is, what should a constructor return when called on a related
> object or subclass?

I don't know where you want to go with this.  If Foo is a class,
Foo(...) should return a foo instance constructed from the arguments.

> Getting back to my specific example, I've got this function:
> 
>     def grep(f, regex):
>         f = file(f)
>         regex = re.compile(regex)
>         for line in f:
>             if regex.search(line):
>                 yield line
> 
> I was originally passing in file handles or filenames, demonstrating to
> the students that calling a constructor on an existing object returns
> that object if it doesn't need to do any real work.

But that's not even a rule!  str() and tuple() can return the
original, but only if has exactly the required type.  list() always
returns a new list -- that's its *purpose*.

> Alex suggested that
> I use fileinput.input(), whereupon my function blew up.
> 
> The meta question is, what kind of programming style do we want to push?
> More smarts on top or bottom?

Teach them Python, please.

--Guido van Rossum (home page: http://www.python.org/~guido/)