File names and file objects [was Re: My Python annoyances]

[Alex Martelli]

Steven D'Aprano <steve at REMOVE.THIS.cybersource.com.au> wrote:
   ...
> What do people think about functions that accept either a file name or a
> file object?
> 
> def handle_file(obj):
>     if type(obj) == str:
>         need_to_close = True
>         obj = file(obj, 'r')
>     else: 
>         need_to_close = False
>     do_something_with(obj.read())
>     if need_to_close:
>         data.close()
> 
> Good idea? Bad idea? Just a matter of personal preference?

Acceptable as an idea, but a disaster in terms of this specific
implementation (as coded, it would reject a Unicode string, or any other
string-like object, for example).  Also, if all you're going to do with
the file is .read() it in one big gulp, there's no real advantage to
this approach, either.

Assuming the way you're going to use the file-like object is subtler
(e.g., loop line by line so that huge files can be processed without
overwhelming memory), then a better implementation may be warranted:

def handle_file(file_or_path):
    try:
        f = open(file_or_path)
        finis = f.close
    except TypeError:
        f = file_or_path
        def finis(): pass
    try:
        for line in f:
            ...
    finally:
        finis()

This version accepts anything that open is happy with, or else any
sequence of lines, including but not limited to a file or file-like
object open for reading.  Now this, it seems to me, is a helpful
approach to polymorphism.


Alex